Serratia: opportunistic pathogen of increasing clinical importance

Serratia marcescens can become a formidable nosocomial (hospital acquired) pathogen, and is reported increasingly in the world literature. However, it is only a recently recognized problem in Australia. Serratia can carry an antibiotic-resistance plasmid, and, after entry of the organism into very sick patients, it may be hard or impossible to eliminate. Initial experience of Serratia in 34 consecutive cases isolated in a three-months period is presented. Rapid increase in the number of Serratia infections occurred after the appearance of a resistant strain. Urinary infection was the commonest presentation (91% of cases). The presence of an indwelling urinary catheter in a debilitated patient was the major predisposing factor. Significant bacteraemia followed in four cases with one death. Contamination of burns (surfaces) and surgical wounds was found in four cases. Serratia strains were found to be highly resistant to most antimicrobial agents in routine sensitivity testing, 20% being fully resistant to all tested agents, and nalidixic acid being the most effective inhibitor in the remainder. With bacteriocin typing of Serratia, two types were found to be dominant. These two bacteriocin types were not identified among strains isolated from other sources such as soil, water and local hospitals. Pharyngeal carriage was found in only one case, but faecal excretion of Serratia was found in 11 cases and may be a significant portal of dissemination. Cross-infection from a hospital reservoir of resistant organisms is postulated. A model of cross-infection is also proposed, and methods of control are discussed. In view of the established danger of Serratia in the hospital setting, its isolation can no longer be ignored.