Gram Negative Bacilli: Enterobacteriaceae Overview - Medical Microbiology

Table of Contents

• Enterobacteriaceae: Major Genera
• Introduction to Enterobacteriaceae
• Antigenic Structure of Enterobacteriaceae
• Escherichia coli (E. coli)
• Salmonella
• Shigella
• Pseudomonas aeruginosa
• Klebsiella Species
• Enterobacter Species
• Serratia Species
• Proteus Species
• Morganella Species
• Providencia Species

Enterobacteriaceae: Major Genera

• Escherichia
• Shigella
• Salmonella
• Edwardsiella
• Citrobacter
• Yersinia
• Klebsiella
• Enterobacter
• Serratia
• Proteus
• Morganella
• Providencia

Introduction to Enterobacteriaceae

• Enterobacteriaceae are Gram-negative, rod-shaped (bacilli) bacteria.
• They ferment rather than oxidize D-glucose, producing acid and often gas as end products.
• They reduce nitrate to nitrite.
• They grow readily on 5% sheep blood or chocolate agar under carbon dioxide or ambient air conditions.
• They are facultative anaerobes, capable of growing in both the presence and absence of oxygen.
• They are catalase positive.
• They are cytochrome oxidase negative.
• They grow readily on MacConkey (MAC) and eosin methylene blue (EMB) agars.
• Most species grow well at 35°C, while Yersinia grows best at 25°C to 30°C.
• They are typically motile by peritrichous flagella, except Shigella and Klebsiella, which are non-motile.
• They do not form spores.
• Coliforms are members that rapidly ferment lactose.
• Noncoliforms are members that do not ferment lactose or ferment it slowly.

Antigenic Structure of Enterobacteriaceae

• Enterobacteriaceae possess a complex antigenic structure that includes:
• Somatic or O antigen.
• Flagellar or H antigen.
• Capsular or K antigen.

Somatic or O Antigens

• O antigens form the outermost part of the cell wall lipopolysaccharide and consist of repeating polysaccharide units (15–20 sugars).
• Each genus within Enterobacteriaceae is associated with specific O groups, and a single organism may possess multiple O antigens.
• The number of O antigens is extensive, e.g., approximately 1500 types in Salmonella and 150 types in E. coli.
• Most Shigella species share one or more O antigens with E. coli.
• E. coli may also cross-react with some Providencia, Klebsiella, and Salmonella species.
• O antigens are resistant to heat and alcohol and are usually detected through bacterial agglutination tests.

Flagellar or H Antigen

• H antigens are located on the bacterial flagella and can be denatured or removed by heat or alcohol.
• Within a single serotype, flagellar antigens may exist in two forms (phase 1 and phase 2), and bacteria can switch between these forms, a phenomenon called phase variation.
• The presence of H antigens on the bacterial surface may interfere with agglutination by anti-O antibodies.

Capsular or K Antigen

• K antigens are found external to O antigens in some, but not all, Enterobacteriaceae.
• Some K antigens are polysaccharides (e.g., K antigens of E. coli), while others are proteins.
• K antigens may be associated with virulence; for example, E. coli strains producing K1 antigen are significant in neonatal meningitis, and other K antigens facilitate attachment to epithelial cells before gastrointestinal or urinary tract invasion.
• Klebsiella species possess large polysaccharide capsules (K antigens) that cover the somatic (O or H) antigens, and these can be identified by capsular swelling tests using specific antisera.

Introduction of Escherichia coli

Characteristics of Escherichia coli

• Escherichia coli (E. coli) is a Gram-negative rod-shaped bacterium.
• It is a facultative anaerobe, meaning it can grow in both the presence and absence of oxygen.
• E. coli is part of the normal intestinal flora of humans and warm-blooded animals.
• It protects the intestinal tract from infections by competing with pathogenic bacteria.
• It assists in digestion and maintains a balanced gut environment.
• E. coli produces small amounts of vitamins, particularly vitamin B12 and vitamin K.
• It colonizes the gastrointestinal (GI) tract of newborns within hours after birth.
• There are more than 700 different serotypes of E. coli identified to date.
• These serotypes are distinguished by variations in surface proteins and polysaccharides.

Clinical Manifestations of Escherichia coli

Urinary Tract Infections (UTIs):

• E. coli is responsible for approximately 80% of all urinary tract infections.

Gastroenteritis:

• Certain strains of E. coli cause intestinal infections, leading to diarrhea, abdominal cramps, and vomiting.

Neonatal Meningitis:

• Some strains, particularly those producing the K1 antigen, are a major cause of meningitis in newborns.

Septicemia (UTI/GTI Origin):

• E. coli can enter the bloodstream from a urinary or gastrointestinal infection, resulting in septicemia.

Spontaneous Peritonitis:

• E. coli may cause spontaneous bacterial peritonitis, especially in patients with liver cirrhosis or ascites.

