Background
Porcine diarrhea during the neonatal and weaning periods is a common problem in pig production farms, resulting in significant mortality and economic losses for pig farmers. The aetiology of post-weaning diarrhea is multifactorial, but an elevated susceptibility to enteric infections due to the altered intestinal microbial balance is believed to play a central role [1]. The micro-organisms most often implicated in post-weaning diarrhea are enterotoxigenic E. coli (ETEC) strains expressing F4 (K88), F5, F6 or F18 fimbriae [2,3]. In the management of piglet gut health around weaning, feed supplementation with lactobacilli has proved beneficial [4-7], although in many cases the molecular mechanisms underpinning the beneficial effects have remained unknown. As adhesion to host tissues is essential for many gastro-intestinal pathogens, the paradigm of competitive exclusion through competition for binding sites has evolved. Therefore, knowledge about Lactobacillus surface components and their roles as adhesins is of major importance when developing strategies based on the administration of commensal bacteria to promote piglet health.
S (surface) layers are the outermost cell envelope structures commonly found on the surface of lactobacilli and other bacterial species. They are composed of numerous identical (glyco)protein subunits (with a 25–71 kDa size in lactobacilli), which form a regular, symmetric and porous array, completely covering the bacterial cell surface. The subunits are held together and connected to the underlying cell surface by non-covalent interactions, and they spontaneously reassemble in vitro by an entropy-driven process, i.e. the subunit proteins are very poorly water-soluble [8]. The biological functions of Lactobacillus S-layer proteins (Slp:s) are not well understood. In some Lactobacillus species, as well as in many other bacteria, S-layer proteins mediate bacterial adherence to host cells or to the extracellular matrix [9-19], but in most cases, the functions of Lactobacillus S-layer proteins have remained unknown.
Unlike in humans, lactobacilli are an essential component of the gastrointestinal microbiota of swine [20,21], with L. amylovorus representing a characteristic species which is especially abundant in piglets [22,23]. The S-layer carrying L. amylovorus strain DSM 16698, isolated from the small intestine of a piglet [24,25], has been shown to exhibit potentially health-promoting effects both in vitro and in weaned piglets in vivo[26,27]. This has raised the question if other strains of this commensal species might also have the potential to be used as probiotic feed additives during the weaning period. Furthermore, another question is related to the role of Slp:s in the probiotic effects of L. amylovorus. We have previously isolated several surface-layer carrying L. amylovorus strains from the small intestine or faeces of pigs and preliminarily characterized them for their putative probiotic properties [28,29]. While simultaneously carrying out the whole genome sequencing of the strains (Kant et al., manuscript in preparation), the present study was undertaken to characterize in detail the putative probiotic properties of these strains and to reveal the role of their divergent S-layer proteins in adherence to porcine intestinal epithelium in vitro. As a comparison, the above-mentioned L. amylovorus strain DSM 16698 of swine intestinal origin, and DSM 20531T, an S-layer carrying strain isolated from silage, were included in the experiments.