REPEATED EPIDEMICS CAUSED BY EXTENDED-SPECTRUM BETA-LACTAMASE-PRODUCING SERRATIA-MARCESCENS STRAINS

An outbreak of Serratia marcescens involving 42 patients admitted to t he general intensive care unit of the Hospital of Varese, Italy, occur red from March 1994 to August 1995. The causative strains were resista nt to oxyimino-cephalosporins and monobactams due to their production of an extended-spectrum beta-lactamase. Another outbreak caused by Ser ratia marcescens strains had occurred in the same unit a few months ea rlier, from February to October 1993, with the strains involved produc ing a novel TEM-derived extended-spectrum beta-lactamase. In order to verify whet-her there were any relationships between isolates from the two epidemics, the strains and their enzymes were characterized. Bioc hemical data and gene amplification experiments showed that the isolat es of the second outbreak harbored a non-conjugative plasmid of approx imately 48kb, codifying for the production of an SHV-derived extended- spectrum beta-lactamase with pI 8.2. Restriction fragment length polym orphism analysis of total genomic DNA by pulsed-field gel electrophore sis of Serratia marcescens isolates unambiguously identified two diffe rent bacterial clones responsible for the two epidemics. Epidemiologic al and microbiological investigations demonstrated the long persistenc e of Serratia marcescens strains and their circulation in other hospit al wards, thus suggesting their possible role as a long-term reservoir for further epidemic spread.