Environmental and physiological determinants of oxytetracycline efficacy against Erwinia amylovora

초록

Fire blight is a bacterial disease of apple and pear caused by Erwinia amylovora and results in serious economic losses worldwide. Although the bactericidal antibiotic streptomycin has been widely studied, comparatively little is known about the population-level responses of E. amylovora to oxytetracycline, a bacteriostatic antibiotic used for fire blight management. In this study, we investigated changes in culturability and physiological subpopulations of E. amylovora following oxytetracycline exposure and evaluated its efficacy under multiple conditions, including nutrient availability, external pH, proton motive force (PMF) perturbation, and antibiotic treatment sequence with streptomycin. Short-term oxytetracycline treatment did not affect the culturability of E. amylovora, whereas long-term treatment caused a gradual decline in culturability. Long-term exposure to oxytetracycline also induced nutrient-dependent physiological shifts in the population. Notably, under nutrient-deficient conditions, the proportion of dead cells increased markedly over time, accompanied by a decrease in viable cells. Viable but nonculturable (VBNC) cells were detected only at a low frequency in samples collected at day 14 under nutrient-deficient conditions. External pH strongly influenced oxytetracycline activity at pH 4.0-5.0 and in immature apple broth resulted in a rapid decline in culturability. Disruption of PMF further accelerated the loss of culturability under nutrient-deficient conditions and was associated with sustained membrane depolarization signals. In sequential antibiotic treatments, oxytetracycline pretreatment reduced the initial bactericidal effect of streptomycin and produced distinct physiological population structures during long-term exposure. Collectively, these results suggest that optimal oxytetracycline efficacy for fire blight control requires consideration of environmental conditions as well as the treatment sequence with streptomycin.

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