Epidemiology and typing of Staphylococcus aureus strains isolated from bloodstream infections

Abstract

We carried out an epidemiological study covering 2,365,067 patient days of hospitalization between 2000 and 2003. During this time, 413 Staphylococcus aureus bloodstream infections occurred. This corresponds to 15% of the 2,676 bloodstream infections observed during this period in the 31 hospitals in our region of France, which has 2.5 million inhabitants. The incidence of nosocomial S. aureus bloodstream infections was 0.11 per 1,000 days of hospitalization. The prevalence of methicillin-resistant S. aureus (MRSA) strains, of which 13% were nonmultiresistant MRSA (NORSA), was 33%, and this percentage was stable over the 4 years. In contrast, the prevalence of S. aureus strains susceptible to methicillin but resistant to quinolones or susceptible to methicillin but multiresistant to antibiotics (EMSSA strains) increased from 4% in 2000 to 23% in 2003. As previously reported, MRSA strains were mostly recovered from nosocomial bloodstream infections, whereas NORSA strains-generally considered to be responsible for community-acquired infections-were always isolated from nosocomial bloodstream infections. Pulsed-field gel electrophoresis (PFGE) analysis of 109 MRSA strains and 15 EMSSA strains demonstrated clonal diffusion of the three major French MRSA clones and revealed considerable genetic heterogeneity among EMSSA strains. Although no epidemiologically related NORSA strains clustered in particular PFGE groups, the distribution of MRSA strains isolated from bloodstream infections according to the portal of entry (vascular devices, pulmonary, and urinary) was not random for the major PFGE clones, suggesting that each MRSA lineage displays particular virulence features.

FIG. 1.

Schematic representation of PFGE restriction patterns with SmaI and the genetic relationships among 109 MRSA and 15 EMSSA strains. Div, division; Pg, pulsogroup; IVD, intravascular device; P, pulmonary; U, urinary tract; Dig, digestive portal of entry; SSI, surgical site infection; E, erythromycin; L. lincomycin; Pef, pefloxacin; Tet, tetracyclin; K, kanamycin; T, tobramycin; AF, fusidic acid; NK, not known. Antibiogroup 1 consists of strains resistant to tobramycin, kanamycin, and pefloxacin (Pef); antibiogroup 2 consists of strains resistant to tobramycin, kanamycin, erythromycin, lincomycin, and pefloxacin; antibiogroup 3 consists of strains resistant to gentamicin, tobramycin, kanamycin, erythromycin, lincomycin, and pefloxacin; antibiogroup 4 consists of strains resistant to tobramycin, kanamycin, erythromycin, and pefloxacin.

FIG. 2.

Multiplex PCR assay for typing of major SCCmec elements of the 12 NORSA strains. Lanes 1 to 12 show the SSCmec types of the 12 NORSA strains (Fig. 1). Lane C shows the SSCmec type IV of the PVL-producing MRSA strain that emerged in France in 2000 (15).