Endotoxin-mediated Toxicity with Septicemia

• When Gram-negative bacteria enter the bloodstream, they release blebs of outer membrane that contain lipopolysaccharide (LPS).
• The endotoxin, specifically the lipid A portion of LPS, plays a major role in triggering toxic effects.
• Lipid A activates the complement system and binds to toll-like receptors (TLRs) present on macrophages, endothelial cells, and epithelial cells.
• This activation leads to the release of numerous cytokines and inflammatory mediators.
• These cytokines and complement components are responsible for the clinical manifestations of bacterial sepsis, which include:
• Fever
• Leukopenia, followed by leukocytosis
• Thrombocytopenia
• Disseminated Intravascular Coagulation (DIC)
• Decreased peripheral circulation and reduced perfusion to major organs shock
• Death
• Leukopenia: Refers to a reduced white blood cell (leukocyte) count.
• Thrombocytopenia: Indicates a reduced platelet count.
• Disseminated Intravascular Coagulation (DIC): A condition involving systemic activation of blood coagulation, leading to fibrin deposition and clot formation in small blood vessels throughout the body.

Enterohemorrhagic E. coli

Antigenic Structure of Enterohemorrhagic E. coli

• E. coli possesses 150 O (somatic), 90 K (capsular), and 50 H (flagellar) antigens.
• Different combinations of these antigens give rise to more than 1000 distinct antigenic types of E. coli.

Pathogenesis of Enterohemorrhagic E. coli

• Reservoir: Found in both humans and animals (particularly cattle).
• Transmission: Occurs primarily via the fecal-oral route, often through contaminated food or water.
• Neonatal meningitis: Can be acquired during childbirth through the birth canal.
• Pili (fimbriae): Facilitate attachment and colonization of the ileal mucosa.
• Cytotonic enterotoxins: Encoded by plasmid or bacteriophage DNA, these toxins stimulate secretion, leading to watery diarrhea.
• Cytotoxic enterotoxins: Also plasmid- or bacteriophage-encoded, these toxins cause tissue damage. The resulting host inflammatory response with lymphocytic infiltration leads to dysentery-like symptoms.
• Plasmid-encoded invasion factors: Allow the bacteria to invade the intestinal mucosa, contributing to tissue injury and disease severity.
• Well-described categories of pathogenic E. coli include:
• Enteroinvasive E. coli (EIEC)
• Enteropathogenic E. coli (EPEC)
• Enterohemorrhagic E. coli (EHEC)
• Enterotoxigenic E. coli (ETEC)
• Uropathogenic E. coli (UPEC)

Enteroinvasive E. coli (EIEC)

• EIEC strains possess invasion factors that enable them to invade and destroy the colonic epithelium.
• This leads to inflammation and tissue damage resembling the effects of Shigella infection.

Enteropathogenic E. coli (EPEC)

• EPEC is a major cause of infantile diarrhea in developing countries.
• It adheres to mucosal cells of the small intestine using chromosomally mediated adherence factors.
• Infection causes loss of microvilli and cellular damage, leading to malabsorption.
• Characteristic attaching and effacing lesions are visible in electron micrographs of small bowel biopsies.
• The infection results in watery diarrhea, which is self-limiting but can become chronic in some cases.

Enterotoxigenic E coli (ETEC)

• ETEC is a common cause of traveler’s diarrhea and an important cause of infant diarrhea in developing regions.
• Possesses colonization factors that promote adherence to small intestinal epithelial cells.
• Produces two major toxins:
• Heat-labile toxin (LT)
• Heat-stable toxin (ST)
• These toxins act primarily in the jejunum and ileum, not the colon.
• ETEC does not invade the mucosa or cause significant inflammation.
• The disease manifests as profuse watery diarrhea without blood or mucus.

Enterohaemorrhagic E. coli (EHEC)

• EHEC strains produce a cytotoxic verotoxin (Shiga-like toxin) that acts on intestinal villi and colonic epithelial cells, resulting in bloody diarrhea, abdominal cramps, and fever.
• A severe complication is the Hemolytic Uremic Syndrome (HUS), especially associated with E. coli O157:H7 strains.
• HUS involves hemolytic anemia due to premature destruction of red blood cells.
• Damaged RBCs clog the renal filtration system, leading to acute kidney failure.
• Thrombocytopenia occurs because platelets adhere to damaged endothelial surfaces.

Uropathogenic E.coli

• UPEC strains possess pili with adhesin proteins that bind specifically to receptors on urinary tract epithelium.
• Infections may be community-acquired or nosocomial (hospital-acquired).
• Females are more prone to urinary tract infections than males.
• Cystitis: Infection limited to the urinary bladder.
• Pyelonephritis: Infection that ascends to the kidneys, causing more severe disease.

Control of E. coli

• Prevention primarily relies on strict sanitary measures to prevent fecal-oral transmission.
• Hand-washing before eating and after using the toilet is essential to minimize infection risk.
• Proper food handling and preparation help prevent contamination and spread of pathogenic E. coli.
• Chlorination of water supplies ensures safe drinking water and reduces transmission through contaminated sources.
• Effective sewage treatment and disposal prevent environmental contamination and waterborne outbreaks.
• Parenteral or oral fluid and electrolyte replacement therapy is critical to prevent dehydration in patients with diarrhea.
• Broad-spectrum antibiotics may be used in chronic, severe, or life-threatening infections, although they are generally avoided in mild diarrheal cases to prevent resistance.
• Maintaining personal hygiene is a key preventive measure in reducing the spread of infection.

