The role of phenotypic variation in rhizosphere Pseudomonas bacteria

Abstract

SummaryColony phase variation is a regulatory mechanism at the DNA level which usually results in high frequency, reversible switches between colonies with a different phenotype. A number of molecular mechanisms underlying phase variation are known: slipped-strand mispairing, genomic rearrangements, spontaneous mutations and epigenetic mechanisms such as differential methylation. Most examples of phenotypic variation or phase variation have been described in the context of host-pathogen interactions as mechanisms allowing pathogens to evade host immune responses. Recent reports indicate that phase variation is also relevant in competitive root colonization and biological control of phytopathogens. Many rhizospere Pseudomonas species show phenotypic variation, based on spontaneous mutation of the gacA and gacS genes. These morphological variants do not express secondary metabolites and have improved growth characteristics. The latter could contribute to efficient root colonization and success in competition, especially since (as shown for one strain) these variants were observed to revert to their wild-type form. The observation that these variants are present in rhizosphere-competent Pseudomonas bacteria suggests the existence of a conserved strategy to increase their success in the rhizosphere.

Coordinated phenotype switching with large-scale chromosome flip-flop inversion observed in bacteria

“…1A) because it grew slowly on routine media and yielded aberrant small colonies (SCs). SCVs are usually associated with persistent and relapsing infections (6,7). During subculture testing, we noted an unexpected feature of the Mu50Ω colony population.…”

“…Since then, being implicated in persistent, relapsing, and antibiotic-resistant infections, SCV has been described for a wide range of bacterial genera and species, including Staphylococcus, Pseudomonas, Burkholderia, Salmonella, Vibrio, Shigella, Brucella, Escherichia, Lactobacillus, Serratia, and Neisseria species (6). Although there are many studies addressing the mechanism of SCV occurrence, where the findings include electron transport deficiencies or menadione, hemin, or thymidine auxotrophicity attributable to the mutations in genes associated with the bacterial metabolic pathway, there are no data available about the genetic events responsible for the reappearance of a rapidly growing form (WT), namely, the normal colony variant (NCV), and the frequently reversible reversion between SCV and NCV (6)(7)(8)(9)(10).…”

Modulation of Host Immunity by Beneficial Microbes

“…Phase variation provides bacteria with a significant advantage of adaptation to different environments and has been extensively documented in several studies as a mechanism that animal pathogens employ to escape immune detection (Kingsley and Baumler 2000). Phenotypic variation is also common among rhizosphere pseudomonads and has been reported as a conserved strategy that bacteria have evolved in order to increase their overall fitness in the rhizosphere (Van den Broek et al 2005). Rhizosphere Pseudomonas bacteria may use antigen variation to reduce their antigenic potential and, therefore, minimize stimulation of the host's immune system.…”

A bacterial symbiont is converted from an inedible producer of beneficial molecules into food by a single mutation in the gacA gene

“…Spontaneous mutations disabling the GacAGacS system have been reported in P. fluorescens that were mainly characterized by a change in colony morphology (33), not unlike the differences in morphology between the PfA and PfB strains in this study. Furthermore, such mutations in pseudomonads are known to occur in the rhizosphere of plants where they display a role in competitive root colonization (34). The point mutation observed in PfB introduces a premature stop codon upstream of the helix-turn-helix motif required for DNA binding, and inability to bind DNA prevents the GacS response regulator from activating transcription (35) (Fig.…”