Streptococcus bovis- An Overview
Streptococcus bovis (S. bovis) is a Gram-positive bacterium that belongs to the bovis/equinus complex of the viridans Streptococci. It is present in the gastrointestinal tract of humans as a normal inhabitant, along with other Streptococcus species. However, it is present in much fewer numbers than other such species.
S. bovis was first isolated from the gastrointestinal tract of cattle or ruminants by Andrewes and Horder in 1906. The species name `bovis` is taken from the Latin word `bovis` which means cow, bull, or ox, indicating the primary host of the species. It belongs to the Group D of the Lancefield antigen grouping and is considered one of the crucial etiologic agent in different Group D Streptococcus infections.
Previously S. equinus and S. bovis were considered as two separate species, but recent studies involving DNA–DNA hybridization have led to the recognition that the names Streptococcus equinus and Streptococcus bovis are subjective synonyms with the specific epithet Streptococcus equinus having priority. However, so far, this has not been adopted in clinical microbiology and the name ‘S. bovis‘ remains widely used.
Despite being a commensal of the gastrointestinal tract in humans and other animals, S. bovis has been associated with numerous diseases, the most important being colorectal cancer. The species is one of the older members of the bovis group which has now been changed to bovis/ equinus complex because of the genetic similarity of the two species.
S. bovis is a facultative anaerobe that can grow in both aerobic and anaerobic conditions. It is catalase-negative and produces lactic acid as the sole or major end product during carbon metabolism. It exhibits various types of hemolysis on blood agar, but mostly α-hemolysis that produces a green-colored zone around the colonies.
S. bovis can cause different forms of infections in humans, such as bacteremia, endocarditis, sepsis, urinary tract infections, and osteomyelitis . There is also a strong association between infection due to S. bovis and colonic neoplasm (as well as other lesions of the gastrointestinal tract). The exact mechanism of pathogenesis and virulence of S. bovis is not yet clearly understood, but it has been suggested that biofilm formation, adhesins, soluble cell-wall antigens, and coagulation factors are involved in the process .
The diagnosis of S. bovis infection is based on the isolation and identification of the organism from clinical specimens, such as blood or urine. Different cultural, biochemical, and molecular methods can be used for this purpose. The treatment of S. bovis infection usually involves antibiotics such as penicillin or ceftriaxone for four weeks or more depending on the severity and site of infection. In some cases, surgery may be required to remove infected valves or devices. In addition, screening for colorectal cancer is recommended for patients with S. bovis infection as a preventive measure.
Streptococcus bovis is a Gram-positive bacterium that belongs to the family Streptococcaceae, which consists of more than 60 species and 12 subspecies of lactic acid bacteria. The genus Streptococcus is classified into different groups based on their cell wall carbohydrate antigens, known as the Lancefield grouping. S. bovis is a member of the Lancefield group D streptococci, which also includes enterococci and other non-enterococcal species.
S. bovis is further classified into the bovis/equinus complex of the viridans streptococci, which are a heterogeneous group of streptococci that are usually α-hemolytic (producing a green zone of hemolysis on blood agar) or non-hemolytic (producing no hemolysis). The bovis/equinus complex includes four major species: S. bovis, S. equinus, S. gallolyticus, and S. infantarius. These species are closely related to each other based on their 16S rRNA gene sequences and DNA-DNA hybridization studies.
However, the taxonomy of S. bovis is still controversial and confusing, as different names have been used for the same or similar strains. For example, S. equinus and S. bovis were considered as two separate species until 2003, when they were recognized as subjective synonyms with the specific epithet S. equinus having priority. However, many clinical laboratories still use the name S. bovis for both species. Similarly, S. gallolyticus was previously known as S. bovis biotype I and S. infantarius as S. bovis biotype II/II/2.
The following table summarizes the current taxonomic classification of S. bovis and its related species:
|S. equinus||-||S. bovis|
|S. gallolyticus||subsp. gallolyticus||S. bovis biotype I|
|S. infantarius||subsp. infantarius||S. bovis biotype II/2|
|subsp. coli||S. bovis biotype II|
Streptococcus bovis is a bacterium that is commonly found in the gastrointestinal tract of humans and animals, especially ruminants such as cows, sheep, and horses . It is one of the normal inhabitants of the gut flora that helps prevent the colonization of pathogenic microorganisms. However, it is present in much fewer numbers than other Streptococcus species.
The exact source of the bacterium is not yet known, but it is assumed that it reaches the gastrointestinal tract via the mouth through different food materials. The bacterium can also be isolated from feces, but the transmission of the bacterium via feces has not been observed yet.
