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

UNLABELLED

Strain-level variation among host-associated bacteria often determines host range and the extent to which colonization is beneficial, benign, or pathogenic. 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 MJ11 with a transposon insertion in the gene. This mutant formed sticky, moderately wrinkled colonies on LBS plates, a condition not known to induce biofilm in this species. A reconstructed mutant displayed the same wrinkled colony, which became smooth when was complemented , indicating the disruption is causal for biofilm formation in MJ11. Transcriptomes revealed differential expression for the biofilm locus in the mutant, relative to the parent, suggesting biofilm may in part involve SYP polysaccharide. However, enhanced biofilm in the MJ11 mutant was not sufficient to allow colonization of a non-native squid host. Finally, moving the mutation into other strains, including the squid symbionts ES114 and ES401, and seawater isolate PP3, revealed strain-specific biofilm phenotypes: ES114 and ES401 mutants displayed minimal biofilm phenotypes while PP3 mutant colonies were more wrinkled than the MJ11 mutant. These findings together define H-NS as a novel regulator of symbiotic biofilm and demonstrate key strain specificity in that role.

IMPORTANCE

This work, which shows how H-NS has strain-specific impacts on biofilm in , 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.