Staphylococci typically appear in grape-like clusters under the microscope, while Streptococci are arranged in chains or pairs.
Staphylococci are catalase-positive, producing bubbles with hydrogen peroxide, whereas Streptococci are catalase-negative.
Staphylococci are facultative anaerobes commonly found on the skin and mucous membranes, often associated with abscesses, wound infections, food poisoning, and toxic shock.
Streptococci, on the other hand, are usually part of the oral, respiratory, and genital flora, and are associated with diseases such as pharyngitis, scarlet fever, rheumatic fever, pneumonia, and neonatal sepsis.
Both groups include commensal species that are harmless under normal conditions but can become opportunistic pathogens when host defenses are compromised.
Differentiation between these two groups is essential for clinical microbiology and guides diagnosis and antibiotic therapy.

Staphylococci

Staphylococci are broadly classified into coagulase-positive and coagulase-negative species.

Classification of Staphylococci:

Coagulase-Positive Staphylococci

Staphylococcus aureus is the primary coagulase-positive species of medical importance.
It produces the coagulase enzyme, which causes plasma to clot by:
Activating prothrombin → thrombin.
Thrombin then catalyzes the conversion of fibrinogen → fibrin clot.
Among human pathogens, the most significant Staphylococci are:
S. aureus – coagulase-positive, highly virulent.
S. epidermidis – coagulase-negative, opportunistic pathogen.
S. saprophyticus – coagulase-negative, associated with UTIs.
S. aureus is differentiated from other Staphylococcal species by:
Mannitol fermentation on Mannitol Salt Agar (MSA) – produces yellow colonies due to acid production.
Hemolysis on blood agar – typically shows beta-hemolysis (clear zone around colonies due to complete RBC lysis).

Coagulase-negative Staphylococci (CoNS):

Staphylococcus epidermidis – common on skin and mucous membranes; associated with prosthetic device infections, endocarditis, bacteremia, and urinary tract infections.
Staphylococcus hominis – colonizes areas with apocrine sweat glands; occasionally implicated in bacteremia and device-related infections.
Staphylococcus capitis – found on scalp, face, and external ear; may cause endocarditis and bloodstream infections, especially in neonates.
Staphylococcus saprophyticus – inhabits the skin, intestine, and vagina; second most common cause of community-acquired urinary tract infections in young women.

General Characteristics of the Staphylococci

Commonly inhabit the skin, anterior nares, and mucous membranes of humans and animals.
Appear as spherical (cocci) cells arranged in irregular, grape-like clusters under the microscope.
Gram-positive organisms that retain crystal violet dye.
Non-spore forming and non-motile (lack spores and flagella).
Some species may possess a capsule, aiding in resistance to phagocytosis.
Facultative anaerobes, capable of growth in both aerobic and anaerobic conditions.
Relatively resistant to drying, heat, and high salt concentrations (can grow on media with up to 10% NaCl).
Currently, about 31 recognized species, with only a few being significant human pathogens (e.g., S. aureus, S. epidermidis, S. saprophyticus).

1. Staphylococcus aureus

Staphylococcus aureus Morphology

Shape: Spherical (cocci), about 0.5–1.5 µm in diameter.
Arrangement: Typically found in irregular grape-like clusters; occasionally seen in pairs or short chains.
Gram reaction: Gram-positive, appearing purple under Gram stain.
Colony characteristics:

On nutrient agar: round, smooth, convex colonies (2–4 mm).
Pigmentation: often produces a golden-yellow pigment (aureus = golden).
On blood agar: shows β-hemolysis (clear zone around colonies).

Motility & spores: Non-motile and non–spore forming.
Capsule: May produce a polysaccharide capsule, enhancing resistance to phagocytosis.
Cell wall: Thick peptidoglycan layer with teichoic acids; contains protein A that binds the Fc region of IgG, interfering with opsonization.
Growth conditions: Facultative anaerobe; tolerates high salt concentrations (up to 10% NaCl) and grows well on Mannitol Salt Agar (MSA), fermenting mannitol (colonies turn yellow).