The bacterium prefers the optimum temperature of the average body temperature of the host, which is around 37°C. It is a facultative anaerobe, which means it can grow in both aerobic and anaerobic conditions. It can tolerate 40% bile but cannot tolerate 6.5% NaCl and pH 9.6.
S. bovis is mostly a benign species that does not cause any harm to the host unless it invades the sterile parts of the body, such as the bloodstream, heart valves, or other organs. Infections associated with S. bovis often result from mucosal disruption, immunosuppression, or medical procedures that allow the bacterium to enter the bloodstream. S. bovis has been associated with various diseases, such as endocarditis, sepsis, urinary tract infections, and colorectal cancer .
The cells of S. bovis are spherical or ovoid in shape with the diameter of the cells being about 0.8 to 1 µm . The cells are arranged in chains, mostly occurring in pairs to moderately long chains . Longer chains are seen with organisms cultured on liquid broth.
The organism is catalase-negative, facultative anaerobe, not capsulated, and usually carries sparsely distributed, long fibrils on the surface.
The cell wall is made up of peptidoglycan and teichoic acid, along with different types of carbohydrates. The peptidoglycan type is Lys–Thr–Ala. The peptidoglycan consists of multiple glycan chains that are cross-linked through short peptides, and the individual glycan is made 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, which is the primary sugar in some Streptococcus species. It contains phosphorylcholine residues in the teichoic acids of its cellular envelope.
The cell surface of S. bovis has a variety of proteins and polysaccharides that are either covalently or non-covalently bonded to the cell wall, much like other gram-positive bacteria. The cell surface can be divided into four further categories: (1) LPXTG-like proteins, (2) pseudopili, (3) surface lipoproteins, and (4) capsule.
LPXTG-like proteins are proteins that are covalently attached to the peptidoglycan layer by a sortase enzyme. These proteins are involved in adhesion, biofilm formation, and immune evasion. Some examples of LPXTG-like proteins in S. bovis are Pil1, Pil3, SbsA, SbsB, SbsC, SbsD, SbsE, SbsF, SbsG, and SbsH.
Pseudopili are long filamentous structures that protrude from the cell surface and are composed of pilin subunits that are covalently linked by sortase enzymes. These structures are involved in adhesion, colonization, and biofilm formation. S. bovis has three pilus gene clusters: pil1, pil2, and pil3.
Surface lipoproteins are proteins that are anchored to the cytoplasmic membrane by a lipid moiety and exposed on the cell surface. These proteins are involved in various functions such as nutrient acquisition, signal transduction, and immune modulation. Some examples of surface lipoproteins in S. bovis are GlsB, GlsC, GlsD, GlsE, GlsF, GlsG, GlsH, GlsI, GlsJ, GlsK, GlsL, GlsM, GlsN, GlsO, GlsP, GlsQ and GlsR.
Capsule is a polysaccharide layer that surrounds the cell wall and protects the bacteria from phagocytosis and complement-mediated killing. The capsule of S. bovis is composed of glucose and galactose residues linked by β-1-4 glycosidic bonds.
Streptococcus bovis is a fastidious bacterium that requires enriched media for optimal growth. It can grow on various types of media, but the most commonly used ones are blood agar and chocolate agar, which allow the observation of hemolysis patterns. The following are some of the cultural characteristics of S. bovis on different media:
- Nutrient agar (NA): S. bovis produces white to grey colored colonies of an average size of 1 mm in diameter. The colonies are round with raised elevation and an entire margin. Growth is mostly poor and requires air with supplied carbon dioxide.
- Blood agar (BA): S. bovis produces smooth, non-pigmented, convex colonies with entire margin on blood agar. Growth occurs readily on blood agar and exhibits α-hemolysis, which is the production of a green-colored zone around the colonies due to the partial lysis of red blood cells . Pronounced greening is observed on chocolate agar.
- Brain heart infusion agar (BHIA): S. bovis grows well on BHIA and forms small, greyish-white, translucent colonies with a smooth surface and entire edge.
- Trypticase soy agar/broth with defibrinated sheep blood (TSA/TSB): S. bovis grows well on TSA/TSB and forms small, white, opaque colonies with a smooth surface and entire edge.
S. bovis is a facultative anaerobe, so it can grow in both aerobic and anaerobic conditions, but it prefers air with 5% carbon dioxide at 37°C. It can tolerate 40% bile but cannot tolerate 6.5% NaCl and pH 9.6. It does not grow at 10°C, but it can grow at 45°C.
