Streptococcus infantarius: Current Insights and Health Impact

Streptococcus infantarius is a bacterial species found in the human gut, with emerging research suggesting it may have roles beyond commensal colonization, including possible links to colorectal disease.

Understanding its biological characteristics, gut colonization, and clinical relevance is essential for assessing its impact on human health.

Classification And Biological Features

Streptococcus infantarius belongs to the Streptococcus bovis/Streptococcus equinus complex (SBSEC), a group of Gram-positive, facultatively anaerobic bacteria within the Streptococcaceae family. Phylogenetic analyses using 16S rRNA sequencing confirm its close relationship to Streptococcus lutetiensis and Streptococcus gallolyticus, though it exhibits distinct metabolic and virulence-associated traits.

A defining feature of S. infantarius is its ability to ferment carbohydrates like lactose and sucrose, producing lactic acid. This trait supports its adaptation to various host environments. Unlike some SBSEC members, it has a higher tolerance for bile salts, aiding its survival in the gastrointestinal tract. It is frequently isolated from human and animal intestines and fermented dairy products, where it contributes to acidification and microbial stability.

Genomic studies reveal that S. infantarius possesses adhesins and biofilm-associated proteins that enhance its ability to adhere to epithelial surfaces, supporting its persistence in the gut microbiota. Certain strains also harbor stress resistance genes, including those involved in oxidative stress response and antimicrobial peptide resistance, providing a survival advantage in fluctuating conditions within the digestive system.

Colonization In The Human Gut

S. infantarius colonizes the gastrointestinal tract through adhesion, metabolic adaptation, and interactions with other microorganisms. Its presence has been documented in both healthy individuals and those with gastrointestinal conditions. Studies using 16S rRNA sequencing and culture-dependent methods consistently identify it in fecal samples, particularly in populations with high consumption of fermented dairy products, indicating dietary influences on its prevalence.

Once in the gut, S. infantarius adheres to the intestinal mucosa through surface-expressed adhesins, enabling resistance to peristaltic clearance. Genes encoding pili and extracellular matrix-binding proteins facilitate attachment to epithelial cells. Biofilm formation in certain strains may further enhance persistence by providing protection against environmental fluctuations and antimicrobial compounds.

Its metabolic activity plays a fundamental role in colonization. By fermenting dietary carbohydrates into short-chain fatty acids (SCFAs) like lactic acid, it influences local pH and microbial composition. SCFAs modulate gut homeostasis, impacting microbial diversity and epithelial function. Individuals with higher S. infantarius levels often exhibit shifts in microbiota composition, though whether this is a cause or consequence remains under investigation.

Potential Association With Colorectal Disease

The presence of S. infantarius in the gut has drawn attention due to its potential link to colorectal disease. Epidemiological studies report a higher prevalence in individuals diagnosed with colorectal cancer (CRC) and precancerous lesions, raising questions about its role in disease progression. Some strains exhibit genetic traits linked to pathogenicity, influencing epithelial integrity and cellular signaling pathways involved in tumorigenesis.

Research suggests S. infantarius may contribute to colorectal disease beyond passive colonization. Certain strains produce metabolites that affect the intestinal epithelium, including pro-inflammatory compounds and fermentation byproducts linked to oxidative stress. Disruptions in redox balance have been implicated in DNA damage and abnormal cell proliferation, processes underlying colorectal carcinogenesis. In vitro studies show that conditioned media from S. infantarius cultures can promote colorectal cancer cell proliferation, though precise molecular interactions remain under investigation.

Geographic and dietary patterns further complicate its relationship with colorectal disease. In regions where fermented dairy products are common, S. infantarius is frequently isolated from both food sources and gut microbiota. Some researchers suggest its presence may be benign or even beneficial under normal conditions, while others hypothesize that microbial imbalances—due to diet, antibiotics, or inflammation—could enhance its pathogenic potential. The interplay between host genetics, environmental factors, and gut microbiota likely determines whether S. infantarius acts as a harmless commensal or a contributor to colorectal pathology.

Laboratory Methods Of Detection

Accurate identification of S. infantarius relies on culture-based techniques, biochemical assays, and molecular diagnostics. Traditional microbiological methods isolate the bacterium on selective media like bile esculin agar, differentiating it from other streptococci based on esculin hydrolysis in the presence of bile salts. Colonies typically appear small and grayish, exhibiting alpha- or gamma-hemolysis on blood agar, though hemolytic patterns vary. While culture methods provide a foundational approach, confirmatory testing is often necessary due to phenotypic similarities with other SBSEC species.

Biochemical identification through automated systems like VITEK 2 or API 20 Strep panels enhances diagnostic accuracy by assessing carbohydrate fermentation and enzymatic activity. However, these methods may yield ambiguous results when differentiating S. infantarius from S. gallolyticus and related species. Molecular techniques such as polymerase chain reaction (PCR) and 16S rRNA sequencing improve specificity, with species-specific primers targeting conserved genetic markers like sodA and gyrB genes providing precise differentiation.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) streamlines bacterial identification by generating unique protein spectra distinguishing S. infantarius from other SBSEC members. This method offers rapid and cost-effective identification, particularly in clinical settings. Whole-genome sequencing (WGS) provides even greater resolution, enabling strain-level differentiation, detection of virulence-associated genes, antimicrobial resistance determinants, and phylogenetic relationships. While WGS is primarily used in research, its increasing accessibility suggests a growing role in epidemiological surveillance and clinical diagnostics.