OUTBREAK OF EXTENDED-SPECTRUM BETA-LACTAMASE-PRODUCING SERRATIA-MARCESCENS IN AN INTENSIVE-CARE UNIT

Serratia marcescens has recently been identified as an important etiol ogical agent in nosocomial infections, and is considered to be an oppo rtunistic pathogen agent in immunosuppressed patients undergoing long periods of intensive care. Research carried out in 1991 and 1992 showe d that it was of epidemiological relevance in only 1-2% of clinical is olates at the Ospedale di Circolo, Varese, Italy. However, between 7 F ebruary and 11 October 1993, the incidence of cases attributable to S. marcescens had increased to 5%; 157 strains of Serratia marcescens we re isolated from clinical specimens of 43 patients admitted to an inte nsive care unit; these strains, characterized by epidemic spread, show ed the same pattern of multiresistance to antibiotics including monoba ctams and oxyimino-cephalosporins. During the same period 23 isolates were also recovered from 18 patients admitted to wards other than the intensive care unit; these strains, characterized by a wide range of a ntibiotic susceptibility, were also sensitive to beta-lactam antibioti cs with the exception of first generation cephalosporins. The producti on of extended-spectrum beta-lactamases (ES beta Ls) and their genetic determinism were studied. All the epidemic strains of S. marcescens r esistant to ceftazidime, cefotaxime, ceftriaxone and aztreonam produce d three different beta-lactamases with pI 5.4, 5.5 and 8.4 respectivel y. In contrast, non-epidemic strains produced only a beta-lactamase wi th pI 8.4. The beta-lactamase with pI 5.5 was plasmid-mediated, hydrol izing ceftazidime and aztreonam, showing it to be an ES beta L; while the beta-lactamase with pI 5.4, although plasmid-mediated, did not hyd rolize monobactams or oxyimino-cephalosporins. The beta-lactamase with pI 8.4 was found to be an inducible chromosomal enzyme capable of hyd rolizing cefotaxime and ceftriaxone. Electrophoresis of the plasmid DN A indicated the presence of a similar plasmid of approximate size 54 k b in the resistant epidemic strains; this was found to be conjugative and mediating resistance also to aminoglycosides. Our data indicate th at the plasmid-mediated production of ES beta Ls may contribute to the epidemic spread of Serratia marcescens in high-risk wards.