Molecular and epidemiological evidence for spread of multiresistant methicillin-susceptible Staphylococcus aureus strains in hospitals

Abstract

The excision of the staphylococcal chromosomal cassette mec (SCCmec) from methicillin-resistant Staphylococcus aureus (MRSA) strains results in methicillin-susceptible S. aureus (MSSA) strains. In order to determine the proportion and diversity of multidrug-resistant MSSA (MR-MSSA) strains derived from MRSA strains, 247 mecA-negative isolates recovered in 60 French hospitals between 2002 and 2004 were characterized. The spa types of all strains were determined, and a subset of the strains (n = 30) was further genotyped by multilocus sequence typing. The IDI-MRSA assay was used to test the isolates for the presence of the SCCmec element, which was detected in 68% of all isolates analyzed. Molecular analysis of the samples suggested that 92% of the MR-MSSA isolates were derived from MRSA clones of diverse genetic backgrounds, of which the clone of sequence type 8 and SCCmec type IV(A) accounted for most of the samples. High variations in incidence data and differences in the molecular characteristics of the isolates from one hospital to another indicate that the emergence of MR-MSSA resulted from independent SCCmec excisions from epidemic MRSA isolates, as well as the diffusion of methicillin-susceptible strains after the loss of SCCmec. MR-MSSA could constitute a useful model for the study of the respective genetic and environmental factors involved in the dissemination of S. aureus in hospitals.

FIG. 1.

(A) Schematic representation of SCCmec types I to IV (adapted from reference with permission) with the positions of the primers used in the IDI-MRSA assay (black arrows) and primer Xsau325 in orfX (gray arrows) (for more details, see reference 22). (B) Representation of the structure HVR::IS431::pUB110::IS431::dcs linked to orfX in SCCmec types I_A, II, and IV_A, with the relative positions of the primers used for the characterization of the SCCmec elements in two J-8 MR-MSSA isolates shown.

FIG. 2.

Delineation of the regions, the localization of the hospitals, and the distributions of the spa lineages for 215 MR-MSSA isolates isolated in continental France in 2004.

FIG. 3.

(A) PFGE profiles for eight pairs of MRSA and MR-MSSA strains isolated from the same patients (lane pairs 1 to 8, respectively). Except for the samples from patient 5, the restriction profiles for both isolates originating from each patient are similar but not identical: the MSSA strain loses a DNA fragment at about the 200-kb band, which carries SCCmec in ST8. (B) Four pairs of related MR-MSSA and MRSA isolates after PFGE, Southern blotting, and hybridization with a digoxigenin-labeled mecA probe. The probe hybridizes with the 200-kb band in MRSA but not with bands of smaller sizes in MR-MSSA. Asterisks, lanes with MR-MSSA strains.

FIG. 4.

Distribution of P_S/R ratio values for 21 hospitals. The dashed-dotted line corresponds to the median.