Introduction to Salmonella

Characteristics of Salmonella

• Salmonella are Gram-negative, non–spore-forming, motile rod-shaped bacteria.
• They possess peritrichous flagella, enabling active motility.
• They do not ferment lactose, distinguishing them from many other Enterobacteriaceae.
• They produce hydrogen sulfide (H₂S), which can be detected on selective media such as TSI agar.
• Antigenic structure includes:
• O (somatic) antigen – present on the cell wall.
• H (flagellar) antigen – associated with bacterial flagella.
• Vi (capsular or virulence) antigen – found on the capsule and contributes to pathogenicity.
• Important species and serovars include:
• Salmonella enterica serovar Typhi (S. typhi)
• Salmonella enterica serovar Paratyphi (S. paratyphi)
• Salmonella enterica *serovar Typhimurium (S. typhimurium)

Pathogenesis of Salmonella

• Salmonella is sensitive to stomach acid, therefore a large number of organisms must be ingested to survive passage through the acidic environment of the stomach.
• Upon reaching the small intestine, adhesins on the bacterial surface bind to specific receptors on the intestinal epithelial cells.
• This attachment triggers endocytosis, allowing the bacteria to be engulfed into epithelial cells.
• Once inside, the bacteria are either destroyed by macrophages or survive and multiply within them, leading to infection.
• After multiplication, Salmonella can enter the bloodstream, causing bacteremia.
• Through the bloodstream, S. typhi can spread to various organs, including the lymph nodes, gallbladder, liver, spleen, and other tissues.
• Some individuals may become chronic carriers of S. typhi, continuously shedding the bacteria in their stools for years and serving as a source of infection to others.

Invasion of Salmonella

• The invasion of host intestinal cells by Salmonella causes dramatic morphological changes in the host cell, primarily due to the manipulation of the host cytoskeleton.
• When Salmonella comes into close contact with the intestinal epithelium, it induces degeneration of enterocyte microvilli, disrupting the normal brush border structure.
• This loss of microvillar structure is followed by intense membrane ruffling at the site of bacterium–host cell contact.
• The membrane ruffling is associated with extensive macropinocytosis, through which the bacteria are internalized into the host cells.
• The entire invasion process occurs rapidly—within a few minutes.
• After internalization, Salmonella becomes enclosed within membrane-bound vesicles inside the host cell.
• Once the invasion is complete, the host cell cytoskeleton returns to its normal organization, while the bacteria continue to survive and replicate within the vesicular compartment.

Clinical signs of Salmonella

• Malaise – a general feeling of weakness and discomfort.
• Constipation – often observed during the early stages of infection.
• Abdominal tenderness – due to intestinal inflammation and infection.
• Fever – a characteristic feature, typically high and sustained.
• Appearance of “Rose spots” – faint, pink, maculopapular rashes that appear on the chest and abdomen, commonly seen in typhoid fever.

Complications of Salmonella Infection

• Intestinal perforation – rupture of the intestinal wall due to deep ulceration, leading to peritonitis.
• Intestinal hemorrhage – bleeding within the intestines as a result of mucosal damage.
• Ulceration of the intestinal mucosa, particularly in the Peyer’s patches of the small intestine, which are key sites of bacterial invasion and inflammation.

Epidemiology of Salmonella

• Salmonella can survive for extended periods in the environment, particularly in water, soil, and food materials.
• Eggs and poultry are among the most important sources of Salmonella strains responsible for human infection.
• Chronic carriers serve as a major reservoir and play a central role in person-to-person (man-to-man) transmission.
• Transmission commonly occurs through the “five F’s” route:
• Feces,
• Fingers,
• Food,
• Flies,
• Fomites (contaminated objects or surfaces).

Laboratory Diagnosis of Salmonella

• Bacterial culture on differential media such as Eosin Methylene Blue (EMB) and MacConkey agar to detect non-lactose fermenting colonies.
• Triple Sugar Iron (TSI) agar test: shows alkaline (red) slant and acid (yellow) butt with H₂S production (blackening).
• Serological test – Widal Test: detects agglutinating antibodies against Salmonella typhi “O” (somatic) and “H” (flagellar) antigens in patient serum.

Introduction to Shigella

Characteristics of Shigella

• Shigella are Gram-negative rod-shaped bacteria that are important human pathogens.
• They are non-lactose fermenters, which helps in differentiating them from many other members of the Enterobacteriaceae family.
• The bacteria are non-motile, lacking flagella for movement.
• They do not produce hydrogen sulfide (H₂S), which is another key distinguishing feature in biochemical identification.
• Shigella possess the “O” (somatic) antigen, which plays a role in serological classification and immune recognition.
• There are four main species of Shigella: Shigella sonnei, Shigella dysenteriae, Shigella flexneri, and Shigella boydii.
• Shigella infections are almost always confined to the gastrointestinal tract, as bloodstream invasion is quite rare.
• The bacteria are highly communicable, meaning that infection can spread easily from person to person, even with a very small infectious dose.
• The pathogenesis of Shigella involves several steps:
• The bacteria invade the mucosal epithelial cells, particularly M cells in the intestinal lining.
• They induce phagocytosis to enter the host cells.
• After being taken up, they escape from the phagocytic vacuole to survive and multiply within the host cell cytoplasm.
• They then multiply and spread laterally within the epithelial cells.
• Finally, they pass directly into adjacent cells, leading to destruction of epithelial layers, tissue damage, and the clinical symptoms of dysentery.

