Abstract
Bacterial surface-layers (S-layers) are crystalline arrays of repeating proteinaceous subunits that coat the exterior of many cell envelopes. S-layers have demonstrated diverse functions in growth and survival, maintenance of cell integrity, and mediation of host interactions. Additionally, S-layers can act as scaffolds for the outward display of auxiliary proteins and glycoproteins. These non-covalently bound S-layer associated proteins (SLAPs) have characterized roles in cell division, adherence to intestinal cells, and modulation of the host immune response. Recently, IgdA (LBA0695), a Lactobacillus acidophilus SLAP that possesses a Group 3 immunoglobulin (Ig)-like domain and GW (Gly-Tryp) dipeptide surface anchor, was recognized for its high conservation among S-layer-forming lactobacilli, constitutive expression, and surface localization. These findings prompted its selection for examination within the present study. Although IgdA and corresponding orthologs were shown to be unique to host-adapted lactobacilli, the Ig domain itself was specific to vertebrate-adapted species suggesting a role in vertebrate adaptation. Using a counterselective gene replacement system, igdA was deleted from the L. acidophilus NCFM chromosome. The resultant mutant, NCK2532, exhibited a visibly disrupted cell surface which likely contributed to its higher salt sensitivity, severely reduced adhesive capacity, and altered immunogenicity profile. Transcriptomic analyses revealed the induction of several stress response genes and secondary surface proteins. Due to the broad impact of IgdA on the cellular physiology and probiotic attributes of L. acidophilus, identification of similar proteins in alternative bacterial species may help pinpoint next-generation host-adapted probiotic candidates.
Highlights
Lactic acid bacteria (LAB) are a diverse group of Gram-positive, non-sporulating, microaerophilic organisms classified by the production of lactic acid as their signature fermentation end product (Claesson et al, 2007)
In the following study we provide evidence that IgdA plays a significant role in cell surface maintenance and, indirectly, host adaptation
The presence of S-layer proteins (Slps) and igdA orthologs was mapped to a lactobacilli phylogenetic tree constructed based on the pyk gene sequence (Figure 1A)
Summary
Lactic acid bacteria (LAB) are a diverse group of Gram-positive, non-sporulating, microaerophilic organisms classified by the production of lactic acid as their signature fermentation end product (Claesson et al, 2007). Within this order are several industry-relevant genera including Lactococcus, Enterococcus, Oenococcus, Pediococcus, Streptococcus, Leuconostoc, and many Lactobacillus species (Makarova et al, 2006). Research has begun to hone in on the molecules and genes associated with these health-promoting properties to augment next-generation probiotic screening efforts (Lebeer et al, 2008; Siciliano et al, 2019). Of particular interest are extracellular proteins, such as surface-layers (S-layers), which can directly interact with and influence the host gastrointestinal tract (Kleerebezem et al, 2010; Lebeer et al, 2010)
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