Extensive intra-hospital spread of a methicillin-resistant staphylococcal clone

Infection by antibiotic-resistant bacteria can pose serious complications to the therapy of cancer patients. The authors introduced DNA fingerprinting techniques for tracking methicillin-resistant Staphylococcus aureus (MRSA) clones recovered at a central cancer hospital of Lisbon (Instituto Português de Oncologia) with the purpose of making an inventory of the MRSA clones endemic during 1995, and compared them with the outbreak-related clones of 1993.

A small group (6 strains) of epidemiologically related MRSA isolated during a suspected outbreak in 1993 and all consecutive single-patient isolates of MRSA (34 strains) recovered between January and November of 1995 from infected patients and health care personnel were characterized using DNA probes and pulsed-field gel electrophoresis.

The six 1993 strains and more than half of all 1995 isolates, including those recovered from the health care personnel, showed DNA fingerprints characteristic of the "Iberian MRSA," a multiresistant clone widespread in Portuguese and Spanish hospitals. Four patients were infected by another MRSA clone previously seen only in hospitals in Brazil.

The epidemic Iberian clone was among the index cases involved with the MRSA outbreak in 1993, and this was found to be endemic in a follow-up survey conducted in 1995, colonizing health care personnel and spreading to most hospital wards. A few isolates of another epidemic clone, the Brazilian MRSA, also were detected among 1995 isolates. A better understanding of the mechanism(s) of epidemicity of these rapidly spreading clones is urgently needed.