Staphylococcus aureus: The Persistent Pathogen

1Queen Elizabeth II Health Sciences Centre and Dalhousie University, Halifax, Nova Scotia; 2Departments of Pathology and Laboratory Medicine, Medicine, and Microbiology and Infectious Diseases, University of Calgary, Calgary, Alberta Correspondence: Dr Lynn Johnston, Department of Medicine, Room 5014 ACC, Queen Elizabeth II Health Sciences Centre, 1278 Tower Road, Halifax, Nova Scotia B3H 2Y9. Telephone 902-473-5553, fax 902-473-7394, e-mail ljohnsto@dal.ca I the last issue of this journal, the recent emergence of community-acquired methicillin-resistant Staphylococcus aureus (MRSA) was reviewed (1). Even with all of the attention paid to the increasing prevalence of methicillin resistance, it is instructive to reflect on the importance of S aureus as a pathogen, regardless of antimicrobial susceptibility, and on the management of S aureus bacteremia. Laupland et al (2) recently examined the epidemiology of invasive S aureus (ISA) infections in a population-based surveillance study within the Calgary Health Region from May 1999 to May 2000. The incidence of ISA infection was 28.4 cases/100,000 persons, with an incidence rate for bacteremia of 17.9 cases/100,000 persons. Of the invasive infections, 46% were nosocomial, and 24% of community-acquired infections were health care-related. Only seven of the 264 patients were infected with MRSA. Patient groups at risk for ISA were those undergoing hemodialysis or peritoneal dialysis; diabetics; injection drug users; alcoholics, and those with HIV; heart disease; chronic obstructive pulmonary disease; stroke; cancer and solid organ transplants. These groups may simply reflect patients with high intensity of exposure to the health care system and instrumentation. The case fatality rate was 19%. Independent risk factors for death in multivariable analysis were bacteremia, respiratory focus of infection, receipt of empiric antimicrobials within 8 h, older age and hypotension. The authors speculated that receipt of empiric antimicrobials within 8 h was a marker for a patient with a greater severity of illness and an inherent higher risk for death. Similar findings were reported by Morin and Hadler (3). In a 1998 population-based study of S aureus bacteremia in Connecticut, the incidence rate was 17 cases/100,000 persons; just slightly lower than the pneumococcal bacteremia rate of 21.2 cases/100,000 persons. Over half (52%) of the 402 cases were hospital-acquired and 62% of the community-acquired cases had some type of contact with the health care system in the previous year. In the remaining community-acquired cases, 85% of the patients had an underlying medical condition. The case fatality rate was 11%, lower than that reported in most studies. Evidence suggests that the rates of nosocomial and communityacquired S aureus bacteremia have risen over time, no doubt related to the increasing complexity of patient problems and the invasive devices used in providing care. In a 12-year study, staphylococci were the leading pathogens of nosocomial bacteremia at the University of Iowa Hospitals and Clinics (4). In that setting, S aureus was one of four pathogens accounting for the increase in overall bloodstream infection rate from 6.7/1000 discharges in 1980 to 18.4/1000 discharges in 1992. A hospital-based study was conducted in Atlanta to compare bloodstream infections due to S aureus during 1980 to 1983 and 1990 to 1993 (5). Steinberg et al (5) found that the rate of nosocomial S aureus bacteremia and community-acquired bacteremia increased from 0.75 cases/1000 discharges to 2.80 cases/1000 discharges and from 0.84 cases/1000 discharges to 2.43 cases/1000 discharges, respectively, between the two time frames. Additional observations from this study were an eight-fold increase in the number of nosocomial device-related infections (50% of bacteremias during 1990 to 1993) and the increasing role of devices in community-acquired bacteremias (0% in 1980 to 1983 and 22% in 1990 to 1993). Sources of community-acquired device-related bacteremias during 1990 to 1993 were implanted ports (49%), hemodialysis catheters (33%), tunneled cuffed catheters (9%) and peripheral lines (5%) (5). A number of studies have examined the clinical features of S aureus bacteremia (SAB). In a population-based study designed to evaluate outcomes of SAB in Denmark, Jensen et al (6) described the epidemiology of SAB and contrasted the features of hospitaland community-acquired cases. The incidence rate was 1.18 cases/1000 admissions, of which 53% (147 cases) were hospital-acquired and 47% (131 cases) were community-acquired. Patients with community-acquired SAB were older, more likely to be injection drug users, and more likely to have skin or an unknown focus, endocarditis and osteomyelitis. The overall mortality rate was 34% and trended toward being higher in patients with community-acquired SAB. The mortality rate was higher in patients with septic shock and lower in hemodialysis patients, but not associated with the type of primary or secondary focus or presence or absence of secondary infection. In a seven-year study of hospitalized patients with SAB in Switzerland during the 1980s, 200 of 281 (71%) cases were hospital-acquired (7). Patients with hospital-acquired SAB were more likely to have an intravascular device, surgical wound, previous hospitalization, alcohol abuse, trauma, an apparent primary site of infection, and a removable focus of infection than patients with community-acquired SAB. The overall mortality was 33.6%, with no significant difference between hospitaland community-acquired cases. However, a higher mortality rate was observed in patients with complicated bacteremia, primary bacteremia, or endocarditis. In a comparison of SAB