Nisin, produced by Lactococcus lactis subsp. lactis, has a broad spectrum of activity against gram-positive bacteria and is generally recognized as safe in the United States for use in selected pasteurized cheese spreads to control the outgrowth and toxin production of Clostridium botulinum. This study evaluated the inhibitory activity of nisin in combination with a chelating agent, disodium EDTA, against several Salmonella species and other selected gram-negative bacteria. After a 1-h exposure to 50 ,ug of nisin per ml and 20 mM disodium EDTA at 37°C, a 3.2to 6.9-log-cycle reduction in population was observed with the species tested. Treatment with disodium EDTA or nisin alone produced no significant inhibition (

The use of many rapid detection technologies could be expanded if the bacteria were separated, concentrated, and purified from the sample matrix before detection. Specific advantages of bacterial concentration might include facilitating the detection of multiple bacterial strains; removal of matrix-associated assay inhibitors; and provision of adequate sample size reduction to allow for the use of representative food sample sizes and/or small media volumes. Furthermore, bacterial concentration could aid in improving sampling techniques needed to detect low levels of pathogens or sporadic contamination, which may perhaps reduce or even eliminate the need for cultural enrichment prior to detection. Although bacterial concentration methods such as centrifugation, filtration, and immunomagnetic separation have been reported for food systems, none of these is ideal and in many cases a technique optimized for one food system or microorganism is not readily adaptable to others. Indeed, the separation and subsequent concentration of bacterial cells from a food sample during sample preparation continues to be a stumbling block in the advancement of molecular methods for the detection of foodborne pathogens. The purpose of this review is to provide a detailed understanding of the science, possibilities, and limitations of separating and concentrating bacterial cells from the food matrix in an effort to further improve our ability to harness molecular methods for the rapid detection of foodborne pathogens.

A method using nisin and a chelating agent to inactivate Salmonella species and other gram-negative bacteria has been developed. The objective of this study was to determine the effect of treatment conditions on the application of this method. Ten gram-negative organisms were used in this study, including six Salmonella species commonly associated with foodborne illness. Organisms were selected on the basis of sensitivity to nisin and a chelating agent. The following parameters were examined: (a) chelating agent, (b) nisin concentration, (c) incubation temperature, and (d) protein interference. Chelating agents included EDTA, ethylenebis (oxyethylene-nitrilo) tetraacetic acid, citric acid monohydrate, and sodium phosphate dibasic. The most effective treatment consisted of 50 to 100 μg/ml nisin applied in combination with 20 mM EDTA or citric acid monohydrate at a temperature range of 30 to 42°C. All of the chelators examined exhibited some inhibitory activity. The addition of bovine serum albumin to the treatments containing nisin and EDTA did not result in a significant decrease in inhibitory action.

Aims: To develop a simple, rapid method to concentrate and purify bacteria and their nucleic acids from complex dairy food matrices in preparation for direct pathogen detection using polymerase chain reaction (PCR). Methods and Results: Plain non-fat yogurt and cheddar cheese were each seeded with Listeria monocytogenes or Salmonella enterica serovar. Enteritidis in the range of 10 1 -10 6 CFU per 11-g sample. Samples were then processed for bacterial concentration using high-speed centrifugation (9700 g) followed by DNA extraction, PCR amplification, and amplicon confirmation by hybridization. Bacterial recoveries after centrifugation ranged from 53 to >100% and 71 to >100% for serovar. Enteritidis and L. monocytogenes, respectively, in the non-fat yogurt samples; and from 77 to >100% and 69 to >100% for serovar. Enteritidis and L. monocytogenes, respectively, in the cheddar cheese samples. There were no significant differences in recovery efficiency at different inocula levels, and losses to discarded supernatants were always <5%, regardless of dairy product or pathogen. Conclusions: When followed by pathogen detection using PCR and confirmation by amplicon hybridization, detection limits of 10 3 and 10 1 CFU per 11-g sample were achieved for L. monocytogenes and serovar. Enteritidis, respectively, in both product types and without prior cultural enrichment. Significance and Impact of the Study: This study represents progress toward the rapid and efficient direct detection of pathogens from complex food matrices at detection limits approaching those that might be anticipated in naturally contaminated products.

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