The composition of lipopolysaccharide (LPS) extracted with aqueous phenol from a virulent English plum isolate of pv. varied according to the partial pressure of oxygen (pO) in the culture medium at the time of harvest. When pO was low, the organism grew slowly and produced smooth LPS bearing rhamnan sidechains. As pO was raised, the rate of growth increased and smooth LPS was replaced by a rough species deficient in rhamnose, which co-extracted with a -glucan. Organization of rhamnose and glucose into separate polymers was shown by the selective susceptibility of the rhamnose-containing polymer to hydrolysis by rhamnanase of the phage A7. By methylation analysis, GC-MS, and H- and C-NMR spectroscopy, the glucan was shown to consist of α(1 → 4)-linked residues with α(1 → 4,6)-branch points and non-reducing terminal residues in the approximate ratio 4:1:1, resembling glycogen in composition. A glucan which co-extracted with LPS using phenol/water from an avirulent plum isolate that was resistant to lysis by phages A1 and A7 was shown by methylation analysis to have a similar structure. Whether the effect on LPS composition was due directly to pO, or was dependent on the rate of growth, has not been established. It is suggested that, because epiphytic growth would entail exposure to high pO, English plum isolates growing on the surfaces of host plants might be unable to produce smooth LPS. Since cell surface composition affects virulence in plant-pathogenic pseudomonads, this effect could account for the observed failure of the English plum isolates to enter the host via leaf scars.


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