General Characteristics of Staphylococcus aureus

Staphylococcus aureus grows in large, round, opaque colonies, often producing a golden-yellow pigment that gives the species its name (aureus = golden).
The optimum growth temperature is 37°C, but it can also grow over a wider temperature range.
It is a facultative anaerobe, capable of surviving and multiplying in both aerobic and anaerobic conditions.
Demonstrates remarkable environmental resilience, being able to withstand:

High salt concentrations (7.5–10%), which allows growth on Mannitol Salt Agar (MSA).
Wide pH variations.
Relatively high temperatures compared to many other non-spore-forming bacteria

Colonies often show β-hemolysis on blood agar, indicating complete lysis of red blood cells.
Produces a wide array of virulence factors that contribute to pathogenicity, including:

Enzymes such as coagulase, hyaluronidase, lipases, DNases, and staphylokinase.
Toxins such as hemolysins, enterotoxins (food poisoning), exfoliative toxins (scalded skin syndrome), and toxic shock syndrome toxin (TSST-1).
Surface proteins like Protein A, which binds the Fc region of IgG and disrupts opsonization and phagocytosis.

Its ability to persist under harsh conditions and produce multiple virulence factors makes S. aureus one of the most important opportunistic and pathogenic bacteria in humans.

Variation in Staphylococci

A culture of Staphylococcus often contains subpopulations of bacteria that differ from the majority in:

Colony characteristics (size, pigment production, hemolysis).
Enzyme production.
Drug resistance profiles.
Pathogenic potential.

The expression of these characteristics can be influenced by in vitro growth conditions such as medium composition, oxygen availability, and pH.
Some isolates may show altered phenotypes, for example:

Smaller colonies (pinpoint colonies).
Loss or reduction of hemolysis.
These variants are termed Small Colony Variants (SCVs).

SCVs are associated with:

Enhanced intracellular survival.
Persistence in host tissues.
Contribution to chronic, relapsing, and antibiotic-resistant infections.

Virulence factors of Staphylococcus aureus

Enzymes

Coagulase – coagulates plasma and blood; produced by ~97% of human isolates; serves as an important diagnostic marker.
Hyaluronidase – breaks down hyaluronic acid in connective tissue, facilitating bacterial spread.
Staphylokinase – dissolves blood clots (fibrinolysis), promoting dissemination.
DNase – hydrolyzes DNA, reducing viscosity of pus and aiding bacterial mobility.
Lipases – break down lipids and oils, enhancing survival and colonization on skin and sebaceous areas.
Penicillinase (β-lactamase) – inactivates penicillin and contributes to antibiotic resistance.

Toxins

Hemolysins (α, β, γ) – lyse red blood cells and also damage leukocytes, platelets, and tissues.
Leukocidin – destroys neutrophils and macrophages, weakening host immune defense.
Enterotoxins – heat-stable exotoxins that cause gastrointestinal distress, nausea, vomiting, and diarrhea (common in food poisoning).
Exfoliative toxin – causes separation of the epidermis from the dermis, responsible for Staphylococcal Scalded Skin Syndrome (SSSS).
Toxic shock syndrome toxin (TSST-1) – superantigen that triggers massive cytokine release, leading to fever, rash, vomiting, shock, and multi-organ failure.

Major Virulence Factors of Staphylococcus aureus

Epidemiology and Pathogenesis of Staphylococcus aureus

Widely distributed in nature and commonly present in human environments.
Can be readily isolated from fomites such as clothing, bedding, door handles, and medical equipment.
Carriage rate among healthy adults is estimated at 20–60%.
Common sites of colonization include the anterior nares, skin, nasopharynx, and occasionally the intestine.
Predisposing factors for infection include:

Poor personal hygiene and malnutrition.
Tissue injury or open wounds.
Preexisting infections that weaken host defenses.
Chronic illnesses such as diabetes mellitus.
Immunodeficiency states (HIV, chemotherapy, steroid use, etc.).

Infections often arise from the host’s own carriage strains (endogenous source) but can also spread by direct contact or contaminated objects.
A major clinical concern is the rising prevalence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), which poses challenges due to multidrug resistance.

Staphylococcal Disease

Infections caused by Staphylococcus aureus range from localized skin infections to serious systemic diseases.

