COMPETITIVE FITNESS OF LISTERIA MONOCYTOGENES SEROTYPES 1/2A AND 4B STRAINS IN MIXED CULTURES WITH AND WITHOUT FOOD IN THE U. S. FOOD AND DRUG ADMINISTRATION ENRICHMENT PROTOCOL

Authors: Gorski, Lisa; Flaherty, Denise; Mandrell, Robert

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2005
Publication Date: 1/5/2006
Citation: Gorski, L.A., Flaherty, D., Mandrell, R.E. 2006. Competitive fitness of listeria monocytogenes serotypes 1/2a and 4b strains in mixed cultures with and without food in the u. s. food and drug administration enrichment protocol. Applied and Environmental Microbiology. 44:214-217.

Interpretive Summary: Enrichment culture is a system that allows one to find a particular bacterium if it is present in a complex system, such as food, that already has a large number of varied bacteria present. It is the first step in detection of foodborne pathogens in foods or food processing plants. One of the common enrichment protocols used to test for the presence of the food borne pathogen Listeria monocytogenes is that found in the U.S. Food and Drug Administration (FDA) Bacterial Analytical Manual (BAM). Thirteen different serotypes of L. monocytogenes have been described with serotype 4b strains most often associated with illness and 1/2a strains most often isolated from foods and processing plants. There have been cases where strains of both serotype were isolated from the same source. There are physiological differences between these two serotypes, including differences in chemical and temperature tolerance and stress, such that we thought there may be differences in how the two serotypes respond during enrichment culture. We compared three strains of L. monocytogenes serotype 4b and five strains of serotype 1/2a in direct competition with each other in mixed-culture in the FDA-BAM enrichment protocol with and without added food to mimic the conditions that occur during attempts to isolate Listeria species from contaminated foods. We asked if when two strains of two different serotypes were in direct competition in a common enrichment culture, would one serotype be better fit, or take over the enrichment culture. Using a colony immunoblot procedure, and analyzing over 112,000 colonies, differences in strain fitness were observed, but were not attributable to serotype. Rather, the data suggested that differences in strain fitness were associated with the source of the strains: environmental strains were more fit overall compared to clinical strains. In 22 of the 24 instances in which a human isolate paired with a non-human, environmental isolate yielded a statistically different result, the environmental isolate, regardless of the serotype, was more fit in the FDA-BAM enrichment protocol. These results may have serious implications for epidemiogical studies of L. monocytogenes, especially during outbreaks.

Technical Abstract: One of the common enrichment protocols used to test foods for the presence of the food borne pathogen Listeria monocytogenes is that found in the U.S. Food and Drug Administration (FDA) Bacterial Analytical Manual (BAM). Thirteen different serotypes of L. monocytogenes have been described with serotype 4b strains most often associated with illness and 1/2a strains most often isolated from foods and processing plants. There are physiological differences between these two serotypes, including a recent study that showed there was a bias in favor of the isolation of 1/2a strains in another commonly used enrichment medium. We compared three strains of L. monocytogenes serotype 4b and five strains of serotype 1/2a in direct competition with each other in mixed-culture in the FDA-BAM enrichment protocol with and without added food to mimic the conditions that occur during attempts to isolate Listeria species from contaminated foods. Using a colony immunoblot procedure, and analyzing over 112,000 colonies, differences in strain fitness were observed, but were not attributable to serotype. Rather, the data suggested that differences in strain fitness were associated with the source of the strains: environmental strains were more fit overall compared to clinical strains. In 22 of the 24 instances in which a human isolate paired with a non-human, environmental isolate yielded a statistically different result, the environmental isolate, regardless of the serotype, was more fit in the FDA-BAM enrichment protocol. These results may have serious implications for epidemiogical studies of L. monocytogenes.