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Environmental factors influence L. monocytogenes survival and attachment on surfaces.

Scientists at USDA-ARS and Purdue University set out to determine which factors enable L. monocytogenes to survive and colonize on surfaces when the bacteria are inoculated from drying droplets. They were able to demonstrate that droplet inoculums are dramatically affected by environmental conditions and result in a mixed distribution of microenvironments, some of which promote adhesion or survival of the bacteria.

In their tests, L. monocytogenes F4244 and its green fluorescence protein derivative, L. monocytogenes pNF9, were grown in tryptic soy broth and modified Welshimer's broth. The investigators used atomic force microscopy and epifluorescence microscopy to assess the spatial distribution of bacteria as they dried from droplets. They also examined the extent of bacterial survival within the droplet area, and how the bacteria may attach to a surface. Environmental factors were considered, such as temperature, presence of nutrients and relative humidity.

In order to reduce the number of medium components, cultures were dialyzed against sterile water. Approximately 2 X [10.sup.6] CFU per ml of culture was spotted on hydrophobic glass slides. The drying pattern showed different areas with clumped cells, and cells within a medium component-based matrix.

To confirm cell viability, the scientists placed agar plugs on the surface and, upon growth, viable cells formed patterns similar to those observed in the dried spots. Moreover, similar experimental conditions involving relative humidity and temperature-rich media allowed bacteria to survive after 84 hours of drying. Minimal media-grown bacteria survived for 72 hours.

Similar bacterial preparations involving available nutrients which were stored at refrigeration temperatures and higher relative humidity survived for more than 10 days. A majority of the bacteria that survived was found within dried medium-based matrices. When washed with a peristaltic pump at a flow rate of 5 ml per minute for 1 minute, the clumps of cells were preferentially retained.

Further information. David E. Nivens, Department of Food Science, Purdue University, 745 Agriculture Mall Dr., West Lafayette, IN 47907; phone: 765-4940460; email:

Pathogenic microorganisms residing on food or food contact materials are important sources of foodborne contamination.
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Publication:Microbial Update International
Date:Dec 1, 2011
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