- Biochemical characteristics of Streptococcus bovis
The biochemical characteristics of S. bovis can be determined by performing various tests that indicate the metabolic and enzymatic activities of the bacterium. These tests help in the identification and differentiation of S. bovis from other related species. The following table summarizes some of the common biochemical tests and their results for S. bovis:
|Catalase||Negative||S. bovis does not produce catalase enzyme that breaks down hydrogen peroxide into water and oxygen.|
|Oxidase||Negative||S. bovis does not produce oxidase enzyme that transfers electrons to oxygen as the final electron acceptor in the electron transport chain.|
|Indole||Negative||S. bovis does not produce indole from tryptophan by the action of tryptophanase enzyme.|
|Methyl red||Positive||S. bovis produces acidic end products from glucose fermentation that lower the pH of the medium below 4.4, turning the methyl red indicator red.|
|Voges-Proskauer||Negative||S. bovis does not produce acetoin or 2,3-butanediol from glucose fermentation, which react with alpha-naphthol and potassium hydroxide to form a red complex in a positive test.|
|Citrate||Negative||S. bovis does not utilize citrate as a sole carbon source and does not grow on Simmons citrate agar, which remains green in a negative test.|
|Urease||Negative||S. bovis does not produce urease enzyme that hydrolyzes urea into ammonia and carbon dioxide, which raise the pH of the medium and turn the phenol red indicator pink in a positive test.|
|Nitrate reduction||Negative||S. bovis does not reduce nitrate to nitrite or nitrogen gas by the action of nitrate reductase enzyme, which can be detected by adding sulfanilic acid and dimethyl-alpha-naphthylamine or zinc dust in a positive test.|
|Esculin hydrolysis||Positive||S. bovis hydrolyzes esculin into esculetin and glucose by the action of esculinase enzyme, which reacts with ferric citrate to form a black complex in a positive test.|
|Hemolysis||Alpha or gamma||S. bovis produces partial or no hemolysis on blood agar, resulting in a greenish or unchanged zone around the colonies, respectively.|
|Carbohydrate fermentation||Variable||S. bovis can ferment different carbohydrates such as glucose, maltose, sucrose, lactose, cellobiose, and starch, producing acid and gas, which lower the pH of the medium and turn the phenol red indicator yellow in a positive test. However, it cannot ferment xylose, ribose, or arabinose.|
S. bovis is mostly a benign species that remains in the gut of animals as a commensal and helps prevent the colonization of different pathogenic microorganisms. However, it has various structures and proteins that support the colonization and invasion of host tissue surface in immune-compromised individuals. These structures not only help the organism make its way into the body and initiate invasion but also protect the organism from the host immune system. The exact mechanism of pathogenesis associated with S. bovis is not yet known, which is why the virulence factors associated with such infections are also not entirely understood. The following are some of the factors that support the survival and growth of the organism inside the host’s body:
Adhesins/ Surface proteins: S. bovis carries a number of 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 remain on the cell wall of the organism and are specific to particular cells on the host’s body. Genomic sequences of S. bovis further support the existence of three pilus gene clusters assigned as pil1, pil2, and pil3, out of which Pil1 is the first virulence factor experimentally identified. Pil1 is responsible for binding to collagen, affecting biofilm formation, and plays an essential role in the initial attachment and colonization stage of infective endocarditis. Furthermore, intestinal colonization by S. bovis is found to be dependent on Pil3 pilus, which assists bacterial attachment by binding to colonic mucus and fibrinogen in humans .
Biofilm: Some strains of S. bovis are capable of forming biofilms around medical devices like catheters that might result in nosocomial or hospital-acquired infections. Biofilm formation provides protection to the bacteria as it works as a barrier against the immune cells as well as antimicrobial agents. The process of biofilm formation is assisted by the presence of fibrinogen-binding adhesions and several other enzymes .
Soluble cell-wall antigen: In the case of colorectal cancer induced by S. bovis, soluble cell-wall antigens are found to play important roles in inducing inflammation and carcinogenic processes. These proteins or antigens induce interleukin-8 activity in different cells throughout the body. IL-8 is known to induce the over-expression of cyclooxygenase 2 (Cox-2) which in turn leads to increased levels of prostaglandin in Caro-2 cells. The increased formation of oxygen radicals and nitric oxide produces mutagenesis in the cells of the intestinal mucosa, which further promotes the induction of cancer .
Streptococcus bovis remains on the gastrointestinal tract of animals as a commensal but can cause different forms of infections. The exact mechanism of infection and pathogenesis of the disease is not yet clearly understood. Still, it has been assumed that biofilm formation is the major event that allows the growth and survival of the organism. Different virulence factors expressed by the organism aid in the process of pathogenesis.
