The L-form of Pseudomonas syringae pv. phaseolicola has been proved to induce resistance to bean halo blight.Various procedures were tested to induce the L-form of Pseudomonas syringae pv. pisi for its potential use as biocontrol agent of pea bacterial blight. Cell-wall deficient cells were induced in a liquid medium with penicillin following a protocol described for P. s. pv. phaseolicola. Cell growth on solid induction medium developed as typical granular and vacuolated structures, and characteristic colonies were observed in the first transfer. However, there was poor growth in subsequent transfers and some reversion to the parental type. To improve the induction procedure, the following new procedures were applied: (1) viability of cells was monitored during induction. The optimum induction time in liquid medium with penicillin was lower for pv. pisi than for pv. phaseolicola. Viability of L-forms in solid induction medium with penicillin was low and decreased in time. (2) the inducer ticarcillin was combined with clavulanic acid, which prevented the reversion to the parental type and (3) a range of concentrations of penicillin and ticarcillin/clavulanic acid was applied by the spiral gradient endpoint method for calculation of minimum inhibitory concentrations (MIC). Based on the results from these tests an induction method for P. s. pv. pisi L-form is proposed and the relevance of L-form is discussed for practice.
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