Staphylococci appear as clusters (think "staff"). Most are penicillin G resistant. Three major pathogenic species are Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus saprophyticus. Methods of differentiation from streptococci
- Gram staining to reveal physical traits.
- Catalase test to show enzyme catalase, which is present in all staphylococci.
- Culture to show hemolysis ability. Most staphylococci are either beta-hemolytic or not hemolytic. Staphylococcus aureus leaves a gold pigment on sheep blood agar. Remember alpha-hemolytic bacteria leave a green pigment on blood agar.
Of the pathogenic species, only S. aureus is coagulase-positive
Most S. aureus produces the enzyme coagulase, which clots the blood. Fibrin formation around the bacterium can protect it from phagocytosis.
Staphylococcus aureus
- Defensive proteins:
- Protein A binds IgG at the Fc region (the tail rather than the arms), preventing opsonization.
- Coagulase.
- Hemolysins alpha, beta, gamma and delta, which destroy RBCs, neutrophils, macrophages and platelets.
- Leukocidins destroys leukocytes.
- Penicillinase inactivates penicillin.
- Novel penicillin binding protein (transpeptidase) forms the cell wall, and is inhibited by penicillin.
- Tissue "tunneling" proteins:
- Hyaluronidase ("Spreading Factor") breaks down proteoglycans in connective tissue.
- Staphylokinase breaks down fibrin clots.
- Lipase degrades fats and oils, allowing colonization in sebaceous glands.
- Protease destroys tissue proteins.
- Exotoxins:
- Exfoliatin causes the detachment in the epidermal layer, leaves blisters.
- Enterotoxin causes food poisoning, vomiting and diarrhea.
- Toxic Shock Syndrome toxin is a superantigen that binds to MHC II of APCs, causing a massive T cell response (comparable to but deadlier than pyrogenic toxin).
- Diseases:
- Gastroenteritis caused by eating food that contained bacteria and exotoxins.
- Toxic shock.
- Staphylococcal scalded skin syndrome from exfoliatin.
- Severe and destructive pneumonia, but rare.
- Meningitis, cerebritis.
- Osteomyelitis, usually affecting boys under 12.
- Acute endocarditis from infection of heart valve. Bacterial vegetation may spread to the brain or lung based on the valve.
- Septic arthritis from infection of the synovial membrane in joints.
- Skin infections, including impetigo, cellulitis, furuncles or carbuncles, wound infections and blood/catheter infections leading to sepsis.
Staphylococcus epidermidis
Part of normal bacterial flora and lives widely on the skin without causing disease. However, it causes infections if it enters wounds on medical devices such as artificial hip joints or heart valves, or in the bloodstream via catheters.
Staphylococcus saprophyticus
Causes urinary tract infection in sexually active young women.
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