The entry of the organism into the host body occurs through the mouth via different food materials. The bacteria then reaches the gastrointestinal area where it adheres to the epithelial cells of the mucosal layer of the tract. The attachment is initiated by the “microbial surface component recognizing adhesive matrix molecules” (MSCRAMM) and other adhesive proteins that can bind to components of the extracellular matrix of the epithelial cells. The initial attachment is then followed by the binding of the pil1 protein that binds to the collagen of the cells and helps in further colonization of the tract. Attachment and colonization are the initial steps in the pathogenesis of the infections as it ensures that the organism can grow, reproduce, and maintain the invasion.
Colonization of the intestinal tract is then followed by invasion where the bacteria invades the deeper tissue, ultimately reaching the bloodstream and causing infections in different parts of the body. Mucosal disruption allows the bacteria to enter the deeper tissues and the bloodstream, from where they reach the heart valves and cause infective endocarditis. The bacteria can uniquely colonize the thrombin present on the platelets and fibrin, and the bacterial colonies develop with protection from new layers of platelets and fibrin that are formed by stimulation from thromboplastin. The interaction of Pil1 and Pil3 with components of the intrinsic coagulation pathway induces the coagulation of blood which is an essential event during endocarditis. Besides, detection of bacteria by the immune system initiates an inflammatory response which further affects the heart valves and surrounding areas. Meanwhile, soluble cell-wall proteins produced by S. bovis induce the release of IL-8, which causes the over-expression of cyclooxygenase 2 (Cox-2). This, together with the inflammatory response induces the increased formation of oxygen radicals and nitric oxide in the body. The increased levels of these chemicals result in mutagenesis that ultimately leads to cancer. Thus, S. bovis bacteria are engaged in inducing severe inflammatory reactions in colorectal mucosa, causing the release of inflammatory and angiogenic cytokines leading to the formation of free radicals that are involved in the development of all types of human cancers.
S. bovis is mostly a benign species that remains in the gut of animals as a commensal and helps prevent the colonization of different pathogenic microorganisms. However, it can cause various infections in humans, especially in those with underlying conditions or compromised immunity. The most common and serious infections caused by S. bovis are bacteremia and infective endocarditis, which are often associated with colorectal neoplasms or other lesions of the gastrointestinal tract. Other less common infections include meningitis, brain abscess, osteomyelitis, spondylodiscitis, and infections in AIDS patients .
- Bacteremia is the presence of bacteria in the bloodstream, which can result from mucosal disruption or invasion of the intestinal tract by S. bovis. Patients with bacteremia may experience fever, chills, malaise, and other nonspecific symptoms. Bacteremia can also lead to septic shock, which is a life-threatening condition characterized by low blood pressure, organ failure, and high mortality. Bacteremia due to S. bovis is often a marker of underlying colorectal cancer or other gastrointestinal diseases, as about 29% of patients with positive S. bovis bacteremia have tumor lesions in the colon or other sites.
- Infective endocarditis is the infection of the heart valves or the lining of the heart chambers by S. bovis, which can reach the heart through the bloodstream from the intestinal tract or other sources. Infective endocarditis can cause damage to the heart valves and surrounding tissues, leading to heart failure, embolism, abscess formation, and other complications. Patients with infective endocarditis may present with fever, chest pain, changed heart murmur, splenomegaly, petechiae, and other signs of systemic infection. Infective endocarditis due to S. bovis is also strongly linked to colorectal cancer or other gastrointestinal lesions, as about 50% to 80% of patients with S. bovis endocarditis have such conditions .
- Meningitis is the inflammation of the membranes covering the brain and spinal cord by S. bovis, which can spread from the bloodstream or other sites of infection. Meningitis can cause severe headache, neck stiffness, photophobia, altered mental status, seizures, and other neurological symptoms. Meningitis due to S. bovis is rare but can be fatal if not treated promptly.
- Brain abscess is a localized collection of pus in the brain tissue by S. bovis, which can result from direct extension of infection from adjacent structures or hematogenous dissemination from distant foci. Brain abscess can cause focal neurological deficits, increased intracranial pressure, fever, and other signs of infection.
- Osteomyelitis is the infection of the bone by S. bovis, which can occur through direct inoculation of bacteria into the bone during trauma or surgery, or through hematogenous spread from other sites of infection. Osteomyelitis can cause bone pain, swelling, tenderness, reduced mobility, fever, and other signs of inflammation.
