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 aggregates in solution ("flocs") as well as biofilm-like associations at the air-liquid interface (ALI). We hypothesize that the rapid transition from an invasive to a non-invasive state reflects an adaptation of STm to Sal4 IgA exposure. In this report, we performed a genome-wide 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 ALI 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 {Delta}fimW strain confirmed that the loss of FimW 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 ALI specifically 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 protective secretory IgA antibodies influence STm virulence in the intestinal environment.