Infections due to Staphylococci

General Properties

Staphyloccus species are usually round in appearance, are gram positive organisms and do not have endospores. They usually grow in clusters resembling a grape bunch hence the derivative of the name – staphyle. The organisms are facultatively anaerobic meaning they can either be aerobes or anaerobes. They grow between 18-40ºC in a wide variety of media. 16 of the 32 species colonise humans.

Classification

Staphlycocci are classified as a subcategory of Micrococcaceae. The other subcategories are: Micrococcus, Stomatococcus mucilaginosis.

The most medically significant variants of the Staphylcoccus species is: aureus, epidermidis & saprophyticus (discussed in tutorial).

Staph. Aureus (Murray 3^rd Ed. 175)

The colonies of Staph aureus are usually golden in colour, due to the carotenoid pigments formed during the growth. It is also the only species that produces the enzyme coagulase, and is also clumping factor positive (i.e: this is bound coagulase, binds fibrinogen, converts to insoluble fibrin – this causes the aggregation process).

Carriage

The crux of the matter is – this organism is bloody common in the general population, and is more common in the hospitalised population. Organism is most found in one’s genital areas.

Virulence Factors (Murray 3^rd Ed. Pg 175)

This organism has numerous virulence factors which can be broken up into broad categories. You do not need to know each and individual toxins, but a categorical view will be presented a little later.

Capsule: inhibits chemotaxis and phagocytosis, inhibits proliferation of mononuclear cells. Facilitates adherence to catheters and synthetic material.

Coagulase: See above for the function of this enzyme.

Clumping factor: See above for the function of this substance

Protein A: binds to Fc portion of IgG, therefore prevents antibody action

Lipase: hydrolise lipids, an important function for the survival of these organisms in the sebaceous areas of infection

Teichoic acids: mediate the attachment of bacteria to mucosal surfaces

Toxins

Cytolytic: some lyse red blood cells, therefore killing them – or can also lyse neutrophils – therefore releasing toxic substance into surrounding tissues.

Exfoliative & Enterotoxin: bind to class II MHC on macrophages, leads to proliferation of T cells, release of cytokines results ==> systemic effects of disease arises from this process.

Category: Microbiology Notes