Localized cutaneous infections:

Localized cutaneous infectionsoccur when bacteria invade the skin through wounds, hair follicles, or sebaceous glands:

Folliculitis – superficial infection of a hair follicle; often mild and self-limiting, though it can progress to deeper lesions.
Furuncle (Boil) – painful, pus-filled abscess developing from an infected follicle or sebaceous gland.
Carbuncle – a larger, deeper, and more serious lesion formed by the interconnection of multiple furuncles; often associated with systemic symptoms like fever.
Impetigo (Staphylococcal impetigo) – bubble-like vesicles or pustules that rupture, crust, and peel away; highly contagious and most common in newborns and young children.

Staphylococcal Disease Localized cutaneous infection

Systemic infections:

Septicemia – bloodstream infection that can originate from any localized lesion such as skin abscesses or wound infections.
Endocarditis – infection of the endocardium or heart valves, occurring in both normal and prosthetic valves; often associated with intravenous drug use or indwelling medical devices.
Osteomyelitis – bone infection that may arise through hematogenous spread from a distant site or by direct invasion from trauma, surgery, or local wound infections.
Bacteremia – presence of bacteria in the bloodstream, often seeded from another infected site or linked to medical devices (e.g., catheters, prosthetics, IV lines); can progress to sepsis or metastatic abscesses.

Staphylococcal osteomyelitis in a long bone

Toxigenic disease:

Food Intoxication (Staphylococcal Food Poisoning)

Caused by ingestion of preformed, heat-stable enterotoxins in contaminated food.
Rapid onset (1–6 hours) of nausea, vomiting, abdominal cramps, and diarrhea.
Self-limiting illness, usually resolves within 24 hours.
Antibiotic therapy is not required.

Enterotoxins

About 15 enterotoxins (A–E, G–P); all are superantigens.
~50% of S. aureus strains produce one or more enterotoxins.
Heat-stable and resistant to gut enzymes.
Produced when bacteria grow in carbohydrate and protein-rich foods.
As little as 25 µg of enterotoxin B can induce symptoms.
Mechanism: toxins stimulate gut neural receptors, activating the CNS vomiting center → emetic effect.
Genes for enterotoxins, exfoliative toxins, and TSST-1 are carried on a pathogenicity island, often mobilized by bacteriophages.

Staphylococcal Scalded Skin Syndrome (SSSS)

Caused by exfoliative toxins (exfoliatins A & B).
Act as proteases that target desmoglein-1, disrupting epidermal cell junctions.
Clinical features: painful, bright red flush, followed by blistering and desquamation of the epidermis.
Most common in neonates and young children.

Toxic Shock Syndrome (TSS)

Mediated by toxic shock syndrome toxin-1 (TSST-1), a superantigen.
Causes non-specific T-cell activation → cytokine storm → fever, rash, vomiting, diarrhea, hypotension, and multi-organ failure.
Classically associated with tampon use, but also linked to nasal packing, surgical wounds, and device-related infections.
Clinical features (appear within 8–12 hours):

High fever
Hypotension
Diarrhea
Rash (erythroderma)
Multi-organ involvement → shock, renal/hepatic dysfunction

Treatment:

Remove foreign body / clean wound.
Administer appropriate antibiotics (e.g., clindamycin + vancomycin).
Supportive therapy: IV fluids, monitoring organ function.
In severe cases → methylprednisone (corticosteroid) to inhibit TNF-α synthesis.

Kawasaki Syndrome (possible association with S. aureus superantigens)

Acute systemic vasculitis in early childhood.
Usually self-limiting, but coronary artery aneurysms occur in 20–30%, leading to morbidity and mortality.
Clinical features:

Persistent high fever
Strawberry tongue
Conjunctivitis
Maculopapular rash
Erythema and edema of hands/feet

Identification of Staphylococcus in Samples

Clinical Sources

Frequently isolated from:

Pus
Tissue exudates
Sputum
Urine
Blood

Laboratory Identification Methods

1. Cultivation

Grows well on nutrient agar, blood agar, and selective media (e.g., Mannitol Salt Agar).
Colonies: round, smooth, convex; S. aureus often produces golden-yellow pigment.

2. Smear (Microscopy)

Gram staining → Gram-positive cocci in clusters ("grape-like").
Non-motile, non-spore forming.

