Sarah Bardsley Capasso Passes Final Defense
Posted: November 24, 2015
Bacterial spot (caused by Xanthomonas arboricola pv. pruni) is the most important bacterial disease of peach and nectarine in the eastern United States. The antibiotic oxytetracycline is used to mitigate the yield-limiting symptoms of this disease. Despite that, yield loss remains high in susceptible stone fruit cultivars, raising concern among growers over the development of antibiotic resistance in the causal pathogen. Previous surveys of the stone fruit orchard bacterial community indicated the presence of oxytetracycline-resistant epiphytic bacteria. This was significant because epiphytic or nontarget bacteria often carry more antibiotic resistance genes and acquire them before their pathogenic neighbors. Under a strong selection pressure such as repeated antibiotic applications, transfer of resistance genes from epiphytic bacteria to pathogenic bacteria is favored. Therefore, when evaluating antibiotic resistance development in pathogenic bacteria, nontarget bacteria must also be considered. The overall goal of this research was to determine the consequences of repeated oxytetracycline applications in commercial Pennsylvania stone fruit orchards, including management factors related to the incidence of bacteria carrying tetracycline resistance genes and the sensitivity of X. arboricola pv. pruni isolates to oxytetracycline. Tetracycline resistance genes, tetA, tetB, tetC, were found in epiphytic bacteria recovered from commercial orchards. While the distribution of resistance genes significantly differed among the sampled orchards, oxytetracycline use was not a significant factor related to the incidence of resistance genes. Nevertheless, the tetracycline resistance genes conferred a high level of resistance (MIC > 450 ug/ml oxytetracycline) and were most commonly associated with Pantoea spp. and Pseudomonas spp. The sensitivity of X. arboricola pv. pruni isolates, remaining high (MIC < 25 ug/ml oxytetracycline), varied significantly among those collected from different orchards. This variability was significantly influenced by the number of oxytetracycline applications used in the field; however, no antibiotic resistance genes were found in these isolates. The molecular mechanism related to the variability of oxytetracycline sensitivity in these isolates is currently unknown.