Modified MacConkey medium which allows simple and reliable identification of Providencia stuartii

Abstract

This work describes a modified MacConkey medium (MCP medium) enabling the simple identification of Providencia stuartii, an emerging nosocomial pathogen. A total of 813 strains, belonging to the families Enterobacteriaceae and Pseudomonadaceae, were tested on MCP medium; all P. stuartii strains were phosphatase positive, as were 97.5% of Morganella morganii strains, in contrast with all other tested organisms. A simple discriminating test, such as the ornithine or citrate test, allowed identification of strains of these species. We have also compared the reliabilities of P. stuartii identification by commercial kits (API 20E system) by using a standard MacConkey or MCP medium. Sixteen and three-tenths percent of P. stuartii strains were misidentified by using the former procedure, while with the latter all strains were correctly identified. Finally, the MCP medium was used over a 6-month period in our routine clinical laboratory. Of a total of 1,278 seeded urine samples from elderly patients, we isolated 103 P. stuartii strains which were all correctly identified by coupling MCP medium and the API 20E system. Seventeen and one- half percent of these strains were misidentified when the API 20E system was used in combination with standard MacConkey medium.