3. Detection of Cytochrome Oxidase

Staphylococcus species are oxidase-negative (helps differentiate from Micrococcus which is oxidase-positive).

4. Catalase Test

Catalase-positive → production of bubbles when exposed to hydrogen peroxide.
Differentiates Staphylococcus (positive) from Streptococcus (negative).

5. Coagulase Test

S. aureus is coagulase-positive (clot formation in plasma).
Distinguishes pathogenic S. aureus from coagulase-negative staphylococci (CoNS).

6. Test Systems

Automated identification systems (e.g., API Staph, VITEK, MALDI-TOF).
Molecular methods: PCR for mecA gene (MRSA detection).

Clinical Concerns and Treatment of Staphylococcus aureus

High resistance to common antibiotics

Around 95% of S. aureus strains produce penicillinase, making them resistant to penicillin and ampicillin.
Methicillin-resistant S. aureus (MRSA): These strains carry resistance genes against multiple β-lactam antibiotics, posing major clinical challenges.

Multidrug resistance

Certain strains show resistance to almost all major drug classes.
Vancomycin often remains the last line of effective treatment, but reports of vancomycin-intermediate and vancomycin-resistant S. aureus (VISA/VRSA) are emerging.

Surgical management

Localized abscesses must be surgically drained or perforated, since antibiotics alone often fail to penetrate pus effectively.

Systemic infections

Severe infections (e.g., septicemia, pneumonia, endocarditis) require intensive, prolonged antimicrobial therapy, sometimes involving combination regimens.

Clinical implications

Infections caused by resistant strains are associated with higher morbidity, longer hospital stays, and increased mortality.
Infection control measures (hand hygiene, isolation, decolonization protocols) are critical to prevent spread in healthcare settings.

Prevention of Staphylococcal Infections

Universal Precautions: Healthcare providers should strictly follow universal precautions (gloves, masks, sterilization, hand hygiene) to minimize nosocomial (hospital-acquired) infections.
Hygiene & Cleansing: Regular hand washing, proper wound care, and disinfection of contaminated surfaces reduce transmission.
Screening & Isolation: Screening high-risk patients (e.g., carriers of MRSA) and isolating infected individuals help control spread.
Rational Antibiotic Use: Avoiding unnecessary antibiotics reduces the emergence of resistant strains.
Decolonization Strategies: In certain cases, carriers may be treated with topical antimicrobials (e.g., mupirocin) or antiseptic washes (chlorhexidine) to reduce colonization.

2. Staphylococcus epidermidis

Source/Type of Infection: Mostly hospital-acquired (nosocomial).
Sites of Infection: Frequently infects intravenous catheters, prosthetic heart valves, joint prostheses, and other implanted medical devices due to its ability to form biofilms.
Clinical Manifestations:

Sepsis in neonates.
Prosthetic valve endocarditis.
Infections related to indwelling devices.

3. Staphylococcus saprophyticus

Source/Type of Infection: Usually community-acquired.
Clinical Manifestation: A common cause of urinary tract infections (UTIs), especially in sexually active young women.
Differentiating Feature: Unlike S. epidermidis, it is novobiocin resistant (a key lab test for distinction).

Streptococci

General Characteristics of Streptococci

Shape & Arrangement: Gram-positive spherical/ovoid cocci, usually in long chains; may also appear in pairs.
Spore & Motility: Non–spore-forming, nonmotile.
Surface Structures: Some species can form capsules and slime layers.
Oxygen Requirement: Facultative anaerobes.
Enzymes: Lack catalase but possess a peroxidase system (distinguishes them from staphylococci).
Colony Morphology: Produce small, nonpigmented colonies.
Environmental Sensitivity: Relatively sensitive to drying, heat, and disinfectants.
Gram Reaction Variability: Although gram-positive, older cultures may lose gram-positivity and appear gram-negative (especially after overnight incubation).

Classification of streptococci

Classification of streptococci is based on several criteria observed over time.

Colony morphology and hemolytic reactions on blood agar help in differentiation.
Serologic specificity of the cell wall group-specific substance (Lancefield antigens) and other capsular or cell wall antigens is a major basis.
Biochemical reactions and resistance to various physical and chemical factors provide further classification.
Ecologic features, including natural habitat and pathogenic potential, are also considered.

