Table of Contents
- What is Streptococcus bovis?
- Classification of Streptococcus bovis
- Habitat of Streptococcus bovis
- Morphology of Streptococcus bovis
- Cultural characteristics of Streptococcus bovis
- Biochemical characteristics of Streptococcus bovis
- Virulence factors of Streptococcus bovis
- Pathogenesis of Streptococcus bovis
- Clinical manifestations of Streptococcus bovis
- Lab diagnosis of Streptococcus bovis
- 1. Cultural characteristics and Biochemical characteristics
- 2. Rapid identification kits
- 3. Molecular diagnosis
- Treatment of Streptococcus bovis
What is Streptococcus bovis?
Streptococcus bovis is a Gram-positive bacterium that is part of the bovis/equinus complex within the viridans group of Streptococci.
- It naturally inhabits the gastrointestinal tract of humans, alongside other Streptococcus species, but in much smaller numbers compared to other species.
- Previously, S. equinus and S. bovis were considered distinct species, but recent DNA–DNA hybridization studies have determined that these names are subjective synonyms, with Streptococcus equinus taking nomenclatural priority.
- Despite this, the name ‘S. bovis‘ is still widely used in clinical microbiology.
- Although S. bovis is typically a commensal organism in the gastrointestinal tracts of humans and other animals, it has been linked to several diseases, the most notable being colorectal cancer.
- S. bovis is one of the original members of the bovis group, which has since been reclassified as the bovis/equinus complex due to genetic similarities between the species.
- The name 'bovis' is derived from the Latin word for cow, bull, or ox, indicating the primary host of the species.
- S. bovis was first isolated from the gastrointestinal tract of cattle or ruminants by Andrewes and Horder in 1906.
- It is classified under Group D of the Lancefield antigen grouping and is recognized as a significant etiological agent in various Group D Streptococcus infections.
Classification of Streptococcus bovis
- The genus Streptococcus consists of more than 60 species and 12 subspecies that are closely related to other lactic acid bacteria, producing lactic acid as the sole or major end product during carbon metabolism.
- The family Streptococcaceae is classified within the branch of Eubacteria characterized by low (< 50 mol%) G+C content, based on their 16S rRNA gene sequence analysis.
- The primary morphological characteristic of this genus is Gram-positive cocci arranged in chains, resulting from division on successive planes parallel to each other.
- S. bovis was one of the first species categorized in the bovis group, which was later renamed the bovis/equinus group due to genetic similarities.
- It has been proposed that S. bovis and S. equinus are synonymous, with S. equinus preferred due to taxonomical priority.
- S. bovis is also part of the viridans group Streptococci, known for producing a green-colored zone by α-hemolysis on blood agar. It belongs to Group D of the Lancefield antigen grouping.
- The following is the taxonomical classification of S. bovis:
Taxonomical Classification of Streptococcus bovis
Domain: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Streptoococcaceae
Genus: Streptococcus
Species: S. bovis
Habitat of Streptococcus bovis
- Streptococcus bovis is a regular inhabitant of the human gastrointestinal tract, although it is present in much lower numbers compared to other Streptococcus species.
- Additionally, it has been isolated from the alimentary tracts of cows, horses, sheep, and other ruminants.
- Members of the bovis/equinus complex are known to colonize the rumen, crop, and cloaca of animals, as well as the colon of humans. The fecal carriage rate of these organisms in humans ranges from 5% to over 60%.
- S. bovis is highly prevalent among a wide range of domesticated and wild animals, including bears, piglets, rodents, dogs, sea otters, and some birds.
- While the exact source of the organism is not yet known, it reaches the gastrointestinal tract via the mouth. S. bovis has also been isolated from feces in some cases, but transmission via feces has not been observed.
- Infections associated with S. bovis often occur when the organism enters sterile parts of the body.
- The optimal temperature for S. bovis is the average body temperature of its host.
Morphology of Streptococcus bovis
- Streptococcus bovis cells are spherical or ovoid, with a diameter of approximately 0.8 to 1 µm.
- The cells typically arrange themselves in chains, occurring mostly in pairs to moderately long chains, with longer chains observed when cultured in liquid broth.
- This cellular arrangement results from division along successive planes parallel to each other, similar to rod-shaped cells.
- S. bovis is a catalase-negative, facultative anaerobe, non-capsulated, and usually features sparsely distributed, long fibrils on its surface.
- Its cell wall is composed of peptidoglycan and teichoic acid, along with various carbohydrates. The peptidoglycan type is Lys–Thr–Ala.
- The peptidoglycan consists of multiple glycan chains cross-linked by short peptides, with each glycan composed of alternating β-1,4-linked units of N-acetylglucosamine and N-acetylmuramic acid.
