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Applied and Environmental Microbiology, June 2001, p. 2410-2420, Vol. 67, No. 6
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.6.2410-2420.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Influence of the Natural Microbial Flora on the Acid Tolerance Response of Listeria monocytogenes in a Model System of Fresh Meat Decontamination Fluids

John Samelis,1 John N. Sofos,1,* Patricia A. Kendall,2 and Gary C. Smith1

Center for Red Meat Safety, Department of Animal Sciences,1 and Department of Food Science and Nutrition,2 Colorado State University, Fort Collins, Colorado 80523

Received 30 November 2000/Accepted 20 March 2001

Depending on its composition and metabolic activity, the natural flora that may be established in a meat plant environment can affect the survival, growth, and acid tolerance response (ATR) of bacterial pathogens present in the same niche. To investigate this hypothesis, changes in populations and ATR of inoculated (105 CFU/ml) Listeria monocytogenes were evaluated at 35°C in water (10 or 85°C) or acidic (2% lactic or acetic acid) washings of beef with or without prior filter sterilization. The model experiments were performed at 35°C rather than lower (<= 15°C) temperatures to maximize the response of inoculated L. monocytogenes in the washings with or without competitive flora. Acid solution washings were free (<1.0 log CFU/ml) of natural flora before inoculation (day 0), and no microbial growth occurred during storage (35°C, 8 days). Inoculated L. monocytogenes died off (negative enrichment) in acid washings within 24 h. In nonacid (water) washings, the pathogen increased (approximately 1.0 to 2.0 log CFU/ml), irrespective of natural flora, which, when present, predominated (>8.0 log CFU/ml) by day 1. The pH of inoculated water washings decreased or increased depending on absence or presence of natural flora, respectively. These microbial and pH changes modulated the ATR of L. monocytogenes at 35°C. In filter-sterilized water washings, inoculated L. monocytogenes increased its ATR by at least 1.0 log CFU/ml from days 1 to 8, while in unfiltered water washings the pathogen was acid tolerant at day 1 (0.3 to 1.4 log CFU/ml reduction) and became acid sensitive (3.0 to >5.0 log CFU/ml reduction) at day 8. These results suggest that the predominant gram-negative flora of an aerobic fresh meat plant environment may sensitize bacterial pathogens to acid.

* Corresponding author. Mailing address: Center for Red Meat Safety, Department of Animal Sciences, Colorado State University, Fort Collins, CO 80523. Phone: (970) 491-7703. Fax: (970) 491-0278. E-mail: John.Sofos{at}colostate.edu.

Applied and Environmental Microbiology, June 2001, p. 2410-2420, Vol. 67, No. 6
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.6.2410-2420.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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