Bifidobacterium longum: Morphology, Biochemical Characteristics, Probiotic Benefits, Laboratory Diagnosis, and Clinical Importance

Introduction
Taxonomy and Classification
Morphology and Microscopy
Cultural and Growth Characteristics
Biochemical and Identification Tests
Pathogenesis and Virulence Factors
Virulence Factors (Functional or Beneficial Factors)
Epidemiology and Transmission
Beneficial Effects
Laboratory Diagnosis
Treatments
Prevention and Control
Conclusion
References

Introduction to Bifidobacterium longum

Bifidobacterium longum is a type of probiotic that resides in the human gastrointestinal tract.
It is particularly abundant in infants and remains a significant component of the adult gut microbiota.
It survives in low-oxygen (anaerobic) conditions.
It ferments carbohydrates, such as oligosaccharides, into lactic acid and acetic acid.
This fermentation process creates an acidic environment that inhibits the growth of harmful pathogens.
It supports digestion.
It increases nutrient absorption.
It strengthens the intestinal barrier.
It plays a crucial role in modulating the immune system.

Taxonomy and Classification of Bifidobacterium longum

Domain: Bacteria
Kingdom: Eubacteria (sometimes not used in modern classification systems)
Phylum: Actinobacteria (also historically referred to as Actinomycota)
Class: Actinomycetia (formerly Actinomycetes in older systems)
Order: Bifidobacteriales
Family: Bifidobacteriaceae
Genus: Bifidobacterium
Species: Bifidobacterium longum

Morphology and Microscopy of Bifidobacterium longum

It is a Gram-positive bacterium.
It is rod-shaped, ranging from short to long forms, with characteristic Y- or V-shaped bifurcations.
It occurs singly, in pairs, or in short chains.
It is pleomorphic, showing variable shapes, including curved or club-shaped rods.
It is non-motile.
It is non-spore-forming.
Under the microscope, it appears as a purple-colored, slightly curved rod.
It is often observed in Y- or V-shaped arrangements during microscopic examination.

Cultural and Growth Characteristics of Bifidobacterium longum

It is a strict anaerobe.
The optimum growth temperature is 36–38°C.
The optimum pH for growth is 6.5–7.
It grows well on enriched media such as de Man–Rogosa–Sharpe (MRS) agar supplemented with cysteine.
On MRS agar, it forms small to medium-sized, convex, smooth, circular colonies.
On blood agar, it produces small to medium-sized colonies that are smooth, convex, white to cream in color.
On blood agar, colonies may appear soft and sometimes slightly mucoid.

Biochemical and Identification Tests of Bifidobacterium longum

Gram staining: Positive
Catalase: Negative
Oxidase: Negative
O/F (Oxidation/Fermentation): Fermentative
Indole: Negative
Motility: Negative
Gas production: Negative
Gelatin hydrolysis: Negative
Nitrate reduction: Negative

Fermentation of carbohydrates:

Amylose: Variable
Glucose: Positive
Galactose: Positive
Maltose: Variable
Mannose: Negative
Lactose: Positive
Mannitol: Negative
Ribose: Negative
Starch: Negative
Cellobiose: Negative
Sucrose: Variable
Xylose: Negative
Sorbitol: Negative
Raffinose: Negative
Trehalose: Negative

Enzymatic reactions:

Fructose-6-phosphoketolase: Positive
Arabinosidases: Positive
Glucosidases: Positive
Glutamate dehydrogenase: Positive
Glutamine synthetase: Positive
Hexosaminidases: Positive
ONPG (β-galactosidase): Positive

Pathogenesis and Virulence Factors of Bifidobacterium longum

General Nature

Bifidobacterium longum is a normal commensal flora of the human gastrointestinal tract, especially abundant in infants and also present in adults.
It is generally non-pathogenic and rarely causes disease.
Instead of causing disease, it plays a protective role in the host.
In immunocompromised individuals, it may rarely cause opportunistic infections, although this is extremely uncommon.

Maintenance of Gut Microbiota Balance

Competes with harmful microorganisms for space and nutrients.
Helps maintain a healthy intestinal microbial ecosystem.

Inhibition of Pathogenic Bacteria

Produces organic acids such as lactic acid and acetic acid, which lower intestinal pH.
Produces bacteriocin-like substances that suppress pathogens such as E. coli and Salmonella spp.

Enhancement of Intestinal Barrier Function

Strengthens tight junctions of intestinal epithelial cells.
Prevents translocation of harmful microbes and toxins across the intestinal barrier.

Modulation of the Immune System

Stimulates anti-inflammatory immune responses.
Increases IgA production.
Helps balance immune cell activity.
Reduces risk of allergies and inflammatory diseases.

Improvement of Digestion

Ferments dietary fibers and complex carbohydrates.
Produces short-chain fatty acids that provide energy to colon cells.