- The cell wall contains ribitol teichoic acid and lacks significant amounts of rhamnose, the primary sugar in some Streptococcus species. It includes phosphorylcholine residues in the teichoic acids of its cellular envelope.
Cultural Characteristics of Streptococcus bovis
- Most Streptococcus species, including Streptococcus bovis, do not grow profusely on traditional media like Nutrient Agar and require media supplemented with specific carbohydrates and nutrients.
- Media such as Blood agar and Chocolate agar are commonly used to identify S. bovis by observing its hemolysis patterns.
- For more selective isolation, media like Brain Heart Infusion Agar and Trypticase soy agar/broth with defibrinated sheep blood can be used.
- As a facultative anaerobe, S. bovis exhibits abundant growth in the presence of air with 5% carbon dioxide at 37°C.
- No growth occurs at 10°C, but growth is positive at 45°C. The organism can tolerate 40% bile but cannot grow in 6.5% NaCl or at pH 9.6.
Nutrient Agar (NA)
On Nutrient Agar, S. bovis forms white to grey colonies with an average size of 1 mm in diameter. The colonies are round, with a raised elevation and an entire margin. Growth is generally poor and requires air with supplied carbon dioxide.
Blood Agar (BA)
On Blood Agar, S. bovis typically forms smooth, non-pigmented, convex colonies with an entire margin. Growth occurs readily, exhibiting various types of hemolysis, predominantly α-hemolysis. A green-colored zone of hemolysis about 1-2 mm wide is observed. On Chocolate Agar, pronounced greening is also noted.
Biochemical characteristics of Streptococcus bovis
The biochemical characteristics of S. bovis can be tabulated as follows:
Fermentation
Enzymatic Reactions
Virulence Factors of Streptococcus bovis
Streptococcus bovis is generally a benign species that resides in the gut of animals as a commensal, helping to prevent the colonization of various pathogenic microorganisms. However, it possesses several structures and proteins that aid in the colonization and invasion of host tissue, particularly in immunocompromised individuals. These structures not only facilitate the organism's entry and invasion but also shield it from the host's immune system.
While the exact mechanisms of pathogenesis associated with S. bovis are not fully understood, several factors are known to support its survival and growth within the host:
1. Adhesins/Surface Proteins
S. bovis has numerous genes coding for "microbial surface component recognizing adhesive matrix molecules" (MSCRAMM) and other adhesive proteins that can bind to components of the extracellular matrix (ECM) of host cells. These molecules are located on the cell wall and are specific to particular host cells.
Genomic sequences of S. bovis indicate the presence of three pilus gene clusters, identified as pil1, pil2, and pil3. Pil1 is the first virulence factor experimentally identified and is responsible for binding to collagen, affecting biofilm formation, and playing a crucial role in the initial attachment and colonization during infective endocarditis. Additionally, intestinal colonization by S. bovis depends on the Pil3 pilus, which aids bacterial attachment by binding to colonic mucus and fibrinogen in humans.
2. Biofilm Formation
Certain strains of S. bovis can form biofilms around medical devices like catheters, potentially leading to nosocomial or hospital-acquired infections. Biofilm formation provides protection to the bacteria by acting as a barrier against immune cells and antimicrobial agents. The process is supported by fibrinogen-binding adhesins and several other enzymes.
3. Soluble Cell-Wall Antigen
In cases of colorectal cancer induced by S. bovis, soluble cell-wall antigens play a significant role in triggering inflammation and carcinogenic processes. These proteins or antigens stimulate interleukin-8 (IL-8) activity in various cells throughout the body. IL-8 induces the overexpression of cyclooxygenase 2 (Cox-2), which leads to increased levels of prostaglandin in Caro-2 cells. The heightened formation of oxygen radicals and nitric oxide causes mutagenesis in the cells of the intestinal mucosa, further promoting the development of cancer.
Pathogenesis of Streptococcus bovis
Streptococcus bovis exists in the gastrointestinal tract of animals as a commensal but can cause various infections. The exact mechanism of infection and pathogenesis is not fully understood, but biofilm formation is believed to be a crucial factor for the organism's growth and survival. Several virulence factors expressed by S. bovis contribute to its pathogenic process.
1. Colonization
- The bacteria enter the host body through the mouth via different food materials. Once in the gastrointestinal tract, S. bovis adheres to the epithelial cells of the mucosal layer.
- The initial attachment is facilitated by "microbial surface component recognizing adhesive matrix molecules" (MSCRAMM) and other adhesive proteins that bind to components of the extracellular matrix of epithelial cells.
