The conserved global regulator H-NS has a strain-specific impact on biofilm formation in Vibrio fischeri symbionts

Strain-level variation among host-associated bacteria often determines host range and the extent to which colonization is beneficial, benign, or pathogenic. Vibrio fischeri is a beneficial symbiont of the light organs of fish and squid with known strain-specific differences that impact host specificity, colonization efficiency, and interbacterial competition. Here, we describe how the conserved global regulator, H-NS, has a strain-specific impact on a critical colonization behavior: biofilm formation. We isolated a mutant of the fish symbiont V. fischeri MJ11 with a transposon insertion in the hns gene. This mutant formed sticky, moderately wrinkled colonies on LBS plates, a condition not known to induce biofilm in this species. A reconstructed hns mutant displayed the same wrinkled colony, which became smooth when hns was complemented in trans, indicating the hns disruption is causal for biofilm formation in MJ11. Transcriptomes revealed differential expression for the syp biofilm locus in the hns mutant, relative to the parent, suggesting biofilm may in part involve SYP polysaccharide. However, enhanced biofilm in the MJ11 hns mutant was not sufficient to allow colonization of a non-native squid host. Finally, moving the hns mutation into other V. fischeri strains, including the squid symbionts ES114 and ES401, and seawater isolate PP3, revealed strain-specific biofilm phenotypes: ES114 and ES401 hns mutants displayed minimal biofilm phenotypes while PP3 hns mutant colonies were more wrinkled than the MJ11 hns mutant. These findings together define H-NS as a novel regulator of V. fischeri symbiotic biofilm and demonstrate key strain specificity in that role. ImportanceThis work, which shows how H-NS has strain-specific impacts on biofilm in Vibrio fischeri, underscores the importance of studying multiple strains, even when examining highly conserved genes and functions. Our observation that knocking out a conserved regulator can result in a wide range of biofilm phenotypes, depending on the isolate, serves as a powerful reminder that strain-level variation is common and worthy of exploration. Indeed, uncovering the mechanisms of strain-specific phenotypic differences is essential to understand drivers of niche differentiation and bacterial evolution. Thus, it is important to carefully match the number and type of strains used in a study with the research question to accurately interpret and extrapolate the results beyond a single genotype. The additional work required for multi-strain studies is often worth the investment of time and resources, as it provides a broader view of the complexity of within-species diversity in microbial systems.