Streptococcus bovis

Taxonomy and Nomenclature

Historical Use

Streptococcus bovis was a species that was discovered in 1919 by Sigurd Orla-Jenson from cow dung and milk. It was studied as a distinct species and became a focus of research due to its link to endocarditis in the 1970's. The species was divided into biotypes based upon biochemical characteristics until the later 1990's, when the biotypes began to gain recognition as independent species. These biotypes were not recognized as individual species until 1984, when DNA-DNA hybridization showed that the biotypes were genetically heterogenous. From this point forward, the original taxonomy of S. bovis as an individual species began to be further investigated, until the late 1990's and early 2000's, when it became a term to describe the group of a new species that had once been called Streptococcus bovis.

Streptococcus bovis consisted of biotypes based upon biochemical characteristics. Biotype I was distinguished based on the microbe's ability to ferment mannitol. Those that could not were separated into the Biotype II classification. However, this was further divided based on the ability to degrade bile-esculin, with those that could being distinguished as Biotype II/1, and those that could not as Biotype II/2.

SBSEC

When referencing S. bovis in current research, the term is often used within the Streptococcus bovis Streptococcus equinus Complex (SBSEC). The SBSEC contains strains that were once classified as the species S. bovis, along with other closely related species. The SBSEC complex includes 8 subspecies: S. equinus, S. gallolyticus subsp. gallolyticus, S. gallolyticus subsp. pasteurianus, S. gallolyticus subsp. macedonicus, S. infantarius subsp. infantarius, S. lutetiensis (previously S. infantarius subsp. coli), and S. alactolyticus. A 2022 study isolated a novel species within the complex: S. ruminicola.

Novel Species Link to S. bovis Biotypes

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Microbiology of the SBSEC

Morphology and Cellular Characteristics

The Streptococcus bovis/Streptococcus equinus complex (SBSEC) consists of nonenterococcal, Lancefield group D streptococci. Members of this group are Gram-positive cocci with a thick peptidoglycan wall containing teichoic acids and no outer membrane. Their spherical or ovid-shaped cells grow as chains of cocci that divide along successive planes that are parallel to each other. Chain lengths vary by culture conditions. In liquid culture, cells typically remain attached longer and form chains of varying length. On solid agar, cells separate more frequently, resulting in pairs or shorter chains of cells . They are nonmotile, lack flagella, and do not form endospores. Surface structure vary among members, with some strains producing a capsule, and pilus types differing among species.

Growth and Physiology

SBSEC organisms are thermotolerant facultative anaerobes. They grow optimally at temperatures between 37-45°C with growth completely halting at extremes of 10°C and 50°C. They obtain energy primarily through carbohydrate fermentation, producing lactic acid as a major metabolic end product. Members are catalase-negative and oxidase-negative, reflecting their lack of enzyme activity in biochemical tests. Organisms can grow in 40% bile salts but are typically intolerant to 6.5% NaCl solutions.

Historical Biochemical Identification

The original members of the S. bovis group, that were first recognized as one bacterium, were first isolated from cheese and were placed into the Lancefield group D of streptococci bacteria based on similar morphological characteristics and similarities in preferences for growth mediums. Enterococci strains were also originally classified into the S. bovis group, but were differentiated soon after because of their ability to grow in 6.5% NaCl, hydrolyze arginine, and decarboxylate tyrosine. All members of the SBSEC complex are small-colony, catalase-negative, gram-positive cocci and can all be detected using the Rapid ID 32 strep systems, making them biochemically challenging to differentiate. However, the original identification and classification of the SBSEC complex were based solely on loosely shared morphological characteristics, and it was clear that new methods for identification should be developed in order to accurately diagnose animals and humans infected with the bacteria.

Modern Identification

To differentiate between the eight subspecies, several methods can be used. S. gallolyticus subsp. gallolyticus is mannitol-positive after fermentation, where S. infantarius subsp. infantarius and S. gallolyticus subsp. pasteurianus would be mannitol-negative. S. infantarius subsp. infantarius and S. gallolyticus subsp. pasteurianus test positive for activity in the alpha- and beta-galactosidase enzymes, while S. gallolyticus subsp. gallolyticus would test negative. The beta-glucoronidase enzyme can also be used to differentiate because S. gallolyticus subsp. pasteurianus tests positive and S. gallolyticus subsp. gallolyticus tests negative. Starch degradation is another method for differentiating between the subtypes, because S. infantarius subsp. infantarius typically tests positive for starch degradation while S. gallolyticus subsp. gallolyticus tests negative. For more fast-paced and reliable identification, MALDI-TOF mass spectrometry systems can be used.

Ecology and Natural Habitat

Members of the Streptococcus bovis/Streptococcus equinus complex occur in the gastrointestinal tracts of humans and animals, as well as in fermented foods. The historical designation of "Streptococcus bovis" encompassed organisms subsequently assigned to several species and subspecies that have since been distinguished in greater detail. Their individual ecological characteristics are distinct which is an important variable to consider whilst describing their habitats and relationships with the host organisms.

Human GI Tract

SBSEC organisms are able to thrive in the intestinal microbiota of completely healthy individuals without causing notable symptoms or disease. Reported carriage frequencies vary with each population examined, specimen type, identification method, and species included, all of which making it very difficult to find substantial data on this microbe. Consequently, historical estimates based on cultures identified as "Streptococcus bovis" are not directly comparable with modern studies targeting individual species or subspecies. Lastly, proven intestinal carriage should be distinguished from invasive infection which will be further elaborated on in the heading of "Human Infection."

In the microbiome of the gut, interactions with other intestinal microorganisms may influence varying effects of colonization. Experimental studies have shown the S. gallolyticus subsp. gallolyticus can produce gallocin, a bacteriocin known to inhibit competing enterococci.

Animal Reservoirs

The rumen is an important habitat for SBSEC organisms, which participate in carbohydrate fermentation across a wide array of animal groups.

Geographical Distinctions

Regional differences in SBSEC ecology include associations with traditional fermented foods showing noticeable differences between Europe, West and East Africa, along with East Asia.

Human infection

Organisms historically identified as "Streptococcus bovis" have been implicated in several different human infections all loosely associated with functions of the gastrointestinal tract.

Entry

The main portal of entry for human infection of S. bovis bacteremia is the gastrointestinal tract, but in some cases, entry is through the urinary tract, the hepatobiliary tree, or the oropharynx.

Role in disease

S. bovis is a human pathogen that has been implicated as a causative agent of endocarditis, urinary tract infections, and more rarely, sepsis and neonatal meningitis.

S. bovis has long been associated with colorectal cancer; however, not all genospecies are associated equally. A 2011 meta-analysis on the association between S. bovis biotypes and colonic adenomas/carcinomas revealed that patients with S. bovis biotype I infection had a strongly increased risk of having colorectal cancer (pooled odds ratio: 7.26; 95% confidence interval: 3.94–13.36), compared to S. bovis biotype II-infected patients. This analysis suggests S. bovis should no longer be regarded as a single bacterial entity in clinical practice. Only Streptococcus gallolyticus (S. bovis biotype I) infection has an unambiguous association with colonic adenomas/carcinomas (prevalence range: 33–71%) that markedly exceeds the prevalence of colonic (pre-)malignancies in the general population (10–25%). Nevertheless, research has not yet determined that S. gallolyticus is a causative agent of colorectal cancer, or if pre-existing cancer makes the lumen of the large intestine more hospitable to its outgrowth.