Pathology/Pathogenesis of Shigella

• Shigella species are highly effective pathogens, capable of causing infection with a very low infectious dose (ID₅₀).
• Ingestion of as few as 100 organisms is sufficient to produce disease in humans.
• The bacteria primarily affect the villi of the large intestine, where they initiate infection and tissue damage.
• Shigella is less invasive than Salmonella, as it does not perforate the intestinal wall or enter the bloodstream.
• The infection results in local inflammation and ulceration of the intestinal mucosa.
• Disease begins when Shigella attaches specifically to receptors on M cells in the large intestine, followed by induced phagocytosis that allows bacterial entry.
• Once inside the host cell, Shigella escapes from the phagosome, multiplies in the cytoplasm, and ultimately kills the infected cell.
• Although Shigella are non-motile, they spread efficiently from cell to cell, enabling rapid dissemination across the intestinal epithelium.
• Microabscesses form in the large intestine and terminal ileum, leading to necrosis of the mucous membrane, superficial ulceration, bleeding, and formation of a pseudomembrane over the ulcerated area.
• This pseudomembrane is composed of fibrin, leukocytes, cell debris, necrotic mucosa, and bacteria.
• As the infection resolves, granulation tissue fills the ulcers, and scar tissue develops at the healed sites.
• The movement of bacteria within the mucosa leads to sloughing of the epithelium, causing intense inflammation and bleeding.
• Another key pathogenic mechanism involves the production of Shiga toxin, a potent cytotoxin that contributes to mucosal damage.
• In severe or complicated infections, toxin-mediated damage can lead to hemolytic uremic syndrome (HUS).
• In HUS, red blood cells rupture in capillaries, resulting in hemolytic anemia and kidney failure, which are serious systemic complications.

Mechanism of Movement of Shigella

• After being engulfed by the host cell, Shigella becomes enclosed within a membrane-bound vacuole.
• Unlike Salmonella, Shigella rapidly lyses the vacuolar membrane, escaping into the cytosol where it begins to grow and divide.
• Once in the cytoplasm, the bacterium becomes coated with filamentous actin, forming an actin tail at one pole.
• Through actin polymerization, Shigella is propelled through the cytoplasm at speeds of approximately 0.4 µm per second.
• Upon reaching the plasma membrane, the bacterium causes the formation of a long protrusion that extends into an adjacent cell.
• The neighboring cell internalizes the protrusion, thereby taking up the bacterium.
• Inside the new host cell, Shigella again escapes from the vacuole, initiating another cycle of intracellular infection.
• This unique mechanism allows Shigella to spread directly from cell to cell without exposure to the extracellular environment, helping it evade immune defenses and sustain infection.

Epidemiology of Shigella

• Shigella is a strictly human pathogen, with no animal reservoir existing in nature.
• Transmission takes place via the fecal-oral route, making hygiene and sanitation critical in prevention.
• The four F’s—feces, fingers, food, and flies—serve as the main vehicles for transmission.
• Outbreaks are commonly observed in settings with close person-to-person contact or poor hygiene, such as daycare centers, nurseries, and mental health institutions, where fecal-oral transmission is more likely to occur.

Clinical signs of Shigella

• Headache is a common early symptom of Shigella infection.
• Nausea often accompanies the onset of illness due to intestinal irritation.
• Diarrhea begins as watery stools but later becomes bloody and mucoid as the intestinal mucosa becomes ulcerated and inflamed.

Laboratory Diagnosis of Shigella

• Bacterial culture is performed on differential media such as Eosin Methylene Blue (EMB) and MacConkey (MAC) agar to isolate and identify Shigella colonies.
• Triple Sugar Iron (TSI) agar is used to differentiate Shigella based on its characteristic reactions (no H₂S production and alkaline slant with acid butt).
• Methylene blue staining of a fecal sample helps determine the presence of neutrophils, indicating inflammatory diarrhea typical of Shigella infection.

Introduction to Pseudomonas aeruginosa

Characteristics of Pseudomonas aeruginosa

• Pseudomonas aeruginosa is a Gram-negative, motile, and aerobic bacterium.
• It is nonfermentative, meaning it does not ferment carbohydrates.
• Exists in two distinct forms: planktonic (free-living) and biofilm (surface-attached).
• Shows optimum growth at 37°C, but can also grow at 42°C, demonstrating its adaptability.
• Has minimal nutritional requirements, allowing it to survive in diverse environments, including soil, water, and hospital settings.
• Exhibits three main colony types:
• Rough type – irregular and dry colonies.
• Smooth type – moist and glistening colonies.
• Mucoid type – slimy appearance due to excess alginate production, often associated with chronic infections (e.g., cystic fibrosis).

