Extensive Phenotypic Changes Associated with Large-scale Horizontal Gene Transfer

Abstract

!Horizontal gene transfer often leads to phenotypic changes within recipient organisms independent of any immediate evolutionary benefits. While secondary phenotypic effects of horizontal transfer (i.e. changes in growth rates) have been demonstrated and studied across a variety of systems using relatively small plasmid and phage, little is known about how size of the acquired region affects the magnitude or number of such costs. Here we describe an amazing breadth of phenotypic changes which occur after a large-scale horizontal transfer event (~1Mb megaplasmid) withinPseudomonas stutzeri including sensitization to various stresses as well as changes in bacterial behavior. These results highlight the power of horizontal transfer to shift pleiotropic relationships and cellular networks within bacterial genomes. They also provide an important context for how secondary effects of transfer can bias evolutionary trajectories and interactions between species. Lastly, these results and system provide a foundation to investigate evolutionary consequences in real time as newly acquired regions are ameliorated and integrated into new genomic contexts.

Genomic Background Governs Opposing Responses to Nalidixic Acid Upon Megaplasmid Acquisition inPseudomonas

“…Our previously published experiments thoroughly explored how acquisition of pMPPla107 increases nalidixic acid sensitivity in Pseudomonas stutzeri 28a84 (13), and here we demonstrate that this effect is replicated by a simple modified assay based on Kirby-Bauer disc diffusion assays ( 17, Figure 1A). Since antibiotics diffuse out from the disc into the media forming a concentration gradient, size of the inhibition halo is positively correlated with sensitivity to nalidixic acid.…”

“…Here we demonstrate that the direction of one previously observed phenotype associated with acquisition of megaplasmid pMPPla107, sensitivity to nalidixic acid in Pseudomonas (13), depends upon the interactions between this megaplasmid and genomic background of the strain of interest. Across almost all surveyed strains, acquisition of pMPPla107 sensitizes cells to nalidixic acid, but in P. putida KT2440 this same horizontal transfer event increases resistance to this antibiotic.…”

“…We have previously shown that acquisition of pMPPla107 by P. stutzeri 28a84 leads to dramatic fitness costs when measured by growth rate and by competitive fitness assay, and have demonstrated that growth in the presence of nalidixic acid increases this fitness cost (12,13). We therefore carried out competitive fitness assays of differentially marked P. putida strains Since megaplasmid acquisition by P. putida appears to increase nalidixic acid resistance according to Kirby-Bauer diffusion assays, we tested whether supplementation of media with nalidixic acid could shift the outcomes of competitive fitness assays between wild type and a megaplasmid containing strain.…”

“…We have previously reported numerous phenotypic changes that occur as a consequence of acquisition of a ~1Mb megplasmid, pMPPla107, by Pseudomonas strains (12,13). In addition to fitness costs correlated with slower growth, acquisition of pMPPla107 alters a variety of other phenotypes in some strains including: increased motility, decreased biofilm formation, decreased thermal tolerance, increased sensitivity to an unidentified compound produced by Pseudomonads, and increased sensitivity to quinolone antibiotics like nalidixic acid and ciprofloxacin.…”