Search for: All records

ABSTRACT Streptomycin (Sm) is a commonly used antibiotic for its efficacy against diverse bacteria. The plant pathogenAgrobacterium fabrumis a model for studying pathogenesis and interkingdom gene transfer. Streptomycin-resistant variants ofA. fabrumare commonly employed in genetic analyses, yet mechanisms of resistance and susceptibility to streptomycin in this organism have not previously been investigated. We observe that resistance to a high concentration of streptomycin arises at high frequency inA. fabrum, and we attribute this trait to the presence of a chromosomal gene (strB) encoding a putative aminoglycoside phosphotransferase. We show howstrB, along withrpsL(encoding ribosomal protein S12) andrsmG(encoding a 16S rRNA methyltransferase), modulates streptomycin sensitivity inA. fabrum. IMPORTANCEThe plant pathogenAgrobacterium fabrumis a widely used model bacterium for studying biofilms, bacterial motility, pathogenesis, and gene transfer from bacteria to plants. Streptomycin (Sm) is an aminoglycoside antibiotic known for its broad efficacy against gram-negative bacteria.A. fabrumexhibits endogenous resistance to somewhat high levels of streptomycin, but the mechanism underlying this resistance has not been elucidated. Here, we demonstrate that this resistance is caused by a chromosomally encoded streptomycin-inactivating enzyme, StrB, that has not been previously characterized inA. fabrum. Furthermore, we show how the genesrsmG,rpsL, andstrBjointly modulate streptomycin susceptibility inA. fabrum. 

Microbiota comprise the bulk of life’s diversity, yet we know little about how populations of microbes accumulate adaptive diversity across natural landscapes. Adaptation to stressful soil conditions in plants provides seminal examples of adaptation in response to natural selection via allelic substitution. For microbes symbiotic with plants however, horizontal gene transfer allows for adaptation via gene gain and loss, which could generate fundamentally different evolutionary dynamics. We use comparative genomics and genetics to elucidate the evolutionary mechanisms of adaptation to physiologically stressful serpentine soils in rhizobial bacteria in western North American grasslands. In vitro experiments demonstrate that the presence of a locus of major effect, thenreoperon, is necessary and sufficient to confer adaptation to nickel, a heavy metal enriched to toxic levels in serpentine soil, and a major axis of environmental soil chemistry variation. We find discordance between inferred evolutionary histories of the core genome andnreAXYgenes, which often reside in putative genomic islands. This suggests that the evolutionary history of this adaptive variant is marked by frequent losses, and/or gains via horizontal acquisition across divergent rhizobium clades. However, differentnrealleles confer distinct levels of nickel resistance, suggesting allelic substitution could also play a role in rhizobium adaptation to serpentine soil. These results illustrate that the interplay between evolution via gene gain and loss and evolution via allelic substitution may underlie adaptation in wild soil microbiota. Both processes are important to consider for understanding adaptive diversity in microbes and improving stress-adapted microbial inocula for human use. 

The genetic and molecular basis of flagellar motility has been investigated for several decades, with innovative research strategies propelling advances at a steady pace. Furthermore, as the phenomenon is examined in diverse bacteria, new taxon-specific regulatory and structural features are being elucidated. Motility is also a straightforward bacterial phenotype that can allow undergraduate researchers to explore the palette of molecular genetic tools available to microbiologists. This study, driven primarily by undergraduate researchers, evaluated hundreds of flagellar motility mutants in the Gram-negative plant-associated bacteriumAgrobacterium fabrum. The nearly saturating screen implicates a total of 37 genes in flagellar biosynthesis, including genes of previously unknown function.