Virulence Factors of Pseudomonas aeruginosa

Pseudomonas aeruginosa possesses multiple virulence factors that contribute to its pathogenicity and ability to cause severe infections.

Elastase

• Cleaves collagen, IgG, IgA, and complement components.
• Lyses fibronectin, exposing receptors that promote bacterial attachment to the lung mucosa.
• Disrupts respiratory epithelium and interferes with ciliary function, aiding colonization and tissue damage.

Protease (Alkaline protease)

• Interferes with fibrin formation and lyses fibrin, preventing effective clot formation and aiding bacterial spread.

Exotoxin A

• Inhibits elongation in eukaryotic protein synthesis, leading to cell death.

P. aeruginosa produces three other soluble proteins involved in

invasion:

• A cytotoxin (25 kDa) – a pore-forming protein that damages host cell membranes.
• Two hemolysins – disrupt red blood cells and host cell membranes.
• A phospholipase and a lecithinase – degrade phospholipids in host cell membranes, promoting tissue damage.

Pseudomonas pigments also play a role in virulence:

• Pyocyanin (blue pigment) – impairs nasal ciliary function, disrupts respiratory epithelium, and causes proinflammatory effects on phagocytes.
• Pyoverdin (yellow-green pigment) – acts as a siderophore (iron-binding compound) and fluoresces under UV light, enhancing bacterial survival under iron-limiting conditions.

Possesses a Type III secretion system (T3SS), which injects exo-enzymes directly into host cells to manipulate host signaling and immune responses.

• The four key exoenzymes are:
• Exo S – interferes with cytoskeletal organization.
• Exo T – disrupts cell adhesion and phagocytosis.
• Exo U – phospholipase with cytotoxic effects causing rapid cell death.
• Exo Y – adenylate cyclase enzyme that disrupts ion balance and damages host tissues.

Pathogenesis of Pseudomonas aeruginosa

• Pseudomonas aeruginosa acts as an opportunistic pathogen, meaning it primarily causes infection when host defenses are compromised or when predisposing conditions are present.
• It can infect nearly any body site, particularly in individuals with weakened immunity, chronic illness, or tissue injury.
• Common predisposing factors include:
• Burns and wounds, where the protective skin barrier is lost.
• Cystic fibrosis, where thick mucus allows bacterial colonization in the lungs.
• Indwelling medical devices, such as catheters, ventilators, or IV lines, which provide surfaces for biofilm formation.
• Immunocompromised states, such as cancer, diabetes, or prolonged antibiotic therapy.
• Once inside the host, the bacterium uses adhesins, pili, and biofilm formation to attach to tissues and resist immune clearance.
• It secretes toxins and enzymes (like elastase, exotoxin A, and proteases) that damage host tissues, impair immune responses, and facilitate bacterial spread.
• Biofilm formation allows the bacteria to survive antimicrobial treatments and evade phagocytosis, leading to chronic or persistent infections.
• Overall, P. aeruginosa pathogenesis results from a combination of host susceptibility, bacterial virulence factors, and biofilm-mediated resistance.

Disease of Pseudomonas aeruginosa

1. Endocarditis

• Pseudomonas aeruginosa infects heart valves, leading to bacterial endocarditis.
• Common in:
• Intravenous (IV) drug users.
• Patients with prosthetic heart valves.
• Pathogenesis:
• The organism establishes itself on the endocardium by direct invasion from the bloodstream.
• Causes inflammation, valve destruction, and potential heart failure if untreated.

2. Respiratory Infections

• P. aeruginosa is a major cause of pneumonia, especially in immunocompromised individuals.
• Types of respiratory infections:
• Pneumonia:
• Common in neutropenic cancer patients undergoing chemotherapy.
• Can lead to bacteremic pneumonia, which is often fatal without aggressive treatment.
• Chronic colonization:
• Occurs in patients with cystic fibrosis, leading to persistent lower respiratory tract infections.

3. Cystic Fibrosis (CF) and Pseudomonas aeruginosa

• CF is the most common lethal inherited disorder among Caucasians, with an incidence of 1 in 2,500 live births.
• Key characteristics of CF:
• Pancreatic insufficiency.
• Abnormal sweat electrolyte concentrations.
• Production of very viscid bronchial secretions, leading to mucus stasis.
• Pathogenesis in CF:
• The thick mucus allows Pseudomonas aeruginosa to persist in the lungs.
• Chronic infection results in progressive lung damage and respiratory failure.

4. Bacteremia and Septicemia

• Occur primarily in immunocompromised patients.
• Predisposing conditions include:
• Hematologic malignancies (e.g., leukemia).
• AIDS-related immunodeficiency.
• Neutropenia.
• Diabetes mellitus.
• Severe burns.
• Clinical sign:
• Ecthyma gangrenosum — characteristic skin lesion seen in neutropenic patients with P. aeruginosa septicemia.
• Complications:
• Dissemination of bacteria through the bloodstream to multiple organs.
• High mortality rate without prompt treatment.