Hemolysis of streptococci

Many streptococci can hemolyze red blood cells in vitro to different extents.

β-hemolysis: complete lysis of erythrocytes, producing a clear zone around colonies.
α-hemolysis: partial lysis of erythrocytes with hemoglobin reduction, giving a greenish discoloration.
γ-hemolysis (nonhemolytic): no hemolysis observed.
β-hemolytic streptococci are also called pyogenic streptococci, which include important human pathogens such as S. pyogenes, S. agalactiae, and S. dysgalactiae.

Lancefield classification

Lancefield classification system is based on cell wall antigens, dividing streptococci into 20 serologic groups (A–H, K–U).
The group-specific carbohydrate antigen determines the serologic specificity and is composed of an amino sugar.
Group A streptococci: rhamnose–N-acetylglucosamine.
Group B streptococci: rhamnose–glucosamine polysaccharide.
Group C streptococci: rhamnose–N-acetylgalactosamine.
Group D streptococci: glycerol teichoic acid containing D-alanine and glucose.
Group F streptococci: glucopyranosyl–N-acetylgalactosamine.

Capsular Polysaccharides of streptococci

The antigenic properties of capsular polysaccharides are important for classification.
S. pneumoniae is divided into more than 90 distinct types based on its capsule, while the capsule also serves as the basis for typing group B streptococci (S. agalactiae).

Biochemical Reactions of streptococci

Streptococci can be differentiated using biochemical tests such as:

Sugar fermentation profiles
Enzyme detection tests
Susceptibility or resistance to specific chemical agents
These tests are typically applied after assessing colony morphology and hemolytic patterns.

Human Streptococcal Pathogens

Streptococcus pyogenes
Streptococcus agalactiae
Viridans streptococci
Streptococcus pneumoniae
Enterococcus faecalis

Characteristics of Medically Important Streptococci

Epidemiology and Pathogenesis of streptococci

Humans serve as the sole reservoir, with inapparent carriers playing a key role in transmission. Spread occurs through direct contact, respiratory droplets, contaminated food, and fomites. Entry usually occurs via the skin or pharynx. Infection patterns vary with environmental conditions:

Skin infections → common in warm, humid climates
Pharyngeal infections → frequent in winter and dry conditions
Children are most often affected by cutaneous and throat infections. If untreated, systemic infections and serious complications may develop.

1. Group A Streptococcus (Streptococcus pyogenes)

Considered the most significant pathogenic streptococcus.
Exists as a strict parasite.
Commonly resides in the throat, nasopharynx, and occasionally on the skin.

Colony Morphology of Streptococcus pyogenes

Cells are spherical or ovoid cocci arranged in chains.
S. pyogenes shows β-hemolysis.

Cultural Characteristics of Streptococcus pyogenes

Streptococci generally form discoid colonies on solid media, about 1–2 mm in diameter.
Growth and hemolysis of S. pyogenes are enhanced when incubated in 10% CO₂.

Variation of Streptococcus pyogenes

A single strain of Streptococcus may produce colonies with different appearances.
In S. pyogenes, two colony types are common:

Matte colonies: Produce abundant M protein, usually associated with virulence.
Glossy colonies: Produce little M protein, often avirulent.

Virulence Factors of Streptococcus pyogenes

C-carbohydrates: Protect the bacteria from lysozyme action.
Fimbriae: Aid in bacterial adherence to host tissues.
M-protein: Provides resistance against phagocytosis; a key virulence determinant.
Hyaluronic acid capsule: Non-immunogenic, helps bacteria evade immune recognition.
C5a protease: Disrupts complement activity and inhibits neutrophil response.

Toxins and Enzymes

Streptolysins (SLO and SLS): Hemolysins that damage red blood cells and various other host cells.
Erythrogenic (pyrogenic) toxin: Causes fever and the characteristic red rash of scarlet fever.
Superantigens: Overstimulate T cells by binding to MHC class II and T-cell receptors, leading to massive cytokine release, shock, and tissue damage (similar to staphylococcal toxic shock syndrome toxin).