Synthesis of Vitamins

Contributes to the production of certain B vitamins, including folate and B12.

Protection against Gastrointestinal Disorders

Helps reduce diarrhea, constipation, and irritable bowel symptoms.
Supports recovery after antibiotic use.

Reduction of Harmful Metabolites

Lowers production of toxic substances such as ammonia and phenols in the gut.

Virulence Factors (Functional or Beneficial Factors) of Bifidobacterium longum

Unlike pathogenic bacteria, B. longum lacks conventional virulence factors such as toxins, tissue-damaging enzymes, or active invasion mechanisms. Instead, its presence in the gut is associated with beneficial effects, supporting host health rather than causing disease. Its functional and beneficial factors include:

Adhesion Factors

Possesses surface proteins and polysaccharides that help it attach to intestinal epithelial cells.
This attachment enhances colonization in the gut.
It also blocks the adherence of pathogenic microorganisms.

Exopolysaccharides (EPS)

Contribute to colonization and biofilm formation.
Help in immune system modulation.
Provide protection against environmental and physiological stress.

Production of Organic Acids

Produces lactic acid and acetic acid.
These organic acids lower intestinal pH and suppress the growth of harmful pathogens.

Bacteriocin-like Substances

Produces antimicrobial compounds that inhibit pathogenic bacteria.

Enzymatic Activity

Secretes enzymes involved in carbohydrate metabolism and digestion.
Supports host nutrition and promotes gut health.

Immune Modulation Factors

Enhances anti-inflammatory responses.
Stimulates mucosal immunity.

Stress Tolerance Mechanisms

Enables survival in acidic pH and bile salt conditions of the gastrointestinal tract.

Epidemiology and Transmission of Bifidobacterium longum

Unlike pathogenic bacteria, Bifidobacterium longum is a commensal and probiotic bacterium, so its epidemiology mainly focuses on its presence, distribution, and abundance in humans and other hosts rather than disease incidence.
It is a major inhabitant of the human gut microbiota, especially in infants and adults.
In infants, it dominates the gut flora in breastfed infants, comprising up to 90% of bifidobacteria.
In adults, the prevalence of this bacterium is lower, while still maintaining gut microbial balance.
Colonization of B. longum varies according to age, diet, health status, and geography.
It is primarily present in fermented foods, probiotics, and some dairy products.
It can survive in the gut when ingested, contributing to microbiota diversity.
Its prevalence is influenced by geography, antibiotic usage (which lowers its presence), and health conditions such as inflammatory bowel disease, which reduce its abundance.
Since B. longum is a beneficial bacterium, its transmission mainly involves natural colonization rather than disease spread.
Vertical transmission occurs from mother to baby during birth, especially during vaginal delivery.
It is also transferred through breast milk.
Early colonization in the infant gut occurs via maternal vaginal and fecal microbiota.
Environmental and dietary transmission occurs through surrounding environments such as family members and contaminated surfaces carrying bacteria.
It can also be transmitted through food products, including fermented foods like yogurt, kefir, and probiotic supplements.
Gut-to-gut transmission can occur indirectly among individuals through the fecal–oral route, representing normal microbiota exchange rather than pathogenic transmission.

Beneficial Effects of Bifidobacterium longum

Impact on Gut and Digestive Wellness

This bacterium contributes to maintaining the integrity of the intestinal barrier.
The intestinal barrier acts as a selective filter, allowing the passage of nutrients into the bloodstream while blocking harmful substances, toxins, and undigested food particles.
It supports digestive health by aiding in the breakdown (cleavage) of complex carbohydrates and fibers.
This breakdown generates energy and useful compounds such as lactic acid and acetic acid.
These acids help maintain an acidic environment in the colon, making it unfavorable for many pathogens and providing protection against infections.

Support of Gut Barrier Function

B. longum maintains gut barrier integrity by increasing the synthesis of mucin, which forms a protective layer lining the gut.
It promotes the strengthening of tight junctions between intestinal epithelial cells.
This helps seal the gut barrier and reduces the risk of systemic inflammation and immune system overactivation.

Enhancing Nutrient Metabolism

It plays an important role in improving the metabolism and absorption of various nutrients.
It aids in the fermentation of dietary fibers.
It produces short-chain fatty acids (SCFAs) such as butyrate, propionate, and acetate.
SCFAs have systemic effects, including regulation of glucose metabolism, appetite control, and immune responses, thereby contributing to overall metabolic health.

Addressing Inflammation in the Body

The bacterium has the ability to regulate the immune system.
It helps reduce systemic inflammation.

Laboratory Diagnosis of Bifidobacterium longum

Sample Collection

A fecal sample is most suitable for gut colonization studies.
Food samples are useful for detecting probiotics in dairy products and supplements.

