[Critical analysis of isolation, counting and identification methods of Listeria in food industry]. / Analyse critique des méthodes d'isolement, de dénombrement et d'identification des Listeria en agro-alimentaire.

Stimulated by epidemic outbreaks in years 80's and 90's detection methods for Listeria, and L. monocytogenes particularly, have undergone many important progresses. New selective media for isolation and enrichment have been formulated with the aim to improve bacteriological methods and decrease time delays. For identification of Listeria at the species level, some biochemical miniaturised tests allow response within 24 h. Within the immuno-chemical methods as ELISA or ELFA, the revelation step has been shortened and automated. The elaboration of L. monocytogenes specific antibodies begins to upset immunological methods. As far as concern simplification of detection tests, a new generation of small immunological plate-shaped tests has been marketed. Whatever the immunological method considered, intrinsic sensibility is about 10(5) at 10(6) CFU/ml. Only one method allows more sensibility: the immuno-magnetic separation. With a sensibility of 1 CFU/ml, immunocapture allows shortening of enrichment time of 24 h or eliminates this step altogether if the aim is to count the Listeria directly in the sample. In the biological molecular methods, some hybridization tests with nuclear probes allow rapid and specific detection of L. monocytogenes. The polymerase chain reaction has been improved to simplify and shorten the DNA preparation and the revelation of amplified fragments. This method will be soon used in food samples analysis. The elaboration of new methods that could allow the specific detection of L. monocytogenes pathogenic strains of is one of future stake.

Salmonella typhimurium TnphoA mutants with increased sensitivity to biological and chemical detergents.

Salmonella typhimurium is a ubiquitous pathogenic bacterium able to sustain the environmental conditions of the gastrointestinal tract, including biliary salts. To understand the mechanisms involved in bile salt resistance and, more generally, detergent resistance, we investigated S. typhimurium mutants produced with the random mutagenic TnphoA transposon. A total of 3,000 transpositional mutants were isolated. Three strains among the 1,432 first mutants lost the ability to grow in the presence of biological and chemical detergents. They were prototrophic and exhibited normal lipopolysaccharide and outer membrane protein profiles after SDS-PAGE. They did not show sensitivity to dyes but showed very different sensitivities to antibiotics. For each mutant strain, Southern blotting analysis revealed a unique TnphoA insertion at different chromosomal locations. These observations were confirmed by transduction experiments.