Staphylococci
They’re virulent organisms associated with a wide range of human infections, from boils to sepsis.
Morphology
Staphylococci are Gram-positive cocci arranged in grape-like clusters. They’re typically non-capsulated, except for a few strains that have a microcapsule (a thin polysaccharide layer).
Numerous, spherical (cocci), Gram-positive, Staphylococcus aureus bacteria.
Classification
Staphylococci are classified based on the coagulase test:
- Coagulase-positive: Staphylococcus aureus
- Coagulase-negative: Staphylococcus epidermidis and Staphylococcus saprophyticus
All staphylococci are catalase positive.
Coagulase test
Coagulase is an enzyme that converts soluble fibrinogen in plasma into insoluble fibrin.
- How the test is done: A smooth suspension of staphylococci is mixed with plasma.
- Positive result:
- Slide test: visible clumping
- Tube test: clot formation
Catalase test
Catalase breaks down hydrogen peroxide into water and oxygen. This helps bacteria survive inside human macrophages.
- How the test is done: Mix 3% H2O2 with a bacterial inoculum (growth from culture).
- Positive result: brisk bubbling
- Negative result: no bubbling
Human Pathology
S. aureus causes many pyogenic infections, including boils, folliculitis, abscess, cellulitis, septic arthritis, osteomyelitis, impetigo, and sepsis. It also causes toxin-mediated diseases, including food poisoning (enterotoxin), toxic shock syndrome (TSST toxin), and scalded skin syndrome (exfoliatin).
Enterotoxin mediated food poisoning
Symptoms such as nausea, vomiting, abdominal cramps, and diarrhea begin within 30 minutes to 8 hours after ingestion of contaminated food. Commonly implicated foods include sliced meats, pastries, sandwiches, and puddings.
This illness is due to preformed enterotoxin in contaminated food, so antibiotics don’t help.
Toxic shock syndrome
This can present like septicemia, with fever, shock, hypotension, a desquamative reddish macular rash, and end-organ failure. It’s due to TSST toxin, which acts as a superantigen.
Patients often have a history of tampon use, recent childbirth, or staphylococcal infections such as infected burns or wounds.
Scalded skin syndrome
This is mostly seen in young children. It’s characterized by fever, blisters, and skin peeling with a positive Nikolsky sign (skin peels off with gentle pressure). It’s caused by the toxin exfoliatin.
S. epidermidis causes device- or implant-associated infections, including intravenous catheter-associated sepsis, prosthetic heart valve endocarditis, intraperitoneal catheter-associated peritonitis, and prosthetic joint-associated arthritis. This is attributed to glycocalyx production by S. epidermidis.
S. saprophyticus causes urinary tract infections in young sexually active women.
Antibiotics for staphylococci
Beta-lactamase-producing S. aureus can be treated with oxacillin, nafcillin, cephalosporins, amoxicillin plus clavulanic acid, and vancomycin.
MRSA is treated with vancomycin or daptomycin. For minor or skin MRSA infections, treat with trimethoprim plus sulfamethoxazole or clindamycin.
VRSA strains are treated with daptomycin or quinupristin dalfopristin.
S. epidermidis infections are treated with vancomycin.
S. saprophyticus infections are treated with quinolones or trimethoprim-sulfamethoxazole.
Laboratory diagnosis of staphylococcal infections
Important points
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Always do Gram stain and culture of pus from infected sites.
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In toxic shock syndrome, blood cultures may be negative, but cultures from the source of infection will be positive.
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Bacteriophage typing can be used for epidemiological purposes to trace the source of an outbreak.