A new rapid method for the detection of Listeria and Salmonella organisms in foods

Conventional methods for detection of Listeria monocytogenes and Salmonella in foods consist of pre-enrichment, secondary enrichment, followed by biochemical and serological identification. They are time consuming and of little value for screening highly perishable food products. Rapid methods have been developed to shorten the detection time for the pathogens in foods and environmental samples. Most of these methods require either pure culture or high cell numbers (>104) before they can be applied. In this study, a detection procedure was developed, consisting of 6 h pre-enrichment and overnight selective enrichment in the BioSys(TM) instrument, followed by rapid identification of the pathogen by PCR. Two selective liquid media were developed. The selectivity of the Listeria broth was dependent on a mix of antibiotics: acriflavin, moxalactam, polymyxin B, and ceftazidime, and the detection in foods was based on esculin hydrolysis by listeriae and formation of black coloration in the presence of iron in the selective medium. For Salmonella, Tergitol and novobiocin were utilized to suppress growth of competitive foodborne microorganisms. Salmonellae were identified by a combination of positive biochemical characteristics consisting of amino acid decarboxylation, acid production from fermentable carbohydrates, and FeS production.; Presumptive positive results for both pathogens were obtained within 24 h. Short time (6 h) pre-enrichment was sufficient for resuscitation of heat-injured pathogens. The physical characteristics of the foods, levels of background microorganisms, and concentration of antibiotics in selective broth influenced the detection. PCR-based testing kits were successfully combined with each of the detection procedures for confirmation of the pathogens in positive vials. Only presumptive positive samples need confirmation. Thus, false positive results caused by nonviable cells, and cost are reduced. Finally, simultaneous recovery of Listeria and Salmonella and the detection methodologies were developed, thereby facilitating rapid routine testing of food samples that may be contaminated with these two pathogens.