The importance of our inner microbial communities for proper immune responses against invading pathogens is now well accepted, but the mechanisms underlying this protection are largely unknown. In this study, we used Caenorhabditis elegans to investigate such mechanisms. Since very little is known about the microbes interacting with C. elegans in its natural environment, we began by taking the first steps to characterize the C. elegans microbiota. We established a natural-like environment in which initially germfree, wild-type larvae were grown on enriched soil. Bacterial members of the adult C. elegans microbiota were isolated by culture and identified using 16S rRNA gene sequencing. Using pure cultures of bacterial isolates as food, we identified two, Bacillus megaterium and Pseudomonas mendocina, that enhanced resistance to a subsequent infection with the Gram-negative pathogen Pseudomonas aeruginosa. Whereas protection by B. megaterium was linked to impaired egg laying, corresponding to a known trade-off between fecundity and resistance, the mechanism underlying protection conferred by P. mendocina depended on weak induction of immune genes regulated by the p38 MAPK pathway. Disruption of the p38 ortholog, pmk-1, abolished protection. P. mendocina enhanced resistance to P. aeruginosa but not to the Gram-positive pathogen Enterococcus faecalis. Furthermore, protection from P. aeruginosa was similarly induced by a P. aeruginosa gacA mutant with attenuated virulence but not by a different C. elegans-associated Pseudomonas sp. isolate. Our results support a pivotal role for the conserved p38 pathway in microbiota-initiated immune protection and suggest that similarity between microbiota members and pathogens may play a role in such protection.

Background: Iron ions affect the expression of outer membrane protein A (OmpA), a major pathogenic protein in Acinetobacter baumannii.Objective: To analyze the effect of iron ions on the expression of the OmpA protein of A. baumannii and explore its association with the virulence of OmpA.Methods: Site-directed mutagenesis was used to construct ompA gene deletion strains and gene repair strains. The OmpA protein expression of A. baumannii under culture with different contents of iron ions was detected. The virulence of A. baumannii with different OmpA protein expression levels were evaluated in macrophages and mice.Results: OmpA protein levels of the three strains were enhanced under iron-rich conditions. They were reduced in the presence of the iron-chelating agent 2,2′-bipyridine. A. baumannii wild type and + ompA had a remarkable toxic effect on RAW246.7 macrophages (P < 0.05). In contrast, the ΔompA had a significantly reduced toxic effect on RAW246.7 macrophages (P < 0.05). The levels of the inflammatory factors IL-1β, IL-6, IL-8, and TNFα in the mice spleen were significantly increased in the + ompA strain treatment group compared with the ΔompA strain group (all P < 0.05). In addition, the levels were higher in the presence of iron ions than in the presence of the chelating agent.Conclusion: Iron-rich conditions increase the OmpA protein expression of A. baumannii. Strains with high OmpA protein expression were more invasive, which may be a key determinant of A. baumannii infection and pathogenicity. Iron control strategies might be used for the management of A. baumannii.