Evaluating Methods of Improving Recovery of Sub-lethally Injured Salmonella in Low Moisture Foods Treated with Antimicrobial Gas

Abstract

Introduction: Currently in the US, low moisture foods (LMFs) are commonly treated with ethylene oxide to control pathogens, but this can be a hazardous chemical. New treatment methods are being tested for LMFs, including fumigation with chlorine dioxide (ClO2) gas or hydrogen peroxide (H2O2) gas. Pathogenic cells not killed by treatment are a problem due to them still being infectious. Sub-lethally injured cells may be unable to grow in recovery media, resulting in inaccurate enumeration. Method: This study compared the recovery of Salmonella enterica or Enterococcus faecium NRRL B2354 in different media formulations. Inoculated LMFs were treated with ClO2 or H2O2 gas. Four non-selective media types were tested along with the selective media type XLD to determine their effect on log reduction. Also, the suitability of E. faecium NRRL B2354 as a surrogate for S. enterica was tested by comparing their log reductions. Results: For two spice types treated with ClO2 there was a significant difference (P<0.05) of log reduction of S. enterica between XLD and sodium pyruvate supplemented media with an average reduction of 3.43 and 2.21 log CFU/g respectively. H2O2 treated basil leaves showed a significant difference between XLD and all other media types with average reduction of 1.01 log CFU and the other media being under 0.503 log CFU/g. The log reduction values of S. enterica and E. faecium NRRL B2354 showed no statistical difference (P>0.05) to each other. Significance: Results implied that non-selective media has a better capacity to recover sub-lethally injured S. enterica colonies. Also, that E. faecium NRRL B2354 has similar log reduction values to S. enterica implies suitability as a surrogate. This is important as LMF processors using the non-selective media will help to prevent unsafe foods from being released and allow them to use a non-pathogenic bacterium in process validation.