5. Central Nervous System (CNS) Infections

• P. aeruginosa can cause meningitis and brain abscesses.
• Portals of entry:
• From inner ear or paranasal sinus infections.
• Direct inoculation during surgery or invasive diagnostic procedures.
• Hematogenous spread from another infection site (e.g., urinary tract).
• Clinical outcomes:
• Severe neurological symptoms due to abscess formation and inflammation.

6. Ear Infections

• Causes external otitis, commonly known as “swimmer’s ear.”
• Occurs due to moisture retention in the ear canal, providing an environment for bacterial growth.
• Can lead to pain, swelling, and discharge from the ear.

7. Eye Infections

• One of the most common causes of bacterial keratitis (corneal infection).
• Also isolated as a cause of neonatal ophthalmia, occurring in 1–12% of newborn infants.
• Complications:
• Corneal ulceration, vision impairment, or blindness if untreated.

8. Bone and Joint Infections

• Can result from direct inoculation (e.g., trauma or surgery) or hematogenous spread from other infection sites.
• High-risk groups:
• IV drug users.
• Patients with urinary tract or pelvic infections.
• Common forms:
• Chronic contiguous osteomyelitis:
• Involves the vertebral column, pelvis, and sternoclavicular joint.
• Osteochondritis:
• Often follows puncture wounds of the foot.
• May lead to bone necrosis and chronic pain.

9. Urinary Tract Infections (UTIs)

• Usually hospital-acquired (nosocomial).
• Associated with urinary tract catheterization, instrumentation, or surgery.
• Epidemiology:
• 3rd leading cause of hospital-acquired UTIs (about 12% of cases).
• Accounts for nearly 40% of Pseudomonas bacteremias.
• Pathogenesis:
• Highly adherent to bladder uroepithelium.
• Can invade the bloodstream from the urinary tract.
• Clinical features:
• Dysuria, fever, and signs of systemic infection in severe cases.

10. Gastrointestinal Infections

• Can affect any part of the gastrointestinal tract.
• Common forms include:
• Perirectal infections.
• Pediatric diarrhea.
• Gastroenteritis.
• Necrotizing enterocolitis (particularly in premature infants).
• The GI tract may also act as a portal of entry in Pseudomonas septicemia.

11. Skin and Soft Tissue Infections

• Include wound infections, pyoderma, and dermatitis.
• Pseudomonas aeruginosa can cause a wide range of skin infections, both localized and diffuse.
• Also implicated in folliculitis and severe forms of acne vulgaris.
• Predisposing factors:
• Breakdown of the integument:
• Burns, trauma, or dermatitis.
• High-moisture conditions:
• Common in the ears of swimmers, toe webs of athletes, perineal region, and under diapers of infants.
• Also seen in whirlpool and hot tub users.
• Immunocompromised states, especially AIDS.
• Complications:
• Chronic or recurrent infections due to biofilm formation and antibiotic resistance.

Treatment of Pseudomonas aeruginosa

• Pseudomonas aeruginosa is frequently resistant to many commonly used antibiotics, making treatment difficult and often requiring specific antimicrobial agents.
• Although many strains remain susceptible to antibiotics such as gentamicin, tobramycin, colistin, and amikacin, resistant forms of the bacterium have also developed over time, limiting the effectiveness of these drugs.
• For severe infections caused by Pseudomonas aeruginosa, the combination of gentamicin and carbenicillin is frequently used, as this synergistic therapy helps improve treatment outcomes and reduces the risk of resistance during therapy.

Immune Defenses of Pseudomonas aeruginosa

• Phagocytosis by polymorphonuclear leukocytes (neutrophils) plays a crucial role in providing resistance against Pseudomonas aeruginosa infections.
• Antibodies directed against somatic antigens and exotoxins also contribute significantly to the host’s recovery and defense mechanisms.
• Once P. aeruginosa infection is established, the production of specific antibodies, such as antitoxins, becomes important in helping to control the progression of the disease.
• Cell-mediated immunity appears to play a minimal role in providing resistance or defense against Pseudomonas aeruginosa infections.

Introduction to Klebsiella, Enterobacter, & Serratia sp.

Characteristics of Klebsiella, Enterobacter, & Serratia sp.

• Common Habitat: Usually found in the intestinal tract of humans and animals.
• Associated Infections: Cause a wide variety of infections, including pneumonia, wound infections, and urinary tract infections (UTIs).
• General Characteristics:
• Some species are non-motile.
• Simmons citrate test: Positive.
• H₂S production: Negative.
• Phenylalanine deaminase test: Negative.
• Some species are weakly urease positive.
• Methyl Red (MR) test: Negative.
• Voges-Proskauer (VP) test: Positive.