- This attachment is followed by the binding of the pil1 protein to collagen, aiding further colonization of the tract. These steps are crucial for ensuring the organism can grow, reproduce, and maintain the invasion.
2. Invasion
- After colonizing the intestinal tract, the bacteria invade deeper tissues, eventually entering the bloodstream and causing infections in various body parts.
- Mucosal disruption allows the bacteria to penetrate deeper tissues and the bloodstream, from where they reach heart valves and cause infective endocarditis.
- The bacteria uniquely colonize thrombin on platelets and fibrin, with bacterial colonies protected by new layers of platelets and fibrin formed by stimulation from thromboplastin.
- The interaction of Pil1 and Pil3 with components of the intrinsic coagulation pathway induces blood coagulation, an essential event during endocarditis.
- Additionally, the immune system's detection of bacteria triggers an inflammatory response, further affecting heart valves and surrounding areas. Soluble cell-wall proteins produced by S. bovis induce the release of IL-8, leading to the over-expression of cyclooxygenase 2 (Cox-2).
- This, combined with the inflammatory response, results in increased formation of oxygen radicals and nitric oxide. These elevated levels cause mutagenesis, ultimately leading to cancer.
- Thus, S. bovis bacteria induce severe inflammatory reactions in the colorectal mucosa, causing the release of inflammatory and angiogenic cytokines and forming free radicals that contribute to the development of various types of human cancers.
Clinical Manifestations of Streptococcus bovis
Streptococcus bovis was once considered a lower-grade pathogen frequently implicated in bacteremia and endocarditis. In addition to infective endocarditis, S. bovis infections have been suggested to occur in various sites outside the colorectum, such as osteomyelitis, discitis, and neck abscesses. Depending on patients' geographic locations, other infections like colorectal adenoma, cholecystitis, cholangitis, and biliary tract diseases have also been linked to S. bovis.
1. Infective Endocarditis
- Infective endocarditis, a condition associated with S. bovis, occurs when the organism induces blood coagulation by binding to fibrinogen-binding proteins and collagen in the blood.
- The organism travels through the bloodstream and can bind to pre-existing endothelial injuries exposing extracellular matrix components such as fibronectin, laminin, and collagen.
- Patients with endocarditis typically experience flu-like symptoms, including chills and fever, along with chest pain and changed heart murmurs. Endocarditis and bacteremia are closely associated with colon cancer.
2. Malignancies
- S. bovis bacteremia is linked to malignancy regardless of the site, with approximately 29% of patients with positive S. bovis bacteremia harboring tumor lesions in various organs such as the colon, duodenum, gallbladder, pancreas, ovary, uterus, lung, or hematopoietic system.
- Chronic inflammation and the production of carcinogenic metabolites induce lesions in the colorectal area, leading to malignancies in affected organs.
Lab Diagnosis of Streptococcus bovis Infections
The laboratory diagnosis of infections caused by Streptococcus bovis starts with sample collection, typically blood, followed by direct examination under the microscope. Diagnosis primarily revolves around identifying the organism, emphasizing its isolation and detection.
1. Cultural and Biochemical Characteristics
- Culturing the organism on selective media and observing colony morphology aids in identification.
- Isolation from clinical specimens occurs on media like blood agar with 5% sheep blood, followed by incubation at 35–37°C for 18–24 hours.
- Biochemical tests further assist in species determination, considering microscopic observations and colony morphology.
2. Rapid Identification Kits
- Clinical labs utilize commercial kits or automated instruments for rapid species determination.
- Microbial cellular fatty acid compositions aid in Streptococcal species identification.
- Common systems include MicroScan Conventional Pos ID, Rapid Pos ID, and BBL Crystal Gram-Pos ID.
3. Molecular Diagnosis
- Molecular methods involve identifying organisms at the molecular level, utilizing unique nucleic acid sequences.
- Polymerase Chain Reaction (PCR) amplifies and detects bacterial DNA, offering detailed identification.
- DNA sequencing determines the bacterial DNA sequence for identification purposes. Ribotyping, employing rRNA restriction fragment polymorphism methods, is another molecular approach.
Treatment of Streptococcus bovis Infections
- Most S. bovis isolates respond well to treatment, typically with penicillin (MIC ≤ 0.1 mg/L), administered intravenously as penicillin G or ceftriaxone for four weeks.
- For uncomplicated cases of native-valve endocarditis, a 2-week therapy combining penicillin G or ceftriaxone with gentamicin is an alternative.
- Treatment is generally straightforward as S. bovis infections are often mild, and most strains are susceptible to common antibiotics. However, in severe cases like infective endocarditis, valve removal or surgery might be necessary.
- Late diagnosis or untreated infections may lead to mucosal lesions turning malignant, necessitating more aggressive treatments.
