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Acid resistance of pathogenic bacteria has serious implications.

Pathogenic bacteria have the ability to adapt to acidic environments and become resistant to exposure to acid or other unrelated stresses. But there have not been many investigations of the ability of pathogens to develop cross protection to stress conditions after they are exposed to alkaline environments.

Given the alkaline nature of some of the detergents and chemical sanitizers used to clean processing equipment where pathogens reside, information on the response of pathogens to alkaline stress and potential cross protection against heat would be useful. This would especially be the case when you're designing interventions for eliminating or controlling pathogens.

Scientists at the Center for Food Safety and Quality Enhancement (University of Georgia, Georgia Experiment Station, Griffin, GA 30223) tested a strain of L. monocytogenes isolated from a drain in a food processing plant for its sensitivity to heat after it was exposed to pH 9 to pH 12 conditions. Bacterial cells treated at pH 12 for 45 minutes were more resistant to thermal inactivation, compared with cells held at pH 7.3 before they were heated.

Alkali-shocked cells survived at least six days at 4 C or 21 C in tryptose phosphate broth (TPB) at pH 11. Cells of L. monocytogenes incubated at 37 C in TPB at pH 10 for 45 minutes, then stored for two or six days at 4 C or 21 C in the same medium, had significantly higher D56[degrees]C values, compared with cells treated and held at pH 7.3.

Exposing cells of L. monocytogenes to 2 parts per million (ppm) to 6 ppm of chlorine for 5 minutes or 10 minutes resulted in injuring a portion of the survivors. Unlike alkali-stressed cells, however, the thermotolerance of chlorine-stressed cells declined compared with the control cells, thus not compromising the effectiveness of thermal processing in achieving the desired reductions in populations.

Contamination of foods by alkali-stressed L. monocytogenes that have survived exposure to processing environments ineffectively cleaned with alkaline detergents or disinfectants may have more serious implications than previously recognized. Alkaline pH-induced cross protection of L. monocytogenes against heat has the potential to enhance the bacteria's survival in minimally processed foods that have received mild heat treatment that would have otherwise have been lethal to nonstressed cells.

Further information. Michael Doyle; phone: 770-228-7284; fax: 770-229-3216; email:
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Publication:Microbial Update International
Date:Feb 1, 2001
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