Phase Variation. Another mechanism of immune evasion is to alter the expression or composition of antigenic structures recognized by the host. Two large structures found on the P. mirabilis outer membrane which generate a humoral response in mice include fimbriae and flagella (61, 84). The MR/P fimbria are phase variable through a mechanism of site- specific inversion. The mrp operon, encoding the MR/P fimbria, is transcribed divergently from mrpI encoding the recombinase MrpI (85). The mrp operon is preceded by a σ70 promoter within an invertible repeat which is under MrpI control (85). When the σ70 promoter faces the mrp operon, the MR/P is phase ON. When the σ70 promoter faces mrpI, MR/P is phase OFF (51). The mrp promoter orientation is fixed in the absence of MrpI suggesting it is the sole recombinase for the operon (51). Another element affecting MR/P expression is the last gene in the mrp operon, mrpJ, which encodes a transcriptional regulator. MrpJ activates expression of the mrp operon promoting MR/P phase ON (86). Additionally, there are 14 paralogs of mrpJ in P. mirabilis: 10 paralogs are a part of fimbrial operons and 4 mrpJ paralogs are orphans (87). It is unknown if any of the MR/P paralogs or the other fimbrial systems in P. mirabilis are phase variable, but altering expression of any of these genes could contribute to fimbrial diversity and subsequent fitness within the host. The effect of the phase variable mrp operon is not limited to MR/P fimbriation as it extends to the repression of motility. Locking MR/P in phase OFF increases swarming motility (50), and artificial overexpression of 12 mrpJ paralogs thwarts motility to different degrees, with some paralogs preventing swarmer cell differentiation altogether (87). MrpJ and its paralog UcaJ directly bind upstream of the master flagellar operon, flhDC, mediating direct repression (88). The other mrpJ paralogs contain a conserved DNA-binding domain, but it is unknown if they can bind flhDC. Transcriptome profiling of P. mirabilis during urinary tract infection revealed the mrp genes are the most highly expressed during early infection (1-3 days) but their expression decreased in later infection (3-7 days). Flagellar genes were the most down-regulated genes in early infection, but in later stages of infection their expression increased (52). Overall, P. mirabilis is fervently committed to inversely expressing fimbriae and flagella. Altering the expression of these structures has implications for infection a...
Phase Variation. Phase variation has been defined by Xxxxxxxx et al. (211) as a process of reversible, high-frequency phenotypic switching that is mediated by DNA mutations, reorganisation or modification. Phase variation is used by several bacterial species to generate population diversity that increases bacterial fitness and is important in niche adaptation including immune evasion. Phase variation occurs at a high frequency of >10-5 switches per cell per generation (98) and can result in reversible ON or OFF switching of traits or in the variation of surface phenotypes. The main advantage of phase variation, by altering the expression of traits or by varying the character of these traits, in the survival of micro- organisms, is the ability to adapt to an environment or to cope with host defense systems. Phase variation is one of the mechanisms enabling pathogens to survive in the host by escaping the immune respons (52). This is illustrated by the fact that phase variation poses a problem in vaccine production due to the high frequency of variation in epitopes exposed by the pathogen (163, 188). Although phase variation, or antigenic variation, is primarily being associated with host-pathogen interactions, a number of reports describe phase variation in a broader context. These reports show that phase variation is also involved in the production of exo-enzymes and secondary metabolites (38, 250) indicating that phase variation can have a much broader impact on the ecology of bacteria, affecting a high number of traits and processes, and therefore phase variation is not only relevant in host-pathogen interactions but also in more ecological and industrial processes. The aim of this thesis is to study the mechanism of phase variation in antagonistic Pseudomonas bacteria in relation to efficient biocontrol of phytopathogens.
