Tracing the Origin of an Outbreak of Methicillin-Resistant Staphylococcus aureus Infections in a Portuguese Hospital by Molecular Fingerprinting Methods

Seventy-six methicillin-resistant Staphylococcus aureus (MRSA) isolates were collected from July 1992 to May 1995 at a 400-bed district hospital in the northeast of Portugal. During the second half of the surveillance period, in July of 1994, an outbreak was detected in the orthopedic ward. Thirty-three (out of the 76) MRSA strains were recovered only in this ward during the outbreak period. All strains were characterized by a variety of genomic fingerprints. Hybridization of Cla I and Sma I restriction digests with the mee A - and Tn 554 -specific DNA probes was used to identify polymorphism and determine chromosomal location of these determinants, and pulsed-field gel electrophoretic analysis of Sma I digests was used to determine chromosomal backgrounds. All strains recovered during the outbreak in the orthopedic ward were found to belong to a single clone that carried the mecA polymorph I, Tn 554 type E in a macrorestriction background called H (clone I::E::H1), which was identified in 18 patients, and 5 health care personnel and from a fomite sample, and was traced to a single transfer patient admitted to the hospital at the beginning of the outbreak. The new clone I::E::H1 differed only in the macrorestriction profile from the MRSA clone previously dominant in this hospital, known as Iberian epidemic clone I::E::A, which has already been identified in several Spanish and Portuguese hospitals.