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The biochemical, serological, and epidemiological characteristics of 95 strains of Serratia marcescens isolated at the Boston City Hospital were examined. Several strains were shown to be endemic, and the majority of isolates were cultured from urine or respiratory secretions. Serratia species were highly resistant to polymyxin B and the cephalosporins, and various proportions were also resistant to other antibiotics including kanamycin, but all of the isolates were sensitive to gentamicin. The appearance of resistance to kanamycin and nalidixic acid among endemic strains was demonstrated. The nosocomial nature of Serratia infections, particularly those involving the urinary tract, was confirmed. Many clinical bacteriology laboratories currently fail to identify the nonpigmented strains.

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The in vitro antimicrobial activity of tobramycin and gentamicin was compared against 362 (3,5,6,10,13,14,18 RESULTSIn vitro susceptibility of various microorganisms to tobramycin and gentamicin. The relative susceptibility of various microorganisms to tobramycin is shown in Fig. 1. S. aureus was the most susceptible of all of the bacteria tested; the MIC for all strains was less than 0.075 ,ug/ml. The Escherichia coli, Pseudomonas, and Klebsiella-Enterobacter strains, and the indole-positive strains of Proteus except P. rettgeri, had comparable susceptibility: the MIC for 85 to 95%, of all of these organisms