Staphylococcal Infections

Study Notes

Staphylococci may be refractory to treatment due to biofilm formation.
S. lugdunensis is a virulent organism causing a disease spectrum similar to S. aureus, with phenotypic characteristics like hemolysis and clumping factor.
S. saprophyticus is typically nonpigmented, novobiocin-resistant, and nonhemolytic, causing urinary tract infections in young women.

Localized staphylococcal infections appear as pimples, hair follicle infections, or abscesses with intense, localized, painful inflammatory reactions that undergo central suppuration and heal quickly when pus is drained.
S. aureus infection can result from direct contamination of wounds, such as postoperative staphylococcal wound infections or infections after trauma (chronic osteomyelitis subsequent to an open fracture, meningitis after skull fracture).
Disseminated S. aureus infection can lead to endocarditis, acute hematogenous osteomyelitis, meningitis, or pulmonary infection.
Food poisoning caused by staphylococcal enterotoxin is characterized by a short incubation period (1-8 hours), violent nausea, vomiting, diarrhea, and rapid convalescence without fever.
Toxic shock syndrome is manifested by an abrupt onset of high fever, vomiting, diarrhea, myalgias, a scarlatiniform rash, and hypotension with cardiac and renal failure in severe cases.

Specimens for testing include surface swabs, pus or aspirate from an abscess, blood, endonasotracheal aspirate, expectorated sputum, or spinal fluid.
Gram-stained smears of pus or sputum show typical staphylococci as Gram-positive cocci in clusters.
Specimens planted on blood agar plates give rise to typical colonies in 18 hours at 37°C, but hemolysis and pigment production may not occur until several days later and are optimal at room temperature.
S. aureus ferments mannitol, but other staphylococci do not.

The catalase test involves placing a drop of 3% hydrogen peroxide solution on a slide with bacterial growth; the formation of bubbles indicates a positive test result.
The coagulase test involves mixing citrated rabbit or human plasma with an equal volume of broth culture or growth from colonies on agar, and incubating at 37°C; clot formation in 1-4 hours indicates a positive test result.

Broth microdilution or disk diffusion susceptibility testing should be done routinely on staphylococcal isolates from clinically significant infections.
Resistance to penicillin G can be predicted by a positive test result for β-lactamase; approximately 90% of S. aureus produce β-lactamase.
Resistance to nafcillin (and oxacillin and methicillin) occurs in about 65% of S. aureus and approximately 75% of S. epidermidis isolates.

Preventive measures aim to control reinfection and eliminate the carrier state in patients subject to recurrent infection, such as chronic furunculosis.
Clothes and bedding that may cause reinfection should be dry-cleaned or washed at a high temperature (70°C or higher) to destroy staphylococci.

Staphylococci produce catalase, which converts hydrogen peroxide into water and oxygen.
Coagulase binds to prothrombin, initiating fibrin polymerization, and may deposit fibrin on the surface of staphylococci, altering their ingestion by phagocytic cells.
Clumping factor is cell wall bound and responsible for adherence to fibrinogen and fibrin.
Other enzymes produced by staphylococci include hyaluronidase, staphylokinase, proteinases, lipases, and β-lactamase.
Hemolysins include α-hemolysin, β-toxin, δ-toxin, and γ-hemolysin, which act on eukaryotic cell membranes, human red blood cells, and white blood cells.
Exfoliative toxins A and B are heat-stable and heat-labile, respectively, and disrupt biologic membranes.