Adaptation of genetically monomorphic bacteria: evolution of copper resistance through multiple horizontal gene transfers of complex and versatile mobile genetic elements.

Copper-based antimicrobial compounds are widely used to control plant bacterial pathogens. Pathogens have adapted in response to this selective pressure. Xanthomonas citri pv. citri, a major citrus pathogen causing Asiatic citrus canker, was first reported to carry plasmid-encoded copper resistance in Argentina. This phenotype was conferred by the copLAB gene system. The emergence of resistant strains has since been reported in Réunion and Martinique. Using microsatellite-based genotyping and copLAB PCR, we demonstrated that the genetic structure of the copper-resistant strains from these three regions was made up of two distant clusters and varied for the detection of copLAB amplicons. In order to investigate this pattern more closely, we sequenced six copper-resistant X. citri pv. citri strains from Argentina, Martinique and Réunion, together with reference copper-resistant Xanthomonas and Stenotrophomonas strains using long-read sequencing technology. Genes involved in copper resistance were found to be strain dependent with the novel identification in X. citri pv. citri of copABCD and a cus heavy metal efflux resistance-nodulation-division system. The genes providing the adaptive trait were part of a mobile genetic element similar to Tn3-like transposons and included in a conjugative plasmid. This indicates the system's great versatility. The mining of all available bacterial genomes suggested that, within the bacterial community, the spread of copper resistance associated with mobile elements and their plasmid environments was primarily restricted to the Xanthomonadaceae family.

High-Quality Draft Genome Sequences of Two Xanthomonas Pathotype Strains Infecting Aroid Plants.

We present here the draft genome sequences of bacterial pathogens of the Araceae family, Xanthomonas axonopodis pv. dieffenbachiae LMG 695 and Xanthomonas campestris pv. syngonii LMG 9055, differing in host range. A comparison between genome sequences will help understand the mechanisms involved in tissue specificity and adaptation to host plants.

Draft Genome Sequence of Xanthomonas axonopodis pv. allii Strain CFBP 6369.

We report here the draft genome sequence of Xanthomonas axonopodis pv. allii strain CFBP 6369, the causal agent of bacterial blight of onion. The draft genome has a size of 5,425,942 bp and a G+C content of 64.4%.

Beauveria brongniartii on white grubs attacking sugarcane in South Africa.

Beauveria brongniartii (Saccardo) Petch fungal infections were observed on the melolonthid Hypopholis sommeri Burmeister (Coleoptera: Scarabaeidae) at two sites (Harden Heights and Canema) in the sugarcane producing area of the northern KwaZulu-Natal Midlands of South Africa. To initially identify the disease-causing fungus, 17 different fluorescently-labelled microsatellite PCR primers were used to target 78 isolates of Beauveria spp. DNA. Microsatellite data resolved two distinct clusters of Beauveria isolates which represented the Beauveria bassiana s.s. (Balsamo) Vuillemin (17 isolates) and B. brongniartii (60 isolates) species groups. These groupings were supported by two gene regions, the nuclear ribosomal Internal Transcribed Spacer (ITS) and the nuclear Bloc gene of which 23 exemplar Beauveria isolates were represented and sequenced. When microsatellite data were analysed, 26 haplotypes among 58 isolates of B. brongniartii were distinguished. Relatively low levels of genetic diversity were detected in B. brongniartii and isolates were shown to be closely related. No genetic differentiation was observed between the Harden Heights and Canema populations; they thus may be considered one, structured and fragmented population over a distance of 5.5 km. Historically high levels of gene flow from swarming H. sommeri beetles is the proposed mechanism for this observed lack of genetic differentiation between populations. Microsatellite analyses also showed that B. brongniartii conidia were being cycled from arboreal forest to subterranean sugarcane habitats and vice versa in the environment by H. sommeri life stages. This is the first record of this species of fungus infecting H. sommeri larvae and adults in South Africa.