Epidemiological differentiation of Serratia marcescens: typing by bacteriocin production

A new method for comparing and differentiating strains of S. marcescens is described which has proved useful in determining the epidemiology of hospital infections. Strains were grown in Trypticase soy broth, and bacteriocin production was induced with mitomycin C for 5 hr. The bacteriocin lysates were then spotted onto nine standard indicator strains, which were chosen with the aid of computer analysis from the 118 indicators tested. After 24 hr at 37 C, zones of inhibition due to bacteriocins were recorded. One hundred twentynine strains were differentiated into 72 different bacteriocin production patterns, but 11 strains were nontypable. None of the 45 other strains of Enterobacteriaceae produced bacteriocins. Bacteriocin production was a stable epidemiological marker. Colonial mutants always had identical patterns, as did the same strain which has passed from patient to patient through cross-infection. The new technique does not require any specialized equipment and can be used in laboratories with limited budgets. The applications of the new method in cross-infection studies and as a supplement to serological typing are discussed.