Stress-induced survival strategies enable Salmonella Enteritidis to persistently colonize the chicken oviduct tissue and cope with antimicrobial factors in egg white: A hypothesis to explain a pandemic

Background: Egg-associated transmission to humans seems to be characteristic of the Salmonella serotype Enteritidis, explaining why this particular serotype has caused a worldwide pandemic since the mid ‘80s. Salmonella Enteritidis is much more capable to persistently colonize the laying hen reproductive tract and to survive in the hostile egg white, as compared to other serotypes. Presentation of the hypothesis: It is hypothesized that stress-induced survival mechanisms enable the serotype Enteritidis to persistently colonize the oviduct without causing damage and excessive inflammation, and to cope with the antimicrobial compounds present in egg white. Testing the hypothesis: To test the hypothesis first of all Salmonella Enteritidis genes that are essential for colonization of the oviduct and survival in eggs need to be identified. Comparative genomics tools should be used to identify genes or pathogenicity islands that are present in Salmonella Enteritidis and not in the multiple non egg-contaminating serotypes. High-throughput signature-tagged-mutagenesis approaches, coupled to micro-array detection of the genes that lead to an attenuated phenotype when mutated is proposed as an ideal tool to identify genes involved in oviduct colonization and egg white survival. Identifying the stressors and antibacterial molecules in the oviduct and in the egg white that limit colonization or survival of non-Enteritidis serotypes is a second important objective that can theoretically be achieved using screenings of expressed oviduct cDNA libraries for their antibacterial activity against strains from multiple serotypes. Finally, the effect of contact with these stressors in the oviduct or egg white on Salmonella gene expression will need to be analyzed, in order to clarify whether serotype Enteritidis-specific regulation of certain stress-survival pathways are either or not present. Implications of the hypothesis: Knowledge on the pathogenesis of egg infections would furthermore give insights that might be extrapolated to other biological interactions, in which a highly specialized bacterial pathogen resists the host response in a specific biological niche. In addition, this info can be of value in developing early warning criteria to identify emerging egg-associated Salmonella strains and in developing safe live attenuated vaccine strains. Correspondence: [email protected] Department of Pathology, Bacteriology and Avian Diseases, Research Group Veterinary Public Health and Zoonoses, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, B-9820 Merelbeke, Belgium Van Immerseel Gut Pathogens 2010, 2:23 http://www.gutpathogens.com/content/2/1/23 © 2010 Van Immerseel; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Background Salmonella is worldwide one of the most important causes of zoonotic disease, associated with consumption of contaminated food. Most important food vehicles for Salmonella are eggs and egg products. The serotype typically associated with eggs and egg products is Salmonella Enteritidis. This serotype is highly prevalent in live poultry, both in broilers and laying hens. In live chickens, also many other Salmonella serotypes are detected, but egg-associated transmission to humans seems to be a characteristic that is specifically reserved for the serotype Salmonella Enteritidis, explaining why this particular serotype has caused a worldwide pandemic since the mid ‘80s [1,2]. Eggs can be contaminated on the outer shell or internally. Outer shell contamination is the result of environmental contamination after shedding of the pathogen by the animal. Internal egg contamination can in principle occur following by eggshell and eggshell membrane penetration, but it is believed that most often colonization of the reproductive tract and incorporation in the forming egg is the main route [3]. Egg formation in the oviduct takes about 24 hours during which the sequential addition of molecules takes place in different compartments of the reproductive tract while the forming egg is migrating from the ovary to the vagina, before being laid. Indeed, the yolk is produced in the ovary, the infundibulum (most upper part of the oviduct) captures the yolk, the magnum segment produces the egg white, the isthmus deposits the eggshell membranes, the uterus forms the shell and the vagina is involved in oviposition. Depending on the oviduct segment in which Salmonella bacteria are present, the bacteria can thus be incorporated in different parts of the egg. Yolk contamination would lead to extensive growth [4]. This would result in a drop in egg production. Therefore it is generally believed that eggs are mostly contaminated in the egg white, in which antibacterial factors are present that limit bacterial multiplication. Until now, it has not been clearly explained why the serotype Enteritidis specifically has been implicated in egg contamination. It is clear however that Salmonella Enteritidis can contaminate the reproductive tract and seems to be a better colonizer of the oviduct environment as compared to other serotypes [5]. Secondly, it has been shown by comparing different strains from multiple serotypes that in general, Salmonella Enteritidis strains cope better with the antimicrobial properties of egg white as compared to other serotypes [6]. More efficient oviduct colonization and better survival in egg white are ideal characteristics for a pathogen that is transmitted to egg-consuming hosts. Despite this knowledge, the molecular mechanisms behind this behavior of Salmonella Enteritidis are still not unraveled. Some Salmonella genes that are involved in oviduct colonization and egg white survival have been identified, but genome-wide screening tools, complemented with indepth studies on the role of specific genes, would be most welcome to get a better profile of the interaction of Salmonella Enteritidis with the oviduct tissue and egg white. Genes shown to be highly expressed in the oviduct or essential for oviduct colonization that are described until now are mostly major virulence genes that contribute to colonization of any organ, and are not exclusively related to oviduct colonization [3]. In the egg white, numerous antimicrobial compounds have been identified. These include molecules that degrade microbial components, such as lysozyme, antibacterial peptides, such as avian b-defensins, lipopolysaccharide binding and bactericidal/permeability increasing proteins (LBP-BPI proteins) that bind LPS and permeabilize the cytoplasmic membrane, such as ovocalyxin-36, molecules decreasing bioavailability of cations and vitamins, such as ovotransferrin, and many others [7-9]. Strikingly, the activity of these molecules to Salmonella and the potential protection mechanisms of Salmonella against these molecules are very poorly investigated under the conditions encountered by the bacteria it the chicken reproductive tract. In this paper, it is proposed that stress-induced survival mechanisms enable the serotype Enteritidis to persistently colonize the oviduct without inducing damage and excessive inflammation, and to cope with the antimicrobial compounds and thus survive in egg white. It is proposed that studies should be carried out to a) analyze the virulence mechanisms that allow Salmonella Enteritidis to persistently colonize the oviduct tissue and b) to identify egg white molecules that have antibacterial activity against Salmonella and unravel the mechanisms that protect Salmonella Enteritidis against these environmental insults, thus enabling its survival in egg white. Presentation of the hypothesis The Salmonella serotype Enteritidis has caused a worldwide pandemic due to its ability to persistently colonize the oviduct tissue of laying hens and survive in the hostile egg white environment. The Salmonella Enteritidis Oand H-antigens, and thus the LPS and flagellar antigens, are discriminating Enteritidis from many other serotypes (from other serogroups). It could thus very well be that the LPS structure of Salmonella serogroup D, containing the serotype Enteritidis, could be more resistant to penetration of antibacterial molecules, such as cationic antimicrobial peptides (CAMPs) present in egg white. However, this does not explain why strains from serotype Enteritidis and not other serogroup D serotypes have a tropism for eggs. It is hypothesized that stressinduced survival mechanisms play a role in the ability to Van Immerseel Gut Pathogens 2010, 2:23 http://www.gutpathogens.com/content/2/1/23 Page 2 of 5