A Salmonella enterica serovar Typhimurium genome-wide CRISPRi screen reveals a role for type 1 fimbriae in evasion of antibody-mediated agglutination.

The O5-specific monoclonal IgA antibody, Sal4, mediates the conversion of Salmonella enterica serovar Typhimurium (STm) from virulent, free-swimming cells to non-motile, multicellular biofilm-like aggregates within a matter of hours. We hypothesize that the rapid transition from an invasive to a non-invasive state is an adaptation of STm to Sal4 IgA exposure. In this report, we performed a genome-wide CRISPR interference (CRISPRi) screen to identify STm genes that influence multicellular aggregate formation in response to Sal4 IgA treatment. From a customized library of >36,000 spacers, ~1% (373) were enriched at the top of the culture supernatant after two consecutive rounds of Sal4 IgA treatment. The enriched spacers mapped to a diversity of targets, including genes involved in O-antigen modification, cyclic-di-GMP metabolism, outer membrane biosynthesis/signaling, and invasion/virulence, with the most frequently targeted gene being fimW, which encodes a negative regulator of type 1 fimbriae (T1F) expression. Generation of a STm ΔfimW strain confirmed that the loss of FimW activity results in a hyperfimbriated phenotype and evasion of Sal4 IgA-mediated agglutination in solution. Closer examination of the fimW mutant revealed its propensity to form biofilms at the air-liquid interface in response to Sal4 exposure, suggesting that T1F "primes" STm to transition from a planktonic to a sessile state, possibly by facilitating bacterial attachment to abiotic surfaces. These findings shed light on the mechanism by which IgA antibodies influence STm virulence in the intestinal environment.