Investigation of nosocomial infections by plasmid analysis.

Molecular biological techniques, including agarose gel electrophoresis, restriction endonuclease analysis and DNA-DNA hybridization (Southern blotting and heteroduplex analysis) are being applied very successfully to the investigation of nosocomial infections. 'Plasmid fingerprinting' by electrophoresis, with or without restriction endonuclease analysis, can be used to identify epidemic strains of bacteria and 'epidemic plasmids' which have spread through several different bacterial species. This technique is rapid and inexpensive, and can be applied to drug-sensitive as well as resistant strains. This approach is especially useful for investigation of organisms for which no standard typing system is available. DNA-DNA hybridization techniques can be used to study the evolution of plasmids in the hospital environment, and to demonstrate the presence and spread of translocatable DNA sequences (transposons) carrying drug resistance determinants from plasmid to plasmid within a bacterial cell.