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.

Incidence and level of exotoxin A in proteolytic cell-free cultures of clinical isolates of Pseudomonas aeruginosa collected from hospitals in Riyadh, Saudi Arabia.

Mice lethality bioassay revealed that out of 102 proteolytic cell-free cultures of Pseudomonas aeruginosa only 17 (15%) were lethal with values of 19-48 LD50/ml. Antitoxin A antiserum neutralized the lethality providing evidence that the main lethal compound in the cell free cultures was exotoxin A. The neutralization also ruled out the lethal contribution of protease, elastase and phospholipase c. The presence of exotoxin A in a proteolytic environment may indicate less efficient proteolytic action at the stage of toxin production.

The role of extracellular products of Pseudomonas aeruginosa in the pathogenesis of infection; an experimental study employing intraperitoneal diffusion chambers.

Exotoxin A is currently thought to be the principal lethal factor in experimental Pseudomonas aeruginosa infection. This belief is founded on the demonstrable toxicity of purified preparations, and on detection of the toxin in the tissues of burned animals infected with Ps. aeruginosa. In the present study, strains of Ps. aeruginosa differing in their ability to produce exotoxin A and other virulence factors in vitro were enclosed within vinyl diffusion chambers and implanted i.p. into mice. Strains which produced much exotoxin A in vitro were not significantly more virulent when enclosed in chambers than strains which produced little exotoxin. In all cases, diffusion of exotoxin A produced within the chamber was impeded by dense adherence of the omentum. It is concluded that, although absorption of exotoxin A may be an important factor in causing death after infected burns, it is not necessarily equally significant in other types of infection.