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 greater risk of subsequent infection than are noncolonized individuals.
Some diseases increase the risk of S. aureus infection; diabetes, for example, combines an increased rate of S. aureus colonization and the use of injectable insulin with the possibility of impaired leukocyte function. Individuals with congenital or acquired qualitative or quantitative defects of polymorphonuclear leukocytes (PMNs) are at increased risk of S. aureus infections; this group includes neutropenic patients (e.g., those receiving chemotherapeutic agents), those with chronic granulomatous disease, and those with Job's or Chédiak-Higashi syndrome. Other groups at risk include individuals with skin abnormalities and those with prosthetic devices.
Overall, S. aureus is a leading cause of nosocomial infections. It is the most common cause of surgical wound infections and is second only to CoNS as a cause of primary bacteremia. Increasingly, nosocomial isolates are resistant to multiple antibiotics. In the community, S. aureus remains an important cause of skin and soft tissue infections, respiratory infections, and (among injection drug users) infective endocarditis. The increasing prevalence of home infusion therapy is another cause of community-acquired staphylococcal infections.
Most individuals who develop S. aureus infections are infected with their own colonizing strains. However, S. aureus may also be acquired from other people or from environmental exposures. Transmission most frequently results from transient colonization of the hands of hospital personnel, who then transfer strains from one patient to another. Spread of staphylococci in aerosols of respiratory or nasal secretions from heavily colonized individuals has also been reported.
In the past 10 years, numerous outbreaks of community-based infection caused by methicillin-resistant S. aureus (MRSA) in individuals with no prior medical exposure have been reported. These outbreaks have taken place in both rural and urban settings in widely separated regions throughout the world. The reports document a dramatic change in the epidemiology of MRSA infections. The outbreaks have occurred among such diverse groups as children, prisoners, athletes, Native Americans, and drug users. Risk factors common to these outbreaks include poor hygienic conditions, close contact, contaminated material, and damaged skin. The community-associated infections have been caused by a limited number of MRSA strains. In the United States, strain USA300 (defined by pulsed-field gel electrophoresis) has been the predominant clone. While the majority of infections caused by this community-based clone of MRSA have involved the skin and soft tissue, 5–10% have been invasive. USA300 is also responsible for an increasing number of nosocomial infections. Of concern has been the apparent capacity of community-acquired MRSA (CA-MRSA) strains to cause serious disease in immunocompetent individuals.
S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at sites of both local and metastatic infections. This classic pathologic response to S. aureus defines the framework within which the infection will progress. The bacteria elicit an inflammatory response characterized by an initial intense infiltration of PMNs and a subsequent infiltration ...