Streptococcus mitis
A Microbial Biorealm page on the genus Streptococcus mitis
Classification
Higher order taxa
Bacteria; Firmicutes; Bacilli; Lactobacillales; Streptococcaceae;
Species
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NCBI: Taxonomy |
Streptococcus mitis
Description and significance
Streptococcus mitis are commensal bacteria that colonize hard surfaces in the oral cavity such as dental hard tissues as well as mucous membranes and are part of the oral flora. They are usually arranged in short chains in the shape of cocci (10). These Gram-positive bacteria are not usually pathogenic but commonly cause bacterial endocarditis, which is the inflammation of an inner layer of the heart. S. mitis are alpha hemolytic, meaning it can break down red blood cells. S. mitis are not motile, do not form spores and lack group-specific antigens (2). S. mitis live optimally at temperatures between 30 and 35 degrees Celsius, making them mesophiles. They are facultative anaerobes, which is a bacterium that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation in the absence of oxygen (7).
Genome structure
The genome of S. mitis has been sequenced and consists of a circular chromosome with about two million bp that varies with different strains. Its GC and AT content are respectively 40.4% and 59.1%. There are a total of 2222 genes of which 2149 are protein coding genes (3).
The genes encoding the lipoproteins Pb1A and Pb1B in S. mitis are clustered close to the genes that are very similar to the streptococcal phages r1t, 01205 and Dp-1. This implies that Pb1A and Pb1B might be located within a prophage (4). To test this possibility, mitomycin C and UV light were used because both can induce the lytic cycle of many phages. Cultures of S. mitis were exposed to this and a significant increase expression of Pb1A and Pb1B were detected by Western blot analysis. Phage particles were visible in the cultures of S. mitis, which was named SM1. This phage had a DNA genome of about 35 kb. All these experiments concluded that Pb1A and Pb1B are encoded by a lysogenic bacteriophage (4).
Cell structure and metabolism
4.1 Cell Structure
As demonstrated by electron microscopy, S. mitis strains usually carry sparsely distributed, long fibrils, and their cell surfaces are often regarded as being soft. The electrophoretic softness and fixed negative charge density of -1.2 to -4.3×106 Cm-3 in the polyelectrolyte layer of S. mitis strains, were determined by the soft particle analysis using measured electrophoretic mobilities (5).
There is a very high frequency of occurrence of extracellular surface structures on S. mitis strains and a variety of appendages with different lengths up to several microns have been found (5). Between different strains, the density of appendages on cell surfaces can vary significantly (5).
S. mitis is characterized by its C-polysaccharide cell wall and a teichoic acid-like polysaccharide. The teichoic acid-like polysaccharide contains a heptasaccharide phosphate repeating unit that neither consists of ribitol nor glycerol phosphate as normally seen in teichoic acids (6). The C-polysaccharide of S. mitis contains, in each repeating unit, two residues of phosphocholine and both galactosamine residues (6).
4.2 Metabolism
S. mitis is a facilitated anaerobe which makes its metabolism very versatile. The utilization and synthesis of intracellular glycogen and its catabolism to lactate has been detected in S. mitis. The glycogen-like polysaccharide functions as the only source of utilizable energy in S. mitis (7).
When an exogenous energy source is absent, the break down of polysaccharide provides S. mitis with energy in a utilizable form, for cells that have polysaccharide increased in β-galactosidase activity when induced with thiomethyl galactoside (8). When induced in a similar manner, cells that lack polysaccharide, and a polysaccharide-negative variant of S. mitis did not increase in β-galactosidase activity. The only substrate for the endogenous metabolism of S. mitis is intracellular polysaccharide (8).
Ecology
S. mitis is a part of the normal mammal flora. They usually inhabit the mouth, throat, and nasopharynx. Certain strains of S. mitis have the ability to produce IgIA1 protease and bind salivary alpha-amylase, which are two properties that are determinants for streptococcus viridans, which are a large group of generally non-pathogenic, commensal, streptococcal bacteria. Some S. mitis that produce neuraminidase tend to colonize mucosal surfaces, although the production of this enzyme is not required for successful colonization (9). However, neither immunoglobulin A1 protease activity nor the ability to bind α-amylase from saliva was a preferential characteristic of persistent genotypes. The major origin of new clones occupied by S. mitis can be found in the respiratory tract (9).
Pathology
S. mitis is usually an etiologic agent in odontogenic infection and endocarditis and only in some cases have been acknowledged as respiratory pathogens. The most common host is humans. The major interaction in the pathogenesis of infective endocarditis is the direct binding of bacteria to platelets (10). S. mitis is a commensal organism that is closely related to the pathogen Streptococcus pneumoniae, the causative agent of otitis, pneumonia, sepsis and meningitis. Homologous recombination between these species has been observed and the transfer of genetic determinants from S. mitis to S. pneumoniae contributes to penicillin resistance in the pathogen (10).
Numerous phages are known to carry determinants that increase virulence to the bacterial host. These factors have been predominantly secreted toxins, such as the streptococcal erythrogenic toxin, staphylococcal enterotoxin A, diphtheria toxin, and cholera toxin (10). Other phage-encoded virulence determinants include extracellular enzymes such as staphylokinase and streptococcal hyaluronidase, enzymes that alter the antigenic properties of the host strain, and outer membrane proteins that increases serum resistance (10). It is likely that Pb1A and Pb1B bind platelets directly, although the mechanism by which PblA and PblB mediate platelet binding by S. mitis has not been illustrated. Thus, the encoding of PblA and PblB by lysogenic SM1 may represent a class of phage-mediated virulence determinants (10).
Application to Biotechnology
In a small number of S. mitis isolates, a cholesterol-dependent cytolysin named mitilysin has been detected. The mitilysin gene was sequenced from seven isolates of S. mitis. Comparisons with the pneumococcal pneumolysin gene show 15 amino acid substitutions (12). S. mitis appear to release mitilysin extracellularly. Based on enzyme-linked immunosorbent assay and neutralization assay results, one isolate of S. mitis may produce a hemolytic toxin in addition to mitilysin (12). As genetic exchange is known to occur between S. mitis and Streptococcus pneumoniae, this finding may have implications for the development of vaccines or therapies for pneumococcal disease that are based on pneumolysin and its properties (12).
Current Research
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References
Edited by student of Rachel Larsen
