Variable-number tandem repeat analysis and multilocus sequence typing data confirm the epidemiological changes observed with Staphylococcus aureus strains isolated from bloodstream infections

Abstract

Since 2000, our geographical region in France systematically surveys bloodstream infections (BSI) due to Staphylococcus aureus. This survey involves 39 health care institutions (HCIs) encompassing 6,888 short-stay beds and was performed during two 3-month periods during 2007 and 2008. The study periods of this survey identified 292 S. aureus isolates causing BSI. Extensive molecular characterization, including genotyping as well as toxin, agr, and staphylococcal cassette chromosome content determinations, allowed us to describe epidemiological evolution in comparison to that discussed in our previous study. Our main epidemiological observation shows that the incidence of BSI remained constant but that methicillin (meticillin)-resistant S. aureus strains with a wider variety of genetic backgrounds now harbor pyl, as has already been reported in different European countries. We noticed stable numbers of BSI episodes involving community-acquired methicillin-sensitive S. aureus (MSSA), whereas a drastic increase in the number of strains harboring the tst gene was recorded. The increase in the number of tst gene-harboring strains is related to known hospital-acquired MSSA isolates and appears related to epidemic episodes in specific HCIs. Monitoring the increase in prevalence of specific strains helps us understand where the standard precautions are not satisfactorily applied or do not efficiently prevent the spread of epidemic MSSA strains in these HCIs. The recent increases in incidence of these strains call for particular vigilance to avoid the spread of potentially virulent MSSA strains harboring the tst gene and for continuance of this strategy of BSI surveillance.

FIG. 1.

Map of France and the central region.

FIG. 2.

Characteristics of strains and BSI according to VNTR groups. The dendrogram was calculated using the MLVA data. On the basis of their MLVA fingerprints, the isolates segregated into different clusters, showing a distance limit of 0.1 for clonality and a distance limit of 0.2 for related isolates, as previously described (10). Results of other molecular tests, including agr and SCCmec typing as well as determination of toxin content, are also indicated. Based on previous determinations, the origins of isolates composing each cluster (either HA or CA strains) appear in the figure. NK, not known.

FIG. 3.

Characteristics of strains and BSI according to PFGE groups. The dendrogram was calculated using the relationships between pulsotypes determined by the unweighted-pair group method using arithmetic means and the Adanson pulsogrouping program (dissimilarity). Results of other molecular tests, including agr and SCCmec typing as well as determination of toxin content, are also indicated. Based on previous determinations, the origins of isolates composing each cluster (either HA or CA strains) appear in the figure. NK, not known.