International clones of methicillin-resistant Staphylococcus aureus in two hospitals in Miami, Florida

Abstract

A total of 202 methicillin-resistant Staphylococcus aureus (MRSA) single-patient isolates recovered between January and June 1998 in two hospitals in Miami, Florida, were characterized by a combination of several molecular typing techniques: multilocus sequence typing, spaA typing, pulsed-field gel electrophoresis, and determination of the structure of the SCCmec element. The overwhelming majority of the isolates-187 of 202, or 93%-belonged to one of three internationally spread epidemic clones which were identified on the basis of their multilocus sequence type (ST) as E-MRSA-16 (ST36), the New York clone V (ST8), and the New York/Japan clone (ST5; SCCmec II) and its single- and double-locus variants. The rest of the isolates (15 of 202, or 7%) were more genetically diverse and were each recovered from a few patients only. Of the 23 MRSA strains isolated from confirmed human immunodeficiency virus-positive patients, as many as 17 (or 70%) belonged to a single ST8 clone carrying SCCmec type IV. The data provide further evidence for the conclusion of earlier studies that most MRSA disease in hospitals is caused by relatively few pandemic clones.

FIG. 1.

Molecular types of MRSA isolates recovered in two Miami hospitals. MRSA isolates recovered in hospitals C and D were characterized by MLST, and the frequencies of the various sequence types (ST) are plotted as percentages of the total number of MRSA isolates. The number of strains with the indicated genotype at the indicated hospital is given above each bar.

FIG. 2.

(Top) PFGE patterns of the various sequence types of MRSA isolates recovered in the two Miami hospitals. Chromosomal DNAs were isolated, restricted by SmaI, and separated by PFGE. (Bottom) The various strains used as sources of the chromosomal DNA are identified by their MLST, sequence type (ST), and spaA type. The hospital source of each isolate is indicated by the letter C or D in the strain designation; strains NTCT8325 and BK2464 are included for comparison.

FIG. 3.

Recovery of the three major MRSA clonal types and sporadic MRSA sequence types (STs) from various clinical sites of disease in the two Miami hospitals. The frequencies of association of the MRSA clonal types ST36 (light shading), ST8 (solid), and ST5 (105, 231, 83) (dark shading) and of sporadic STs (open) with the different clinical sites of disease are shown. Resp, respiratory tract.