Xanthan-deficient mutants of Xanthomonas axonopodis pv. citri, the bacterium responsible for citrus canker, were generated by deletion and marker exchange of the region encoding the carboxy-terminal end of the first glycosyltransferase, GumD. Mutants of gumD did not produce xanthan and remained pathogenic in citrus plants to the same extent as wild-type bacteria. The kinetics of appearance of initial symptoms, areas of plant material affected, and growth of bacteria inside plant tissue throughout the disease process were similar for both wild-type and mutant inoculations. Moreover, exopolysaccharide deficiency did not impair the ability of the bacteria to induce hypersensitive response on non-host plants. Apart from variations in phenotypic aspects, no differences in growth or survival under different stress conditions were observed between the xanthan-deficient mutant and wild-type bacteria. However, gumD mutants displayed impaired survival under oxidative stress during stationary phase as well as impaired epiphytic survival on citrus leaves. Our results suggest that xanthan does not play an essential role in citrus canker at the initial stages of infection or in the incompatible interactions between X. axonopodis pv. citri and non-host plants, but facilitates the maintenance of bacteria on the host plant, possibly improving the efficiency of colonization of distant tissue.

We previously associated the emergence of carbapenem resistance in Acinetobacter baumannii with the loss of an outer membrane (OM) protein designated CarO. CarO was found essential for L L-ornithine uptake: CarO-deficient strains were specifically impaired to grow only on L L-ornithine, and failed to incorporate L L-[ 14 C] ornithine from the medium. L L-arginine, and histidine and lysine to a lower extent, could effectively compete for L L-[ 14 C] ornithine uptake. L L-ornithine also reduced A. baumannii sensitivity to imipenem, suggesting that both compounds compete for uptake. The overall results indicate that CarO participates in the selective uptake of L L-ornithine, carbapenems, and other basic amino acids in A. baumannii.