Patient factors contributing to the emergence of gentamicin-resistant Serratia marcescens.

One hundred forty nosocomial Serratia marcescens infections (including 76 cases of bacteremia) were identified by prospective surveillance from 1975 through 1977 and retrospective chart review from 1968 through 1974. Thirty-four cases (24 per cent) involved gentamicin-resistant strains. All gentamicin-resistant strains appeared after 1974. Ninety per cent of the patients had undergone surgery, and 88 per cent had received prior antimicrobial therapy. The emergence of gentamicin-resistant S. marcescens paralleled the increase in usage of gentamicin. Prior use of gentamicin for more than two days in an individual patient was a significant risk factor (P = 0.0002) for being infected with a Serratia that was gentamicin-resistant. Other factors which separated gentamicin-resistant Serratia infections from gentamicin-sensitive Serratia infections were (1) urinary site of infection (P = 0.0005), (2) urinary catheter (P = 0.002), (3) endotracheal tube or tracheotomy (P = 0.03) and (4) increasing duration of hospitalization (P less than 0.05). Thirty-three of 34 (97 per cent) patients with gentamicin-resistant strains had urinary catheters. Specific measures to control infection were effective in decreasing the incidence of infections caused by gentamicin-resistant Serratia.