Characterization of a novel trimethoprim resistance gene, dfrA28, in class 1 integron of an oligotrophic Acinetobacter johnsonii strain, MB52, isolated from River Mahananda, India.

The present work has validated the capability of using diluted Luria broth (10(-3)) and its amendment with antibiotics to quantify proportions of antibiotic-resistant oligotrophic bacteria contained in river water. The assay conditions for assessing fractions of antibiotic-resistant oligotrophic bacteria, especially the type and concentration of antibiotic exposure by determining LD(50) concentration for each antibiotic using sensitive oligotrophic strain, have been standardized. The presence of an array of two gene cassettes including a novel dihydrofolate reductase gene, dfrA28, within the variable region of the class 1 integron in an oligotrophic test strain, MB52, in a population of oligotrophic bacteria isolated from Mahananda River water, has been described. The predicted protein consisting of 172 amino acids exhibited 76.4% amino acid identity with DfrA1. A gene for streptomycin resistance, aminoglycoside adenyl transferase aadA1, was located downstream of dfrA28. Phylogenetic analysis of MB52 identified it as a strain of Acinetobacter johnsonii.