Application of a novel immunomagnetic separation-bacteriophage assay for the detection of Salmonella enteritidis and Escherichia coli O157:H7 in food.

Salmonella infection is the second most prevalent cause of foodborne illness in most developing countries. Meat, poultry, and dairy products are frequently implicated in outbreaks. The objective of this study was to apply a novel immunomagnetic separation (IMS)-bacteriophage assay to the detection of Salmonella enteritidis in artificially inoculated skimmed milk powder, chicken rinses, and ground beef. In all food types tested, the IMS-bacteriophage assay was able to detect an average of 3 CFU of S. enteritidis in 25 g or ml of food sample. Total assay time including pre-enrichment is about 20 h. The results indicate that the IMS-bacteriophage assay is a rapid and sensitive means of detecting S. enteritidis in these foods. The assay was successfully adapted to the detection of Escherichia coli O157:H7 and was able to detect E. coli in ground beef at the lowest inoculation level tested, 2 CFU/g. The assay was also adapted to the simultaneous detection of S. enteritidis and E. coli. The results indicate that the IMS-bacteriophage assay shows promise for the simultaneous detection of these pathogens, but further development work would be necessary to improve sensitivity and produce reliable results at low inoculation levels.

Sensitivity of Listeria monocytogenes to sanitizers used in the meat processing industry.

Nineteen Listeria monocytogenes strains were characterized by automated ribotyping, pulsed-field gel electrophoresis, and plasmid profiling to determine the relationship between genotype and sanitizer resistance. Isolates within a ribogroup had a consistent sensitivity or resistance phenotype except for ribogroup C isolates. All isolates with resistance phenotypes harbored two plasmids. The sensitivity of L. monocytogenes strains to quaternary ammonium compounds (QACs) was correlated with sensitivity to sanitizers and antibiotics with other modes of action. All isolates tested contained the mdrL gene, which encodes an efflux pump that confers resistance to QACs and is both chromosome and plasmid borne.

Postadaptational resistance to benzalkonium chloride and subsequent physicochemical modifications of Listeria monocytogenes.

Many studies have demonstrated that bacteria, including Listeria monocytogenes, are capable of adapting to disinfectants used in industrial settings after prolonged exposure to sublethal concentrations. However, the consequent alterations of the cell surface due to sanitizer adaptation of this pathogen are not fully understood. Two resistant and four sensitive L. monocytogenes strains from different sources were progressively subcultured with increasing sublethal concentrations of a surfactant, benzalkonium chloride (BC). To evaluate the effects of acquired tolerance to BC, parent and adapted strains were compared by using several morphological and physiological tests. Sensitive strains showed at least a fivefold increase in the MIC, while the MIC doubled for resistant strains after the adaptation period. The hydrophobicities of cells of parent and adapted strains were similar. Serological testing indicated that antigen types 1 and 4 were both present on the cell surface of adapted cells. The data suggest that efflux pumps are the major mechanism of adaptation in sensitive strains and are less important in originally resistant isolates. A different, unknown mechanism was responsible for the original tolerance of resistant isolates. In an originally resistant strain, there was a slight shift in the fatty acid profile after adaptation, whereas sensitive strains had similar profiles. Electron micrographs revealed morphological differences after adaptation. The changes in cell surface antigens, efflux pump utilization, and fatty acid profiles suggest that different mechanisms are used by resistant and sensitive strains for adaptation to BC.