Gram Staining

Under the microscope, bacteria appear as purple-colored, slightly curved rods.
They are often seen in Y- or V-shaped arrangements.

Culture

On MRS agar: small to medium-sized, convex, smooth-surfaced, circular colonies are formed.
On blood agar: small to medium-sized, smooth, convex, white to cream colonies are formed.
Colonies may be soft and sometimes slightly mucoid.

Biochemical Tests

After culture, colonies are tested and identified as B. longum based on the following results:

Gram staining: Positive
Catalase: Negative
Oxidase: Negative
O/F: Fermentative
Indole: Negative
Motility: Negative
Gas production: Negative
Gelatin hydrolysis: Negative
Nitrate reduction: Negative

Molecular Methods

16S rRNA gene sequencing is used for accurate identification and differentiation from other bifidobacteria.
PCR-based assays use species-specific primers for B. longum.

Treatment / Clinical Relevance of Bifidobacterium longum

It does not require treatment, as it is not a pathogen.
Instead, it is used therapeutically as a probiotic.
It is used in the prevention and treatment of diarrhea, including infectious and antibiotic-associated diarrhea.
It helps in relieving symptoms of irritable bowel syndrome (IBS).
It is used after antibiotic therapy for gut microbiota restoration.
It supports allergy reduction and modulation of the immune system.

Prevention and Control of Bifidobacterium longum Infection

Prevention and control mainly refer to maintaining its healthy presence in the gut and preventing rare opportunistic infections in vulnerable individuals.

Maintaining Healthy Gut Colonization

Dietary intake: Consume prebiotics such as dietary fibers and oligosaccharides to promote its growth.
Consume fermented foods or probiotic supplements containing B. longum.
Breastfeeding: It supports natural colonization in infants through breast milk.
Avoid unnecessary antibiotics: Antibiotic use can reduce bacterial populations, so unnecessary use should be avoided.

Preventing Rare Opportunistic Infections

High-risk individuals such as immunocompromised patients and premature infants should be carefully monitored when using or may need to avoid live probiotic administration.
Ensure proper sterilization and handling of probiotic products in hospital settings.

Hygiene Measures

Maintain proper hand hygiene.
Practice safe food handling to prevent gut translocation of bacteria in vulnerable individuals.

Conclusion

Bifidobacterium longum is a Gram-positive, anaerobic, non-motile bacterium.
It is a key component of the human gut microbiota, especially in infants.
It is a non-pathogenic bacterium.
It provides various health benefits, including maintenance of gut microbiota balance.
It increases intestinal barrier function.
It produces beneficial metabolites such as short-chain fatty acids (SCFAs).
It modulates the immune system.
Transmission primarily takes place via maternal transfer and dietary intake of probiotic foods.
Laboratory diagnosis is mainly done through culture, microscopy, biochemical tests, and molecular methods.
Overall, B. longum is a protective and beneficial microorganism essential for gut health.
It is widely used in probiotic therapies.

References

Zhao, L., Wang, S., Dong, J., Shi, J., Guan, J., Liu, D., Liu, F., Li, B., & Huo, G. (2021). Identification, characterization, and antioxidant potential of Bifidobacterium longum subsp. longum strains isolated from feces of healthy infants. Frontiers in Microbiology, 12, 756519. https://doi.org/10.3389/fmicb.2021.75651
Aryal, S. (2022, March 8). Biochemical test of Bifidobacterium bifidum. Microbe Notes. https://microbenotes.com/biochemical-test-of-bifidobacterium-bifidum/
Yi, D. H., Kim, Y. T., Kim, C. H., Shin, Y. S., & Lee, J. H. (2018). Isolation and characterization of Bifidobacterium longum subsp. longum BCBL-583 for probiotic applications in fermented foods. Journal of Microbiology and Biotechnology, 28(11), 1846–1849. https://doi.org/10.4014/jmb.1809.09029
Wong, C. B., Odamaki, T., & Xiao, J. Z. (2019). Beneficial effects of Bifidobacterium longum subsp. longum BB536 on human health: Modulation of gut microbiome as the principal action. Journal of Functional Foods, 54, 506–519. https://doi.org/10.1016/j.jff.2019.02.002
Odamaki, T., Bottacini, F., Kato, K., Mitsuyama, E., Yoshida, K., Horigome, A., Xiao, J. Z., & van Sinderen, D. (2018). Genomic diversity and distribution of Bifidobacterium longum subsp. longum across the human lifespan. Scientific Reports, 8(1), 85. https://doi.org/10.1038/s41598-017-18391-x
Massive Bio. (2025, December 3). Bifidobacterium longum: Gut health and inflammation. MassiveBio.com. https://massivebio.com/bifidobacterium-longum-gut-health-and-inflammation-bio/