Extracellular Enzymes

Streptokinase: Converts plasminogen to plasmin, breaking down fibrin clots and aiding bacterial spread; antibodies (antistreptokinase) may develop after infection; also used therapeutically in clot treatment.
Hyaluronidase: Breaks down hyaluronic acid in connective tissue, facilitating spread; antigenic and induces specific antibodies.
Streptodornase (DNase A, B, C, D): Hydrolyzes DNA, reduces pus viscosity, and helps bacteria spread in tissue; antibodies to DNase (especially DNase B) are diagnostic after skin infections.

Clinical Findings of Streptococcus pyogenes

Invasive Infections

The portal of entry determines the clinical manifestation; infection spreads rapidly along lymphatics and may enter the bloodstream.
Erysipelas: Entry via skin; causes massive brawny edema with a sharply raised, red, advancing margin of infection.
Cellulitis: Acute, rapidly spreading infection of skin and subcutaneous tissues after trauma, burns, wounds, or surgery; characterized by pain, swelling, tenderness, and erythema. Unlike erysipelas, lesions are not raised and margins are indistinct.
Necrotizing fasciitis (streptococcal gangrene): Severe infection of subcutaneous tissue with rapid necrosis of skin and underlying layers; often termed “flesh-eating bacteria.”
Puerperal fever: Infection enters the uterus after delivery, causing septicemia originating in endometritis.
Bacteremia/Sepsis: May follow traumatic or surgical wound infections, cellulitis, or rarely pharyngitis; rapidly progressive and potentially fatal.

Localized Infections

Streptococcal sore throat (pharyngitis): Most common β-hemolytic S. pyogenes infection. Bacteria adhere via lipoteichoic acid-covered pili and hyaluronic acid capsule. Characterized by acute nasopharyngitis, tonsillitis, intense redness, and edema of mucous membranes. Does not usually involve the lungs.
Streptococcal pyoderma (impetigo): Superficial skin infection in children; presents as vesicles that rupture and form pus-covered erosions later encrusted. Highly communicable, spreads by contact, and is common in warm, humid climates.

Systemic Invasive Infections

Scarlet fever: Follows throat infection when strains produce pyrogenic toxin. Toxin spread causes high fever and a diffuse bright red rash over face, trunk, and extremities.
Streptococcal toxic shock syndrome (STSS): Severe invasive infection leading to shock, bacteremia, respiratory failure, and multiorgan failure; mortality is ~30%. Often follows minor trauma in otherwise healthy individuals, usually associated with soft tissue infections.

Epidemiology and Pathogenesis of Streptococcus pyogenes

Humans serve as the only natural reservoir.
Carriers may remain asymptomatic.
Spread occurs through direct contact, respiratory droplets, contaminated food, or fomites.
Usual portals of entry are the skin or pharynx.
Type and frequency of infection vary with climate, season, and living conditions.
Skin infections are more common in warm, humid environments.
Pharyngeal infections occur more often in colder, dry climates, especially during winter.
Children are the main group affected by both skin and throat infections.
If left untreated, systemic infections and serious sequelae can develop.

Poststreptococcal Diseases

Poststreptococcal diseases occur after an acute Streptococcus pyogenes infection, leading to nephritis or rheumatic fever.
Latent period: 1–4 weeks.
These diseases are not caused by direct bacterial effects but represent a hypersensitivity response.
Nephritis is more commonly preceded by skin infections.
Rheumatic fever is more commonly preceded by respiratory tract infections.
Rheumatic fever: causes damage to heart muscle and valves; presents with fever, malaise, migratory polyarthritis, and inflammation of all parts of the heart.
Acute glomerulonephritis: initiated by antigen–antibody complexes deposited on the glomerular basement membrane.
Clinical features of acute nephritis: blood and protein in urine, edema, high blood pressure, urea nitrogen retention, and low serum complement levels.

2. Group B Streptococcus (Streptococcus agalactiae)

Group B Streptococcus agalactiae is part of the normal flora of the human vagina, pharynx, and large intestine.
It can be transferred to infants during delivery and cause severe infections.
It is the most prevalent cause of neonatal pneumonia, sepsis, and meningitis.
Pregnant women should be screened and treated to prevent transmission and infection in newborns.
It can also cause wound infections, skin infections, and endocarditis in people who are at risk.
These bacteria are typically β-hemolytic and produce zones of hemolysis that are only slightly larger than the colonies, usually about 1–2 mm in diameter.
Group B streptococci can hydrolyze sodium hippurate and give a positive response in the CAMP test (Christie, Atkins, Munch-Peterson).

