Author
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Guard, Jean |
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Submitted to: Meeting Proceedings
Publication Type: Proceedings Publication Acceptance Date: 3/11/2011 Publication Date: 4/6/2011 Citation: Guard, J.Y. 2011. Improving Food Safety by Understanding the Evolution of Egg-contaminating Salmonella Enteritidis. In: Proceedings of XXX International Seminar of AMEVEA. April 6-8, 2011, Bogota, Columbia. 2011CDROM. Interpretive Summary: Technical Abstract: Improving Food Safety by Understanding the Evolution of Egg-contaminating Salmonella Enteritidis Jean Guard, Veterinary Medical Officer U. S. Department of Agriculture, Athens, GA USA (jean.guard@ars.usda.gov) The curious case of egg contamination by Salmonella enterica serovar Enteritidis S. Enteritidis is currently the world’s leading cause of human food-borne salmonellosis. Its emergence worldwide was well-documented in the early 1980s, but its first appearance was probably during the 1970s (6,7). It is the only Salmonella enterica serotype that routinely contaminates and survives in the internal contents of eggs produced by otherwise healthy hens (8). Other Salmonella enterica serotypes such as S. Pullorum and S. Gallinarum are well-documented for their ability to contaminate the internal contents of eggs, but they sicken birds and do not cause food borne disease in humans (2,9). Comparison of the two prevalent serotypes, S. Enteritidis and S. Typhimurium Egg-contaminating S. Enteritidis has retained a host range essentially as broad as that of S. Typhimurium (11,12). Together, these two serotypes cause approximately 40% of all human illnesses. Comparison of outer membrane properties revealed that strains of S. Enteritidis can produce a capsular-like lipopolysaccharide O-antigen region that mitigates signs of disease in hens (14,15). S. Enteritidis is especially capable of surviving within the internal contents of eggs, because it makes this capsule (13,16). S. Typhimurium does not appear to make this capsule with any efficiency. Playing poker with Salmonella A useful analogy for understanding that both large and small scale genetic events contribute to the ability of S. Enteritidis to contaminate eggs is to consider a deck of cards where constant reshuffling and cutting of the deck eventually results in one individual in a game drawing a combination of higher value. Several hands have the potential to win the game. Certain patterns of genetic inheritance add value over all the other hands in play. This situation accounts for the emergence of one serotype more than another. The winner appears to have linked together traits such as the ability to survive stressful conditions, to grow to high cell density and to access new niches (8). Both S. Enteritidis and S. Typhimurium have a group of genes that give these serotypes an advantage in the environment over the other 1400 serotypes belonging to subspecies I Salmonella enterica. However, S. Enteritidis developed the ability to contaminate eggs and thus it has access to an entirely new niche. Methods used for analysis of evolutionary trends can introduce bias S. Enteritidis appears to be an unusual case of evolution within the Salmonellae, because small scale genetic events are driving the biology which impacts human health. These events are deletions, and nucleotide substitutions that may or may not alter an open-reading frame. All of these events are referred to as single nucleotide polymorphisms (SNPs). For example, the largest SNP discovered in S. Enteritidis is a 215bp deletion that removed the N-terminus and some upstream region of the gene SEN4316. SNPs are not usually detected by DNA-DNA microarray hybridization (3). However, microarrays can be designed to detect SNPs (4,5). Conversely, a SNP that disrupts gene function can be missed by a DNA-DNA microarray even if it registers that the gene is present. Analysis of mRNA is thus a necessary supporting assay to assess which genes are functional. The problem with analysis of mRNA is that gene expression is dependent upon environmental conditions. It is quite possible that a gene could have a very important role within a specific environment that is absent in the conditions for analysis chosen by an investigator. Again, this situation results in a false interpretation. All methods have a limit of dete |
