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[Clinical epidemiology of an outbreak of nosocomial infection caused by Staphylococcus aureus resistant to methicillin and aminoglycosides: efficacy of control measures. Comité de Control de Infecciones].

BACKGROUND
The appearance of outbreaks of nosocomial infections due to methicillin resistant Staphylococcus aureus (MRSA) is a current problem in Spain. The clinical and molecular epidemiology of these outbreaks as well as the efficacy of their control measures are a matter of controversy.


METHODS
An outbreak of MRSA nosocomial infections in the Hospital Clínic i Provincial of Barcelona, a 953-bed University Hospital, with a total of 347 cases from September 1989 to October 1991 is described. The control measures used include prospective and continued surveillance of all MRSA isolations, identification of the reservoir, use of different types of isolation and control of nasal carriers among health care workers. The MRSA strains isolated were studied by standard microbiological procedures, phage-typing and extrachromosomal (plasmid) DNA analysis by means of restriction endonuclease analysis (REAP).


RESULTS
From the Intensive Care Units, the outbreak extended to the medical and surgical wards. Seventy-one percent of the cases corresponded to infected patients and 29% to asymptomatic carrier patients. The MRSA strain responsible of the outbreak had a notable antibiotic multiple-resistance pattern. The studies performed showed that most of the strains belonged to phage group III and were lysed by phage 77. Plasmid DNA analysis showed that 95% of the strains isolated had a unique homogeneous profile. Despite the different control measures used, the MRSA infection has acquired a medium level endemic rate in the Hospital Clínic i Provincial.


CONCLUSIONS
The introduction and spread of methicillin-resistant MRSA in a teaching hospital with more than 500-bed may be rapid and affecting large number of patients. Effective control measures carries multiple problems, which must be addressed with the collaboration of all hospital employees. The molecular typing techniques used (REAP) further identified that the outbreak is due to one single MRSA strain, with an epidemiologic behavior identical to the one showed by the epidemic strains previously described in the United Kingdom and Australia.