Streptococcosis is an infectious disease caused by bacteria of the genus Streptococcus. This disease is most common among horses, guinea pigs, dogs, cats, and fish with symptoms varying based on the streptococcal species involved. In humans, this disease typically involves a throat infection and is called streptococcal pharyngitis or strep throat.
Pathogenesis Occurring in pairs or chains, streptococci are found to be Gram-positive (although older cultures may lose this characteristic), non-mobile, non-spore forming, and catalase-negative. Bacteriophages, also known as phages, of streptococcus within different parameters of temperature, pH, and salinity maintain successfully stable and are lytic. Integrase, transposase, and recombinase coding genes are found to be absent within phages. Streptococcosis can start occurring due to a weak immune system, or by having bacteria enter wounds. Spreading of Streptococcus is often sporadic, and can be done through direct contact (may be done through materials that are likely to carry infection), air transport or (rarely) ingestion.
Classification The bacterial species involved in a streptococcosis infection is typically identified through microscopy of the bacteria to observe their morphology, biochemical tests (e.g., hemolysis ability), and tests on which antibodies are produced by the infected organism. Antibody detection, also known as serologic grouping, categorizes with the labeling of Group A to Group V; it uses differences with cell wall carbohydrates and pili-associated protein. With the use of hemolysis, species are divided within three different categories: incomplete (α hemolytic), complete (β-hemolytic), and no (γ hemolytic) hemolysis detected. Two common species seen are S. agalactiae which has been associated with fish and (more significant) S. suis which has been associated with pigs.
Clinical identification The clinical manifestations of streptococcus infections differ greatly depending on both the host species and group and strain of the bacteria.
Alpha-hemolytic streptococci (S. pneumoniae and viridians) The first group of streptococci is alpha-hemolytic which comprises primarily S.pneumoniae and viridans streptococci. This group is referred to as alpha-hemolysis because the cell membrane of red blood cells is left intact. When cultured, alpha-hemolysis can be deemed present when the agar gel appears greenish. Identifying and diagnosing alpha-hemolytic Streptococcus is done with a sputum gram stain and culture test. Further identification can be done serologically to test for the presence of capsular antigen, which is the dominant structure on the surface of S. pneumoniae. Bile solubility can be used to further distinguish S. pneumoniae from viridans streptococci as S. pneumoniae are bile soluble and viridans streptococci are not. S. pneumoniae are the most significant alpha-hemolytic streptococci and are responsible for several infections including:
Lower respiratory tract infection Pneumonia Septicaemia Meningitis Septic arthritis Sinusitis The identification and diagnosis of these conditions often require a combination of bacteriologic methods with other clinical identification characteristics that are condition-specific.
Beta-hemolytic streptococci (Group A, B, C, D, F, G, and H) In contrast, the beta-hemolytic group of streptococci includes those capable of complete lysis of red blood cells. Beta-hemolytic streptococci are further divided into additional subgroups consisting of: Group A, Group B, Group C, Group D, Group F, Group G, and Group H. Beta-hemolysis is identified by its yellow and transparent appearance on the cultured media. Clinical identification of beta-hemolytic streptococci relies on culturing the bacteria with agar media that has been supplemented with blood. This method allows for beta-hemolysis to be easily identified, which is a critical step in further identification tactics. Identification into subgroups can be done by the Lancefield antigen-determination test which uses antibodies to distinguish B-hemolytic streptococci into different species. An additional method used to identify B-hemolytic streptococci is the PYR test, which is primarily used in distinguishing S. pyogenes from other B-hemolytic strains by testing for the presence of pyrrolidonyl aminopeptidase. Both the Lancefield antigen grouping sera and PYR test are widely available for commercial usage. Each method presents its limitations and studies suggest that a combination of the two protocols be used to achieve the most reliable results.
Group A Group A streptococcal infections are predominantly caused by S. pyogenes. Human pathologies are mostly associated with Group A streptococci and arise most often as respiratory or skin infections. Group A streptococcal infections include:
Streptococcal pharyngitis Impetigo Necrotizing fasciitis Cellulitis Streptococcal toxic shock syndrome Rheumatic fever Post-streptococcal glomerulonephritis The identification and diagnosis of these conditions often require a combination of bacteriologic methods with other clinical identification characteristics that are condition-specific.
Group B Group B streptococcal infections, most commonly associated with S. agalactiae, are extremely prevalent among pregnant women, newborns, and the elderly. Cattle have also been shown to be important reservoir hosts for S. agalactiae. Reports of S. agalactiae have also been identified in several other mammals, fish, and reptiles.
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