Staphylococcus aureus, the most virulent of the many staphylococcal species, has demonstrated its versatility by remaining a major cause of morbidity and mortality despite the availability of numerous effective antistaphylococcal antibiotics. S. aureus is a pluripotent pathogen, causing disease through both toxin-mediated and non-toxin-mediated mechanisms. This organism is responsible for both nosocomial and community-based infections that range from relatively minor skin and soft tissue infections primarily to life-threatening systemic infections.
The “other” staphylococci, collectively designated coagulase-negative staphylococci (CoNS), are considerably less virulent than S. aureus but remain important pathogens in infections primarily associated with prosthetic devices.
Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram's stain (Fig. 135-1). These organisms are catalase-positive (unlike streptococcal species), nonmotile, aerobic, and facultatively anaerobic. They are capable of prolonged survival on environmental surfaces in varying conditions.
Gram's stain of S. aureus in a sputum sample with polymorphonuclear leukocytes. (From ASM MicrobeLibrary.org.© Pfizer, Inc.)
More than 30 staphylococcal species are pathogenic. A simple strategy for identification of the more clinically important species is outlined in Fig. 135-2. Automated diagnostic systems, kits for biochemical characterization, and DNA-based assays are available for species identification. With few exceptions, S. aureus is distinguished from other staphylococcal species by its production of coagulase, a surface enzyme that converts fibrinogen to fibrin. Latex kits designed to detect both protein A and clumping factor also distinguish S. aureus from other staphylococcal species. S. aureus ferments mannitol, is positive for protein A, and produces DNAse. On blood agar plates, S. aureus tends to form golden β-hemolytic colonies; in contrast, CoNS produce small white nonhemolytic colonies.
Biochemical characterization of staphylococci: algorithm of biochemical tests used to discriminate among the clinically important staphylococci. Additional tests are necessary to identify all of the different species.
Determining whether multiple staphylococcal isolates from different patients are the same or different is often relevant when there is concern that a nosocomial outbreak is due to a common point source (e.g., a contaminated medical instrument). Molecular typing methods, such as pulsed-field gel electrophoresis and sequence-based techniques [e.g., staphylococcal protein A (spa) typing], have increasingly been used for this purpose.
S. aureus is a part of the normal human flora; ∼25–50% of healthy persons may be persistently or transiently colonized. The rate of colonization is higher among insulin-dependent diabetics, HIV-infected patients, patients undergoing hemodialysis, and individuals with skin damage. The anterior nares are a frequent site of human colonization, although the skin (especially when damaged), vagina, axilla, perineum, and oropharynx may also be colonized. These colonization sites serve as a reservoir of strains for future infections, and persons colonized with S. aureus are at ...