Non-transmissible and self-transmissible plasmids conferring drug resistance in clinical isolates of Serratia marcescens from hospitals in Riyadh, Saudi Arabia.

Sixty Serratia marcescens isolates were obtained from patient specimens from three different hospitals in the city of Riyadh. These were tested for their antibiotic resistance factors using eleven different antibiotics. Their ability to transfer antibiotic resistance plasmids to a sensitive E. coli strain (RR1) was tested by transformation and conjugation experiments. Agarose gel electrophoresis was used to determine the size and number of the R-plasmids. Southern blotting was used to assess homologies between antibiotic resistance plasmids from different isolates. Among the isolates tested, 36.7% contained plasmids, and all these were from strains isolated from two hospitals. No R-plasmids could be detected among the multiple resistant strains isolated from the third hospital. Among the strains that contained plasmids, approximately 63.6% transferred multiple antibiotic resistance to E. coli and the rest transferred only one antibiotic resistance marker. The majority of strains carrying out the plasmids showed similarities in band number and size. In view of the similarities this group was denoted the predominant group, and selected for further molecular investigations. Restriction endonuclease digests of plasmids from this group gave the same restriction pattern which confirmed that they were closely related. Hybridization experiments using these plasmids and nick-translated 32p-labelled pBR-322 DNA probe, showed that all the large bands (36 kb) are related and exhibit homology with pBR-322.