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Potential resistance to antimicrobials shown by certain bacteria.

An increased prevalence of bacteria resistant to therapeutic antibiotics has led to a greater risk for illnesses that are difficult to treat using currently available antibiotics. The emergence of such antibiotic-resistant organisms should be of concern to food processors who should use more and varied antimicrobial preservation techniques to control foodborne pathogens.

However, there are little or no data on the possible emergence of foodborne pathogens resistant to traditional food antimicrobials, such as organic acids. A few scientists believe that some foodborne pathogens can be resistant to traditional food antimicrobials. And research bears this out. For example, there is a potential for the development of resistance by L. monocytogenes or S. typhimurium to regulatory-approved food antimicrobials at certain concentrations and pH.

Scientists at the University of Tennessee investigated the potential development of resistance to certain antimicrobials. They evaluated the potential mechanisms of microbial resistance associated with specific microorganisms.

Four strains of L. monocytogenes (101, 108, 310 and Scott A) and S. typhimurium DT 104 (2380, 2576, 2582 and 2486) were used in this research. Food antimicrobials evaluated included sodium benzoate, potassium sorbate and sodium lactate. Researchers monitored growth using a micro-broth dilution assay and measuring absorbance at 630 nm. Bacterial strains were exposed to increasing concentrations of each antimicrobial up to 0.5% at pH 6.0 without any pre-stress.

L. monocytogenes showed growth in the presence of increasing concentrations--0.1% to 0.5%--of all the antimicrobial compounds tested, with the exception of 0.5% sodium benzoate. Sodium lactate had little effect on the growth of any strain, except L. monocytogenes 101 at 0.4% to 0.5% levels. S. typhimurium growth was inhibited using more than 0.3% benzoic and more than 0.4% sorbic acid.

As with L. monocytogenes, sodium lactate had little effect on the growth of any strain of S. typhimurium. Both microorganisms showed potential for resistance to sodium lactate and to lower concentrations of benzoate and sorbate.

Further information. P. Michael Davidson, Department of Food Science and Technology, University of Tennessee, 101 Food Safety and Processing, 2605 River Dr., Knoxville, TN 37996; phone: 865-974-0098; fax: 865-974-2750; email:
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Publication:Microbial Update International
Date:Oct 1, 2006
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