An epidemiological study of Serratia marcescens isolates from nosocomial infections by enzyme electrophoresis

Serratia marcescens isolates from 164 patients with suspected nosocomial infection in several hospitals in the greater Paris region were investigated by analysis of the electrophoretically demonstrable allelic variations of gene loci coding for five esterases and five other enzymes. All the loci were polymorphic and the mean number of alleles per locus was 6.1. A total of 72 distinctive electrophoretic types (ETs) representing multilocus genotypes was distinguished. The isolates were divided into two groups according to their resistance to antibiotics: 82 multiresistant isolates (MRI) and 82 relatively susceptible isolates (RSI). Seventy-two MRI (88%) were in four genetically related ETs: ET1, ET2, ET8 and ET9; ET1 was found in 48 isolates, whereas the remaining MRI were in 10 ETs, and all RSI in 61 ETs. Three ETs contained both MRI and RSI. The mean coefficients of genetic diversity for the 10 enzyme loci among ETs and isolates were smaller for MRI than for RSI, while the modal ET of MRI resembled that of RSI. The epidemiological significance of isolates varied according to their ET. Thus, isolates belonging to ET1, ET2 and ET8 were responsible for outbreaks or for sporadic infections, whereas isolates of other ETs were responsible for only sporadic infections. The temporal distribution of ET1 isolates among hospitals identified seven outbreaks in seven clinical departments.