Virtually all methicillin-resistant Staphylococcus aureus (MRSA) infections in the largest Portuguese teaching hospital are caused by two internationally spread multiresistant strains: the ‘Iberian’ and the ‘Brazilian’ clones of MRSA

Objective To determine the nature (clonal type and antibiotic resistance pattern) of methicillin-resistant Staphylococcus aureus (MRSA) strains recovered from the largest teaching hospital in Portugal and to detect temporal trends in clonal types during three consecutive surveillance periods in 1992-93, 1994-95 and 1996. Methods MRSA strains were characterized by chromosomal Sma I macrorestriction patterns using pulsed-field gel electrophoresis (PFGE) and by DNA fingerprints—applied to Cla I digests—capable of probing two specific areas of the staphylococcal chromosome: (1) the vicinity of the mecA gene, and (2) the attachment site(s) and copy number of transposon Tn 554 . The combination of these methods can generate ‘clonal types' useful for epidemiological tracking of MRSA strains. Results During the 1992-93 collection period, 65% of MRSA strains carried the mecA polymorph Tn 554 pattern E and PFGE pattern A (I::E::A)—a clonal type that was used to define the ‘Iberian MRSA', which is widely spread throughout southern Europe. The representation of this clone decreased to 42% in 1994-95 and to 20% in 1996. At the same time, a second multiresistant MRSA strain carrying mecA polymorph XI, Tn 554 type B and PFGE pattern B (XI::B::B)—a clonal type characteristic of the so-called ‘Brazilian MRSA'—increased from 5% in 1992-93 to 36% in 1994-95 and 29% in 1996. Conclusions Throughout the four years of surveillance, the Iberian and Brazilian MRSA types and their single subtype variants together have been responsible for the overwhelming majority (close to 90%) of all MRSA infections in the largest teaching hospital of Portugal. The mechanism of epidemicity of these two multiresistant international MRSA clones remains to be elucidated.