Comparison of genetic backgrounds of methicillin-resistant and -susceptible Staphylococcus aureus isolates from Portuguese hospitals and the community

Abstract

In order to understand the origins of the dominant methicillin-resistant Staphylococcus aureus (MRSA) clones in Portuguese hospitals, we compared the genetic backgrounds of nosocomial MRSA with methicillin-susceptible S. aureus (MSSA) isolates from the same hospitals (n=155) and from the community (n=157) where they were located. Pulsed-field gel electrophoresis, spa typing, multilocus sequence typing, and agr type analysis revealed that the genetic backgrounds correspondent to the dominant MRSA clones in Portuguese hospitals during the last 15 years (Iberian ST 247, Brazilian ST 239, and EMRSA-15 ST 22) were scarcely or not found among the present MSSA collection. The four major MSSA clones encountered (A-ST 30, B-ST 34, C-ST 5, and H-ST 45) correspond, or are very similar, to the background of other international MRSA pandemic clones, i.e., EMRSA-16, New York/Japan, Pediatric, and Berlin clones. However, with the exception of the Pediatric clone, none of these MRSA clones has been detected in Portugal. Our findings suggest the three major MRSA clones identified in Portuguese hospitals have not originated from the introduction of SCCmec into dominant MSSA backgrounds present in the Portuguese nosocomial or community environment but were probably imported from abroad. In contrast, the MRSA Pediatric clone might have originated in our country by the acquisition of SCCmec type IV into MSSA clone C. Furthermore, we provide evidence that the introduction of SCCmec into sensitive clones is most likely a relatively infrequent event that seems to depend not exclusively on the presence of a successful MSSA lineage.

FIG. 1.

PFGE analysis versus spa type analysis of MSSA strains. Distribution of the 312 MSSA isolates into 20 PFGE types is shown. The area of each circle is proportional to the number of isolates included in the PFGE type, and white and gray areas correspond to nosocomial and community isolates, respectively. Subtypes are indicated for each PFGE type as well as the corresponding ST. spa type data, including spa type, spa profile, and spa lineage, are shown for 171 MSSA strains. PFGE types A, B, C, J, H, and T (left side of the figure) correspond to 67% of the total MSSA isolates and belong to the five major MRSA CCs.

FIG. 1.

PFGE analysis versus spa type analysis of MSSA strains. Distribution of the 312 MSSA isolates into 20 PFGE types is shown. The area of each circle is proportional to the number of isolates included in the PFGE type, and white and gray areas correspond to nosocomial and community isolates, respectively. Subtypes are indicated for each PFGE type as well as the corresponding ST. spa type data, including spa type, spa profile, and spa lineage, are shown for 171 MSSA strains. PFGE types A, B, C, J, H, and T (left side of the figure) correspond to 67% of the total MSSA isolates and belong to the five major MRSA CCs.

FIG. 2.

Molecular characterization of MSSA strains and comparison with MRSA pandemic clones. Shown from left to right are (i) a dendrogram indicating the estimated relationships of PFGE types based on Bionumerics analysis, including representatives of seven international pandemic MRSA clones (in bold), and squares filled with the same color or pattern indicate MSSA and MRSA backgrounds showing a relatedness of over 70%; (ii) listing of isolates; (iii) PFGE type; (iv) spa lineage, profile, and type; (v) agr type; (vi) MLST ST and CC. ▴, CC for which no founder could be assigned.

FIG. 3.

Clonal type distribution. Numbers underlined represent the major clonal types in each collection or subcollection. Minor clones (clones harboring a maximum of 5% of all isolates) were classified as “other” clonal types and include STs 1, 9, 10, 12, 20, 22, 25, 97, 106, 121, 188, 573, 580, and 615.