Klebsiella pneumoniae

Characteristics of Klebsiella pneumoniae

• Habitat: Commonly found in the gastrointestinal tract, eyes, respiratory tract, and genitourinary tract.
• Most Common Species: Klebsiella pneumoniae is the most frequently isolated species.
• Capsule: Possesses a polysaccharide capsule that protects the bacterium from phagocytosis and antibiotics, and gives colonies a moist and mucoid appearance.
• Odor: Exhibits a distinctive “yeasty” odor.
• Clinical Significance:
• A frequent cause of both nosocomial (hospital-acquired) and community-acquired pneumonia.
• Community-acquired pneumonia caused by K. pneumoniae is very severe, with rapid onset and high mortality, even with appropriate antibiotic therapy.
• Observed mortality rate: Approximately 50%.
• Invasive Liver Syndrome:
• Reported particularly in Asia.
• Predisposing factors: Diabetes mellitus and capsular strain of Klebsiella.
• Complications: May include bacteremia, meningitis, endophthalmitis, and necrotizing fasciitis.

Virulence Factors of Klebsiella pneumoniae

• The capsule polysaccharide (CPS) is an extracellular toxic complex mainly composed of polysaccharides that play a major role in virulence.
• CPS triggers extensive lung tissue damage, contributing to the severity of infections.
• It provides resistance to complement-mediated killing, helping the bacterium evade the host immune system.
• The capsule also impedes adhesion to and invasion of epithelial cells by sterically preventing receptor-target recognition of bacterial adhesins.
• Klebsiella pneumoniae has the ability to scavenge iron from the surrounding environment using secreted siderophores, such as enterochelin and aerobactin, which support bacterial growth in iron-limited host conditions.
• The bacterium possesses lipopolysaccharide (LPS), which contributes to endotoxin activity and enhances immune evasion.
• It also produces adhesins, both fimbrial and non-fimbrial, which facilitate bacterial attachment to host tissues.
• Klebsiella pneumoniae expresses two types of antigens: approximately 80 (K) capsular antigens and 5 (O) somatic antigens, both of which are important for identification and immune response interactions.

Pathogenesis of Klebsiella pneumoniae

• Klebsiella pneumoniae induces a cytotoxic effect upon infection of human lung epithelial cells, though this effect does not occur in the air passageway.
• The cytotoxic effect is not dependent on capsule serotype, but it requires the presence of live bacteria and is not directly linked to bacterial adhesion.
• The host cell cytotoxicity is believed to be associated with bacterial virulence.
• However, strains with varying capsule levels show different degrees of virulence, indicating that other bacterial factors may also contribute to Klebsiella pathogenicity.
• Adhesins play a crucial role in Klebsiella infections, especially in urinary tract infections (UTIs) and gastrointestinal infections (GTIs), by facilitating bacterial attachment to host tissues.
• Bacteremia caused by Klebsiella pneumoniae has been reported to originate from various infection sites:
• Urinary tract infections (UTI): 27%.
• Gastrointestinal tract infections (GTI): 24%.
• Intravenous sites: 20%.
• Pulmonary system: 15%.
• Mortality in Klebsiella infections is influenced by several factors, including:
• Underlying disease conditions.
• Age of the patient.
• Site of the initial infection.
• Significant biochemical characteristics of Klebsiella pneumoniae include:
• Lactose positive reaction on differential media.
• Most strains are urease positive.
• Non-motile nature of the bacterium.

Treatment of Klebsiella

• Klebsiella species are β-lactamase positive, which means they can inactivate β-lactam antibiotics.
• Ampicillin and amoxicillin can be used in combination with β-lactamase inhibitors, such as clavulanic acid, to enhance their effectiveness.
• Cephalosporins, including cefuroxime, cefotaxime, and fluoroquinolones, are commonly used for treatment.
• In cases of cephalosporin resistance, Klebsiella strains are often sensitive to gentamicin.
• Multidrug-resistant strains are associated with serious hospital-acquired infections, requiring careful antibiotic selection.
• For urinary tract infections (UTIs) caused by Klebsiella, effective treatments include trimethoprim, nitrofurantoin, or an oral cephalosporin.
• For pneumonia, vigorous treatment with aminoglycosides or cephalosporins is recommended.
• A vaccine targeting the lipopolysaccharide (LPS) antigen has also been developed for preventive purposes.

Introduction to Enterobacter species

Characteristics of Enterobacter species

• Enterobacter species comprise 12 recognized species, with E. cloacae and E. aerogenes being the most common clinical isolates.
• These bacteria are frequently isolated from wounds, urine, blood, and cerebrospinal fluid (CSF) samples.
• The primary virulence factor is the endotoxin, which contributes to inflammation and tissue damage.
• Major characteristics include:
• Colonies that resemble those of Klebsiella.
• Motile organisms.
• Methyl Red (MR) negative and Voges-Proskauer (VP) positive reactions.
• Urease negative.
• Enterobacter species are important opportunistic pathogens, frequently implicated in nosocomial (hospital-acquired) infections, especially in patients receiving antimicrobial therapy.
• The presence of serious underlying illnesses such as diabetes, malignancy, or burns significantly increases susceptibility to infection.
• Infections caused by Enterobacter species can be endogenous (originating from the patient’s own flora) or exogenous (acquired from external sources, particularly hospital environments).
  

