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INTRODUCTION

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. It is responsible for numerous nosocomial and community-based infections that range from relatively minor skin and soft tissue infections (SSTIs) 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 select settings, such as infections that involve prosthetic devices.

MICROBIOLOGY AND TAXONOMY

Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram’s stain (Fig. 142-1). 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 the host range for others is quite narrow—i.e., limited to one or two closely related animals.

FIGURE 142-1

Gram’s stain of S. aureus in a sputum sample, illustrating staphylococcal clusters. (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 form small white nonhemolytic colonies. Newer methods such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) are increasingly being used for staphylococcal speciation in some clinical microbiology laboratories.

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 dramatically enhanced the ability to discriminate among clinical isolates.

S. AUREUS INFECTIONS

EPIDEMIOLOGY

S. aureus is both a commensal and an opportunistic pathogen. Approximately 20–40% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized with the same strain. The rate of colonization is elevated among ...

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