Antibiotic resilience in Xanthomonas axonopodis pv. punicae causing bacterial blight of pomegranate

Bacterial blight caused by Xanthomonas axonopodis pv. punicae (Xap) is one of the economically important diseases of pomegranate in India. Under field conditions, the disease is being managed using antibiotics and copper-based compounds but with limited success due to their poor bio-efficacy. The reduced efficacy of antibiotics and copper compounds against field populations of Xap might be due to the development of bactericide resistance through acquired genes. In the present study, ten bacterial blight-infected pomegranate samples were collected from different geographic locations of Karnataka, India, and causal agent Xap was isolated and identified through 16S rRNA sequencing. Streptomycin resistance genes such as rpsL, strA, strB and copper resistance genes copL, copB were detected using gene-specific primers in PCR. All ten isolates were positive for streptomycin resistance genes whereas copper resistance gene copB was absent in three isolates (Xap1, Xap4, Xap6) while copL was absent in Xap4 and Xap6 isolates. Further, in vitro experiments using different concentrations of streptomycin on culture media showed lowest growth inhibition up to 1500 mu g/ml concentration, supporting the molecular evidence of antibiotic resistance. The present study provides preliminary information on the presence of antibiotic resistance genes in the field populations of Xap.