CAMP Test

CAMP Test: Group B streptococci (Streptococcus agalactiae), when streaked perpendicular to Staphylococcus aureus, produce CAMP factor.
The CAMP factor enhances the hemolytic activity of S. aureus, creating a distinct arrow-shaped zone of hemolysis.

Streptococcal tests

Streptococcal infections pose the highest risk to elderly and immunocompromised populations.
Predisposing factors include: diabetes mellitus, cancer, advanced age, liver cirrhosis, corticosteroid therapy, HIV infection, and other immunocompromised conditions.
Major clinical manifestations (in descending order of frequency) are:

Bacteremia
Skin and soft tissue infections
Respiratory infections
Genitourinary infections

Diagnostic Laboratory Tests of Streptococcus

1. Specimens

Type of specimen depends on the site of streptococcal infection.
Common specimens: throat swab, pus, blood (for culture).
Serum is collected for antibody detection.

2. Smears

Smears from pus often show single cocci or pairs, not distinct chains.
Sometimes appear Gram-negative due to non-viability and loss of ability to retain crystal violet.

3. Culture

Suspected samples are cultured on blood agar plates.
Incubation in 10% CO₂ enhances hemolysis.
Blood cultures may yield hemolytic group A streptococci (e.g., in sepsis) within hours to a few days.
Some non-hemolytic streptococci and enterococci grow slowly.

Hemolysis pattern:

Group A streptococci may be presumptively identified by growth inhibition with bacitracin, though more reliable confirmatory tests are preferred.

4. Antigen Detection Tests

Used for rapid identification of group A streptococci.

5. Serologic Tests

Detection based on rise in antibody titers to group A streptococcal antigens.
Important antibodies include:

ASO (Antistreptolysin O) → widely used, especially in respiratory infections.
Anti-DNase and antihyaluronidase → useful in skin infections.
Antistreptokinase, anti-M type-specific antibodies, and others.

3. Group C and D of Streptococci

Group C streptococci infrequently cause human infections
Group D streptococci classified into enterococci and non-enterococci
Enterococci include E. faecalis, E. faecium, E. durans
Normal colonizers of the human large intestine6
Cause opportunistic urinary, wound, and skin infections
Can grow in 6.5% salt solution or bile, and are not killed by penicillin G
Usually non-hemolytic, but occasionally α-hemolytic

4. Group D Streptococci (Enterococci & Non-enterococci)

Non-enterococci (Group D streptococci)

Non-enterococci include Streptococcus bovis
Sensitive to 6.5% salt concentration and killed by penicillin G
Show variable hemolytic patterns
Identification is essential for proper treatment and to prevent complications
Rapid diagnostic tests use monoclonal antibodies reacting with C-carbohydrates
Culture methods include:

Bacitracin disc test
CAMP test
Esculin hydrolysis

Treatment of Streptococcus

Groups A and B streptococci are treated with penicillin and amoxicillin
Long-term penicillin therapy is required for patients with a history of rheumatic fever or recurrent strep throat
Enterococcal infections usually require combined therapy (e.g., penicillin or ampicillin with aminoglycosides)

5. α-Hemolytic Streptococci: Viridans Group

Large complex group includes Streptococcus mutans, S. oralis, S. salivarius
Growth not inhibited by Optochin, and colonies are not soluble in bile (deoxycholate)
Most numerous and widespread residents of gums, teeth, oral cavity, and also found in nasopharynx, genital tract, and skin
Not highly invasive, but dental or surgical procedures facilitate entry into the bloodstream
Cause bacteremia, meningitis, abdominal infections, and tooth abscesses
Most serious infection is subacute endocarditis

Blood-borne bacteria settle and grow on heart lining or valves
Persons with preexisting heart disease are at high risk
Colonization of the heart occurs via biofilm formation

S. mutans produce slime layers that adhere to teeth, forming the basis of plaque
Involved in dental caries
Persons with preexisting heart conditions should receive prophylactic antibiotics before surgery or dental procedures

