Rate of carriage of Serratia marcescens in patients with and without evidence of infection

The epidemiology of Serratia marcescens is poorly understood. We designed a study to investigate carriage sites of the organism, and possible modes of transmission of infection. Using Sorbitol-MacConkey agar with colistin 200 IU/ml and MacConkey agar with a 10 mug colistin disc we performed cultures from various sites in patients already infected with S. marcescens. Over t he same period of time we also investigated all patients in the intensive c are unit (ICU) for colonization with the agent. Environmental screening was performed in the ICU only. Of 37 infected patients, 65% demonstrated carri age at a second site and 43% at multiple sites. Throat carriage was found i n 59%, faecal carriage in 42%, nasal carriage in 31% and urinary carriage i n 22%. Carriage over several weeks was found in 22%. Of 40 ICU patients, 10 % demonstrated nasal and/or throat carriage. Environmental screening yielde d 4 isolates. All ICU patient strains and a strain from the ICU bedpan mace rator were O14:K14 with similar random amplified polymorphic DNA types. The se results show that patients with S. marcescens infection are likely to ca rry the organism at multiple sites and that carriage may be prolonged. A si gnificant level of carriage was also found in non-infected patients in a un it where the organism was prevalent.