Abstract
BackgroundSurface layers (S-layers) are two-dimensional crystalline arrays of repeating proteinaceous subunits that form the outermost layer of many bacterial cell envelopes. Within the Lactobacillus genus, S-layer presence is frequently associated with probiotic-relevant properties such as improved adherence to host epithelial cells and modulation of the immune response. However, recent studies have demonstrated that certain S-layer functions may be supplemented by a novel subset of proteins embedded within its lattice, termed S-layer associated proteins (SLAPs). In the following study, four Lactobacillus acidophilus NCFM SLAPs (LBA0046, LBA0864, LBA1426, and LBA1539) were selected for in silico and phenotypic assessment.ResultsDespite lacking any sequence similarity or catalytic domains that may indicate function, the genes encoding the four proteins of interest were shown to be unique to S-layer-forming, host-adapted lactobacilli species. Likewise, their corresponding deletion mutants exhibited broad, host-relevant phenotypes including decreased inflammatory profiles and reduced adherence to Caco-2 intestinal cells, extracellular matrices, and mucin in vitro.ConclusionsOverall, the data presented in this study collectively links several previously uncharacterized extracellular proteins to roles in the underlying host adaptive mechanisms of L. acidophilus.
Highlights
Surface layers (S-layers) are two-dimensional crystalline arrays of repeating proteinaceous subunits that form the outermost layer of many bacterial cell envelopes
Orthologs were only present in S-layerforming strains, which is in agreement with previous S-layer associated protein (SLAP) studies [19, 32]
Lactobacillus species tend to cluster based on lifestyle [26], mapping of SLAP presence added an additional level of granularity
Summary
Surface layers (S-layers) are two-dimensional crystalline arrays of repeating proteinaceous subunits that form the outermost layer of many bacterial cell envelopes. Within the Lactobacillus genus, S-layer presence is frequently associated with probiotic-relevant properties such as improved adherence to host epithelial cells and modulation of the immune response. Lactobacillus are Gram-positive, non-sporulating, anaerobic or microaerophilic bacteria, with complex nutritional requirements [1]. These versatile microorganisms inhabit diverse environments including dairy, meat and plant fermentations, as well as the gastrointestinal and urogenital tracts of humans and animals [2]. Renowned for their applications in food and feed fermentations, several Lactobacillus species are touted for their health-promoting, probiotic properties [3]. Efficacy tends to correlate with underlying host adaptive mechanisms such as bile and acid tolerance [5, 6], adhesion to mucus and epithelial cells [7, 8], and modulation of the immune response [9, 10] -- characteristics that can oftentimes be linked to the presence of specific extracellular proteins [2, 11, 12].
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