Staphylococcus aureus, the most virulent of the many staphylococcal species, has demonstrated its versatility by remaining a major cause of morbidity and mortality worldwide despite the availability of numerous effective antistaphylococcal antibiotics. S. aureus is a pluripotent pathogen, causing disease through both toxin- and non-toxin-mediated mechanisms. This organism is responsible for numerous nosocomial and community-based infections that range from relatively minor skin and soft tissue infections 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 that are primarily associated with prosthetic devices.
MICROBIOLOGY AND TAXONOMY
Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram’s stain (Fig. 172-1). These organisms (~1 μm in diameter) are catalase-positive (unlike streptococcal species), nonmotile, aerobic, and facultatively anaerobic. They are capable of prolonged survival on environmental surfaces under varying conditions. Some species have a relatively broad host range, including mammals and birds, whereas for others the host range is quite narrow—i.e., limited to one or two closely related animals.
Gram’s stain of S. aureus in a sputum sample. (From ASM MicrobeLibrary.org.© Pfizer, Inc.)
More than 30 staphylococcal species are pathogenic. Identification of the more clinically important species has generally relied on a series of biochemical tests. 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 that detect both protein A and clumping factor also distinguish S. aureus from most 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. Increasingly, sequence-based methods (e.g., 16S rRNA) are being used to identify different staphylococcal 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. More recently, whole-genome sequencing has enhanced the ability to discriminate among clinical isolates.
S. aureus is both a commensal and an opportunistic pathogen. Approximately 30% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized. The rate of colonization is elevated among insulin-dependent diabetics, HIV-infected patients, patients undergoing hemodialysis, injection drug users, and individuals with skin damage. The ...