6. Streptococcus pneumoniae: The Pneumococcus

Causes 60–70% of all bacterial pneumonias
Cells are small, lancet-shaped, arranged in pairs or short chains
Possess a polysaccharide capsule, enabling typing with specific antisera
Lack catalase and peroxidases → cultures die in the presence of O₂

Cultural Characteristics of Streptococcus pneumoniae

Colonies are small, round, dome-shaped initially, later developing a central plateau with an elevated rim
α-hemolytic on blood agar
Growth is enhanced by 5–10% CO₂

Growth Characteristics of Streptococcus pneumoniae

Obtain most energy from glucose fermentation
Rapid production of lactic acid limits growth

Virulence Factors of Streptococcus pneumoniae

Disease results from ability to multiply in tissues
Pneumolysin: cytolytic and inflammatory toxin
Capsule: major virulence factor that prevents/delays phagocytosis
90 capsular types identified
Immunity is type-specific; serum antibodies to capsular polysaccharide provide protection
Immunization with a given capsular polysaccharide confers immunity against that type
Encapsulated strains are at least 10⁵ times more virulent than unencapsulated strains

Cell Wall & Cell Wall Polysaccharide (CWPS) of Streptococcus pneumoniae

Unlike capsule polysaccharide, peptidoglycan and CWPS can independently induce inflammatory responses similar to whole pneumococcal infection

Pneumococcal Proteins of Streptococcus pneumoniae

IgA1 protease: interferes with host mucosal defenses
Neuraminidase: may aid epithelial attachment by cleaving host sialic acid
Pneumococcal surface protein A (PspA):

Shows structural and antigenic variability across strains
Present in most clinical isolates

Epidemiology & Pathology of Streptococcus pneumonia

Carriage rate: 5–50% of people harbor it as normal nasopharyngeal flora
Infections are usually endogenous
Very delicate, does not survive long outside host
Risk groups: young children, elderly, immunocompromised, patients with lung disease/viral infections, and those in close quarters
Pneumonia occurs when bacteria are aspirated into lungs of susceptible hosts → leads to overwhelming inflammatory response
Can also spread to middle ear via the eustachian tube

Pathogenesis of Pneumococcal Disease

Colonization

Streptococcus pneumoniae is commonly carried in the upper respiratory tract of healthy individuals.
Attachment to host cells is suggested to be mediated by a disaccharide receptor on fibronectin present on human pharyngeal epithelial cells.

Adherence and Predisposing Factors

Pneumococci adhere to tracheal epithelial cells, and this adherence is enhanced by prior influenza virus infection, increasing susceptibility.

Invasion and Spread

Mechanisms of translocation from the nasopharynx to the lung (causing pneumonia) or directly into the bloodstream (causing bacteremia/septicemia) are not fully understood.

Bacterial Multiplication and Lysis

Uncontrolled multiplication in the lungs, meninges, or middle ear leads to pneumococcal lysis.
Lysis releases cell wall components and pneumolysin, both of which are key in triggering host responses.
Lysozyme in secretions may contribute by activating autolysin, further promoting bacterial lysis.

Inflammatory Response

Pneumococcal lysis induces strong inflammation:
Directly: by attracting and activating phagocytes.
Indirectly: through complement activation.
This excessive inflammatory response is believed to be a major factor responsible for morbidity and mortality in pneumococcal infections.

Clinical Findings of Streptococcus pneumoniae

Sudden onset with fever, chills, and sharp pleural pain
Sputum: bloody or rusty colored
Spontaneous recovery may occur in 5–10 days
Also causes otitis media, sinusitis, and purulent bronchitis

Diagnosis of Streptococcus pneumoniae

Gram stain: presumptive identification
Quellung test (capsular swelling reaction)
Culture characteristics: α-hemolytic, optochin sensitive, bile soluble, inulin fermentation

Treatment and Prevention of Streptococcus pneumoniae

Traditionally treated with penicillin G or V
Drug resistance increasingly reported
Capsular antigen vaccine: for older adults & high-risk individuals; effective for 5 years
Conjugate vaccine: for children 2–23 months

Gram-Positive Cocci of Medical Importance: Staphylococci & Streptococci - Classification, Virulence, Diseases, Diagnosis & Treatment

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Gram Positive Cocci of Medical Importance