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Strong adherence promotes retention of strains on processing equipment.

Consuming foods contaminated with L. monocytogenes may cause listeriosis. The organism is often present on raw meat ingredients and has been a recurring problem in meat processing facilities.

L. monocytogenes is capable of strongly adhering to equipment or surfaces in meat processing facilities, resulting in the formation of biofilms. Scientists at Oklahoma State University examined select strains of L. monocytogenes isolated from raw and ready-to-eat meats and from surfaces in facilities that process them. Their data suggest that the bacteria's strong adherence ability not only promotes the retention of strains on food processing equipment, but may also enhance invasion and replication in host tissues, causing greater virulence than less adherent strains.

In experiments, the scientists selected various strains and divided them into weak or strong adherent phenotypes. These were grouped using three molecular subtyping methods: multilocus sequence typing, pulsed-field gel electrophoresis and ribotyping.

The researchers tested the adherence and virulence capacity of the strains using in vitro assays and the human cell line, Caco-2. With a primary focus on strains showing strong adherence, an in vivo study was also conducted by orally inoculating mice with the bacteria. Isolates were inoculated into the mice using oral gavage dosing, which involves administering fluids directly into the lower esophagus or stomach using a feeding needle introduced into the mouth and threaded down the esophagus. The scientists used specific strains of L. monocytogenes that were recovered after three, four and five days.

In the in vivo study, the investigators found that strains capable of strongly adhering to abiotic surfaces were more invasive than those strains that were weakly adherent. In addition, a necropsy was conducted to perform a histopathological examination of spleen and liver tissue samples obtained from the infected mice to confirm the virulence.

The scientists indicate that strongly adherent strains invaded the spleen and liver, as was evident from the conspicuous histological lesions in the liver. Comparing the histological lesions in the tissue samples confirmed that only the strongly adhering invasive strains in the human cell line caused greater tissue damage and virulence.

Further information. Peter M. Muriana, Department of Animal Science, Robert M. Kerr Food and Agricultural Products Center, Oklahoma State University, 109 FAPC, Stillwater, OK 74078; phone: 405-744-5563; fax: 405-744-6313; email:
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Publication:Microbial Update International
Date:Oct 1, 2010
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