Single- or double-membrane-bound vesicles and P, Ca, and Fe-containing granules in Xanthomonas citri cultured on a solid medium.

The organelle-like structures of Xanthomonas citri, a bacterial pathogen that causes citrus canker, were investigated using an analytical transmission electron microscope. After high-pressure freezing, the bacteria were then freeze-substituted for imaging and element analysis. Miniscule electron-dense structures of varying shapes without a membrane enclosure were frequently observed near the cell poles in a 3-day culture. The bacteria formed cytoplasmic electron-dense spherical structures measuring approximately 50 nm in diameter. Furthermore, X. citri produced electron-dense or translucent ellipsoidal intracellular or extracellular granules. Single- or double-membrane-bound vesicles, including outer-inner membrane vesicles, were observed both inside and outside the cells. Most cells had been lysed in the 3-week X. citri culture, but they harbored one or two electron-dense spherical structures. Contrast-inverted scanning transmission electron microscopy images revealed distinct white spherical structures within the cytoplasm of X. citri. Likewise, energy-dispersive X-ray spectrometry showed the spatial heterogeneity and co-localization of phosphorus, oxygen, calcium, and iron only in the cytoplasmic electron-dense spherical structures, thus corroborating the nature of polyphosphate granules.

Growth temperature-dependent variation of cell envelope lipids and antibiotic susceptibility in Stenotrophomonas (Xanthomonas) maltophilia.

Clinical isolates of Stenotrophomonas (Xanthomonas) maltophilia showed growth temperature-dependent variation in susceptibility (TDVS) to aminoglycoside antibiotics between 30 degrees C and 37 degrees C, but little or no TDVS effect for polymixin B, colistin, ceftazidime, chloramphenicol and piperacillin. When phenylethanol was added at sub-inhibitory concentrations, the TDVS effect was eliminated. Gas liquid chromatography showed that 13-methyl tetradecanoate (i-15;0), was the predominant fatty acid, and was present in lower proportions in cells grown at 30 degrees C than 37 degrees C, by contrast to the unsaturated acids, which were found in increased proportions in cells grown at 30 degrees C. However, the extent of these shifts in composition did not correlate with the extent of the TDVS effect in individual strains. Membrane analysis by spin label-electron spin resonance spectroscopy showed that strains exhibiting TDVS had significantly decreased membrane fluidity compared with susceptible strains at 30 degrees C. Furthermore, analysis of the outer and cytoplasmic membranes from the strains with TDVS revealed that in organisms grown at 30 degrees C, the outer membrane remained in a more rigid conformation than the cytoplasmic membrane. We conclude that resistance of S. maltophilia to aminoglycoside antibiotics at 30 degrees C correlates with changes in the conformation of the outer membrane so that binding and/or uptake of the antibiotic is inhibited.

Differential binding capacity and internalisation of bacterial substrates as factors in growth rate of Acanthamoeba spp.

Acanthamoeba spp. are free-living predators that selectively feed on bacteria. Adherence of the bacterial food source to the trophozoite membrane is followed by internalisation and digestion. Through co-cultivation of A. castellanii and A. polyphaga, individually, with Xanthomonas maltophilia, Escherichia coli, Staphylococcus epidermidis and Pseudomonas aeruginosa (despite the amoebicidal properties of the latter organism), specificity with regard to the preferred bacterial substrate was judged. X. maltophilia and P. aeruginosa adhered almost immediately forming a multilayered mantle of bacilli around trophozoites of both species of amoebae. E. coli adhered to fewer trophozoites and in smaller numbers. X. maltophilia was readily internalised after co-cultivation for 8 h, whereas P. aeruginosa, E. coli and S. epidermidis were not internalised even after co-cultivation for 24 h. These data suggest that the suitability of a bacterial food source for the Acanthamoeba spp. studied is associated not only with the proclivity with which the bacterial species binds to the trophozoite surface, but also with the rate of its internalisation.