- Spondylodiscitis is the infection of the intervertebral disc and adjacent vertebral bodies by S. bovis, which can occur through hematogenous spread from other sites of infection or direct extension from adjacent structures. Spondylodiscitis can cause back pain, stiffness, radiculopathy, spinal deformity, fever, and other signs of infection.
- Infections in AIDS patients are opportunistic infections that occur in patients with acquired immunodeficiency syndrome (AIDS) due to their impaired immune system. S. bovis can cause various infections in AIDS patients such as bacteremia, endocarditis, meningitis, brain abscess, osteomyelitis, spondylodiscitis, cholecystitis, cholangitis, biliary tract infections and others. These infections can be more severe and difficult to treat in AIDS patients than in immunocompetent patients.
The clinical manifestations of S. bovis infections depend on the site and extent of infection as well as the host factors such as immune status and underlying conditions. Therefore, a thorough diagnostic evaluation is necessary to identify the source and severity of infection as well as any associated complications or comorbidities.
Lab diagnosis of infections caused by S. bovis begins with the collection of samples, which in this case, is blood. The first observation is a direct examination of these samples under the microscope. The diagnosis is mostly related to the identification of the organism; thus, it is mostly centered on the isolation and detection of the organism.
###Cultural characteristics and Biochemical characteristics
Culturing the organism on different selective media and observing the colony morphology on these media provides a basis for the identification of the organism. This also narrows the scope of diagnosis and makes the process more feasible.
Isolation of the organism from primary clinical specimens is achieved on selective culture media like blood agar supplemented with 5 percent sheep blood, following an incubation period of 18–24 h in the air at 35–37°C.
The isolated colonies are then subjected to different biochemical tests which help in the species determination. Depending on the microscopic observation, colony morphology, and biochemical tests, S. bovis can be detected.
Some of the biochemical tests that are used for S. bovis identification are:
- Catalase test: negative
- Bile solubility test: negative
- Growth at 6.5% NaCl: negative
- Growth at 40% bile: positive
- Growth at pH 9.6: negative
- Esculin hydrolysis: positive
- Pyrrolidonyl arylamidase (PYR) test: negative
- Voges-Proskauer test: positive
- Lactose fermentation: negative
- Mannitol fermentation: positive
- Sorbitol fermentation: negative
Rapid identification kits
Many clinical laboratories have started to employ different commercial identification kits or automated instruments that allow rapid determination of bacterial species. Microbial cellular fatty acid compositions are used for the identification of Streptococcal species.
Some of the common automated systems for Streptococcal species identification include MicroScan Conventional Pos ID, Rapid Pos ID and BBL Crystal Gram-Pos ID.
Molecular methods of diagnosis of bacteria usually include tests that help in the identification of the organism at a molecular level. This method utilizes the unique set of nucleic acid sequences present in each organism which provides a more detailed and accurate identification.
One of the most important molecular methods is Polymerase Chain Reaction (PCR) which helps in the amplification and detection of bacterial DNA. Besides, DNA sequencing is performed to determine the DNA sequence of the bacteria that can then be used for its identification.
Ribotyping is yet another molecular method that involves rRNA restriction fragment polymorphism methods.
The treatment of infections caused by S. bovis depends on the type and severity of the infection, the susceptibility of the organism, and the patient`s condition and comorbidities. The following are some general guidelines for the treatment of S. bovis infections:
- Penicillin is the drug of choice for most S. bovis infections, as most strains are susceptible to penicillin (MIC ≤ 0.1 mg/L) . Penicillin can be given intravenously as penicillin G or orally as penicillin V.
- For patients who are allergic to penicillin, alternative agents include ceftriaxone, vancomycin, or daptomycin .
- For patients with infective endocarditis caused by S. bovis, a combination of penicillin G or ceftriaxone and gentamicin is recommended for at least four weeks . However, if the infection is uncomplicated, involves a native valve, and the organism is fully sensitive to penicillin, a two-week course of therapy may be sufficient .
- For patients with bacteremia caused by S. bovis, the duration of treatment depends on the source and extent of the infection, but usually ranges from two to six weeks .
- For patients with colorectal cancer or other gastrointestinal lesions associated with S. bovis infection, surgical resection of the lesion may be required, in addition to antibiotic therapy .
- For patients with other forms of infection caused by S. bovis, such as osteomyelitis, septic arthritis, meningitis, or abscesses, the treatment should be tailored to the specific site and severity of the infection, and may require drainage or debridement of the infected tissue .
The treatment of S. bovis infections should be guided by the results of culture and susceptibility testing, as well as the clinical response of the patient. The choice and duration of antibiotic therapy should be adjusted according to the individual case and the recommendations of an infectious disease specialist.
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