Characterization of the acyl carrier protein gene and the fab gene locus in Xanthomonas albilineans

Validation of an in vitro system for studies of pathogenicity mechanisms in Xanthomonas campestris

Several minimal media capable of inducing pathogenicity genes have been used to study plant-pathogen interactions. An in planta assay to study a closer interaction between the bacteria and the host was also developed and has been employed by our group. In order to determine whether growth medium could be improved to better approximate in planta conditions beyond that offered by the defined minimal medium XVM1, we compared the expression of 20 Xanthomonas campestris pv. campestris (Xcc) genes by quantitative reverse transcription - polymerase chain reaction (qRT-PCR) under in vivo (bacteria recovered from the plant) and in vitro (rich medium NYG, minimal medium XVM1 and XVM1 + leaf extract) growth systems. The results showed a higher expression level of the genes in the in planta system when compared to growth in culture media. In planta growth is closest to a real interaction condition and captures the complexity of the plant cell environment; however, this system has some limitations. The main finding of our work is that the addition of plant extract to XVM1 medium results in a gene expression profile that better matches the in planta profile, when compared with the XVM1 medium alone, giving support to the use of plant extract to study pathogenicity mechanisms in Xanthomonas.

A DHA14 drug efflux gene from Xanthomonas albilineans confers high-level albicidin antibiotic resistance in Escherichia coli

AIMS

Identification of a gene for self-protection from the antibiotic-producing plant pathogen Xanthomonas albilineans, and functional testing by heterologous expression.

METHODS AND RESULTS

Albicidin antibiotics and phytotoxins are potent inhibitors of prokaryote DNA replication. A resistance gene (albF) isolated by shotgun cloning from the X. albilineans albicidin-biosynthesis region encodes a protein with typical features of DHA14 drug efflux pumps. Low-level expression of albF in Escherichia coli increased the MIC of albicidin 3000-fold, without affecting tsx-mediated albicidin uptake into the periplasm or resistance to other tested antibiotics. Bioinformatic analysis indicates more similarity to proteins involved in self-protection in polyketide-antibiotic-producing actinomycetes than to multi-drug resistance pumps in other gram-negative bacteria. A complex promoter region may co-regulate albF with genes for hydrolases likely to be involved in albicidin activation or self-protection.

CONCLUSIONS

AlbF is the first apparent single-component antibiotic-specific efflux pump from a gram-negative antibiotic producer. It shows extraordinary efficiency as measured by resistance level conferred upon heterologous expression.

SIGNIFICANCE AND IMPACT OF THE STUDY

Development of the clinical potential of albicidins as potent bactericidial antibiotics against diverse bacteria has been limited because of low yields in culture. Expression of albF with recently described albicidin-biosynthesis genes may enable large-scale production. Because albicidins are X. albilineans pathogenicity factors, interference with AlbF function is also an opportunity for control of the associated plant disease.