Clinical manifestations of Enterobacter species

• Bacteremia caused by Enterobacter species often occurs when the organism is directly infused into the bloodstream, commonly through contaminated intravenous fluids or catheters.
• Clinical features include:
• Fever
• Central nervous system (CNS) involvement
• Hypotension
• Leukocytosis (increased white blood cell count)
• Lower respiratory tract infections may occur, including:
• Bronchitis
• Lung abscess
• Pneumonia
• Soft tissue infections can develop, particularly in hospitalized or immunocompromised patients.
• Urinary tract infections (UTIs) are also common, often associated with catheterization or other urinary tract instrumentation.
  

Treatment of Enterobacter

• Enterobacter species produce β-lactamase and cephalosporinase enzymes, which make them resistant to many β-lactam and cephalosporin antibiotics.
• They often show some resistance to tetracyclines.
• Most strains remain sensitive to:
• Fluoroquinolones
• Co-trimoxazole (trimethoprim-sulfamethoxazole)
• Carbapenems
• They differ from Serratia species by being sensitive to polymyxins.
  

Introduction to Serratia species

Characteristics of Serratia species

• The genus Serratia includes seven species, but Serratia marcescens is the only clinically important one.
• It is ubiquitous in nature, commonly found in soil, water, hospital floors, rooms, and air.
• Frequently associated with nosocomial infections of the urinary and respiratory tracts.
• Has been implicated in outbreaks of bacteremia in nurseries, cardiac surgery wards, and burn units.
• Demonstrates fair resistance to many antibiotics, making treatment challenging.
• Major characteristics include:
• Slow lactose fermentation
• Production of a characteristic pink pigment, particularly when cultures are left at room temperature.

Treatment of Serratia

• Serratia species are generally resistant to cephalosporins.
• Resistance to ampicillin and gentamicin is variable, depending on the strain.
• Gentamicin is considered the first-line treatment for most Serratia infections.
• In recalcitrant (hard-to-treat) cases, fluoroquinolones or carbapenems are recommended as alternative therapies.

Introduction to Proteus, Morganella & Providencia species

• Proteus, Morganella, and Providencia species are part of the normal intestinal flora of humans.
• These organisms act as opportunistic pathogens, causing infections under favorable conditions such as immunosuppression or hospitalization.
• They possess the ability to deaminate phenylalanine, a key biochemical characteristic used for identification.
• All three genera are lactose negative, distinguishing them from other Enterobacteriaceae in culture-based diagnostics.

Introduction to Proteus species

Characteristics to Proteus species

• Proteus species are Gram-negative bacteria, with Proteus mirabilis and Proteus vulgaris being the most widely recognized human pathogens.
• These species are commonly isolated from urine, wounds, ears, and cases of bacteremia.
• Both P. mirabilis and P. vulgaris produce swarming colonies on non-selective media and emit a distinctive “burned chocolate” odor.
• They are strongly urease positive and phenylalanine deaminase positive, which are key diagnostic features.
• Proteus species exhibit characteristic swarming motility, while urease positivity can be visualized in biochemical tests.
• Members of the Proteeae group are widespread in the environment and form part of the normal flora of the human gastrointestinal tract.
• Although Escherichia coli is the leading cause of uncomplicated cystitis, pyelonephritis, and prostatitis, Proteus ranks third, particularly in hospital-acquired infections.
• Proteus mirabilis has been implicated in bacteremia, neonatal meningoencephalitis, empyema, and osteomyelitis.
• Nosocomial transmission of Proteus mirabilis and Morganella morganii has been documented, including cases in a cardiac surgery unit with septicemia.
• In such outbreaks, no environmental source was identified, but O serotyping confirmed cross-infection among patients by both species.

Treatment of Proteus

• Proteus mirabilis is generally β-lactamase negative, meaning it is usually susceptible to β-lactam antibiotics.
• Proteus vulgaris, however, is usually resistant to penicillins and cephalosporins, but may be sensitive to β-lactamase–stable derivatives such as cefotaxime.
• Infections associated with renal stones caused by Proteus species are usually difficult to treat and often unsuccessful due to persistent bacterial colonization
• Treatment selection for Proteus infections is typically based on laboratory findings and antibiotic susceptibility testing to ensure effective therapy.

Introduction to Morganella species

Characteristics of Morganella species

• The genus Morganella currently includes one species: Morganella morganii, which has two subspecies — morganii and sibonii.
• It is a documented cause of urinary tract infections (UTIs).
• It can also be isolated from other anatomical sites, including wounds, respiratory tract, and bloodstream.
• Urease positive, indicating its ability to hydrolyze urea into ammonia and carbon dioxide.
• Phenylalanine deaminase positive, which helps differentiate it from other Enterobacteriaceae.

Introduction to Providencia species

Characteristics of Providencia species

• Providencia rettgeri is a known pathogen of the urinary tract and has been involved in nosocomial (hospital-acquired) outbreaks.
• Providencia stuartii is associated with nosocomial infections, particularly in burn units, and is frequently isolated from urine samples.
• Both species are phenylalanine deaminase positive, an important biochemical characteristic for identification.
• Human isolates of Providencia species have been recovered from multiple anatomical sites, including urine, throat, perineum, axilla, stool, blood, and wound specimens.