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However, both Streptococcus dysgalactiae and the Streptococcus anginosus group can possess group A antigen as well. Group A streptococci, when grown on blood agar, typically produce small (2–3 mm) zones of beta-hemolysis, a complete destruction of red blood cells. The name group A (beta-hemolytic) Streptococcus is thus also used. [1]
Group A streptococcal infections are a number of infections with Streptococcus pyogenes, a group A streptococcus (GAS). [1] S. pyogenes is a species of beta-hemolytic Gram-positive bacteria that is responsible for a wide range of infections that are mostly common and fairly mild. If the bacteria enters the bloodstream, the infection can become ...
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Similarly, group B streptococcus typically denotes Streptococcus agalactiae, although minor beta-hemolytic group B streptococci like S. troglodytidis exist. [15] While most streptococcal illnesses in humans originate from species adapted to humans, such as S. pneumoniae or S. pyogenes, there are zoonotic species capable of causing infections. [15]
Streptococcus pyogenes, or GAS, displays beta hemolysis. Some weakly beta-hemolytic species cause intense hemolysis when grown together with a strain of Staphylococcus. This is called the CAMP test. Streptococcus agalactiae displays this property. Clostridium perfringens can be identified presumptively with this test.
Streptococci are chains of cocci such as Streptococcus pyogenes. Staphylococci are irregular (grape-like) clusters of cocci (e.g. Staphylococcus aureus). Tetrads are clusters of four cocci arranged within the same plane such as Micrococcus sp.). Sarcina describes a pack-like cuboidal arrangement of eight cocci such as Sarcina ventriculi.
For example, if a microbiologist observes colonies that resemble a Staphylococcus species, they may perform a catalase test to confirm that it belongs to the genus Staphylococcus, and a coagulase test to determine whether it is a coagulase-negative staphylococcus or a more pathogenic species, such as S. aureus. [3]: 101 [8]: 203
The antibody reaction allows these species to be visualized under a microscope. If the reaction is positive, the capsule becomes opaque and appears to enlarge. Photomicrograph of Streptococcus pneumoniae bacteria revealing capsular swelling using the Neufeld quellung test. Notice the two streptococci at the top of the photo that appear to have ...