Gram Positive Cocci Flashcards
What are the 3 broad classes of Gram positive cocci?
- Staphylococcus 2. Streptococcus 3. Enterococcus
Describe the catalase test and its purpose.
Catalase test is used to differentiate staphylococcus from streptococcus and enterococcus. Staphylococcus possesses catalase, coming up positive in the test.
Describe the coagulase test
Staph. aureus produces coagulase, which converts fibrinogen to fibrin. All other staph do not. Positive in the coagulase test = staph. aureus. Coagulase negative staphylococci come up negative in the test.
Staphylococcus aureus is carried by what % of people. and where?
Carried by 20% - 40% of the population in the anterior part of the nose.
Describe staphylococcus aureus infection
Typically pyogenic
- Furuncle (boil)
- Carbuncle (collection of furuncles)
- Impetigo (lesions around mouth and nose)
- Bone and joint infection
- Bacteria and septicaemia
Can progress to endocarditis and organ abcesses
What is staphyloccocus saprophyticus and what does it do?
Coagulase negative staphylococcus, the 2nd most common cause of UTIs in women.
What is staphylococcus epidermis and what does it do?
S. epidermis is normally commensal flora.
Has slime that protects against immune system and aids in virulence.
What is biofilm, and how is it relevant to disease?
Coagulase negative staphyloccic produce biofilm, particularly on infection of foreign devices such as prosthetic heart valves, or orthopaedic joints.
Why is it hard to tell if coagulase negative staph are the pathogenic culprit?
Coagulase negative staphylococci are usually commensal flora. It is often difficult to tell whether they are responsible for disease, or have just contaminated the specimen.
Describe the morphology of staph aureus.
Arranged in grape-like clusters.
Describe the general morphology of streptococci.
Arranged in chains or diplococci
How are streptococci classed?
According to their haemolytic properties.
Beta-haemolytic = full digestion of blood cells, causing blood agar to turn transparent. The virulent species are beta.
Alpha-haemolytic = partial digestion of blood cells, leaving blood agar a mild opaque color. Most alpha species are commensal flora, and are not usually virulent.
How are beta-haemolytic streptococci further divided into classes?
Give an example for the two main classes.
Lancefield grouping. Based on differences in cell wall carbohydrates.
Lancefield group A = streptococcus pyogenes
Lancefield group B = streptococcus agalactiae
For streptococcus pyogenes, describe the Lancefield group, haemolytic class and types of infection it causes.
Lancefield group A
Beta-haemolytic.
Causes suppurative and non-suppurative infections. Primary infection can progress to suppurative complications.
Describe the haemolytic class of streptococcus agalactiae and the infections it causes.
Beta-haemolytic.
Typically colonisers of the gastrointestinal tract and vagina.
Causes post-partum maternal sepsis and neo-natal infections.
