The alternative sigma factor HrpL negatively modulates the flagellar system in the phytopathogenic bacterium Erwinia amylovora under hrp-inducing conditions.

In this work we present evidence of an opposite regulation in the phytopathogenic bacteria Erwinia amylovora between the virulence-associated Type III secretion system (TTSS) and the flagellar system. Using loss-of-function mutants we show that motility enhanced the virulence of wild-type bacteria relative to a nonmotile mutant when sprayed on apple seedlings with unwounded leaves. Then we demonstrated through analyses of motility, flagellin export and visualization of flagellar filament that HrpL, the positive key regulator of the TTSS, also down-regulates the flagellar system. Such a dual regulation mediated by an alternative sigma factor of the TTSS appears to be a level of regulation between virulence and motility not yet described among Proteobacteria.

Fire blight protection with avirulent mutants of Erwinia amylovora.

Fire blight is a necrotic disease caused by the bacterium Erwinia amyiovora, which affects pears, apples and ornamentals including Crataegus, Pyracantha, and Cotoneaster. The disease can be only partially controlled, through the use of resistant genotypes, cultural measures and antibacterial compounds, thus other methods must be investigated. It has long been established that avirulent isolates of the pathogen can control the disease, under experimental conditions. However, field use of avirulent isolates is not acceptable because of their unknown genetic stability. The protective ability under controlled conditions of genetically characterized avirulent insertion mutants of E. amylovora was examined. A bioassay on apple seedlings was used for the determination of the protective ability of 34 insertion mutants (hrp, dsp, ams). Some protective effect could be observed with most of the mutants tested and was dependent on the avirulent/virulent inoculum ratio as well as on the level of virulence of the pathogen; a minimal concentration of the avirulent mutant was necessary to give a significant level of protection. An early competition between avirulent and virulent strains for putative infection sites might be involved. For six of the mutants tested, the protective ability was particularly high and might be related to the alteration of regulatory functions of hrp genes. Results obtained with Ams- and Ams- Hrp- mutants suggested that the bacterial exopolysaccharide might play a role in the protection.