Analysis of epidemiologic markers of nosocomial Serratia marcescens isolates with special reference to the Grimont biotyping system.

Seventy-one strains of Serratia marcescens obtained from hospitalized patients of the Instituto Nacional de la Nutricion in Mexico City and two Virginia hospitals (University of Virginia Medical Center and Norfolk General Hospital) were analyzed to find markers useful for the epidemiologic investigation of outbreaks with this organism. Biotyping with commercial microwell systems (API 20# system [Analytab Products, Plainview, N.Y.] and DMS Rapid NFT [DMS Laboratories, Inc., Flemington, N.J.]) was not useful. Biotyping with the system designed by Grimont (assimilation tests, pigment production, and the ability to reduce tetrathionate broth) was helpful to characterize all strains. Of the 37 Mexican strains, 36 belonged to biogroup A 5/8 and 32 were biotype A8b. The 34 strains from the Virginia hospitals were distributed among six different biogroups and 12 biotypes. Significant differences in antimicrobial susceptibility (50% MIC, microgram/ml) between Mexican and Virginia strains were seen with carbenicillin (256 versus 8), piperacillin (64 versus 4), amikacin (16 versus 2), gentamicin (2 versus 0.5), and tobramycin (16 versus 2). Some Mexican strains showed variability in the susceptibility to amikacin because they were low producers of 6'-N-acetyltransferase type I. The Mexican strains seemed to come from a hospital with cross-infection problems because most were isolated from urine, were multiresistant, and more nonpigmented; in contrast, the strains isolated at University of Virginia Medical Center represent the experience of a hospital with scattered S. marcescens infections. The Grimont biotyping scheme is a useful epidemiologic tool for the clinical microbiologist.