Characterization of a novel class A carbapenemase PAD-1 from Paramesorhizobium desertii A-3-E, a strain highly resistant to β-lactam antibiotics.

Although clinical antibiotic-resistant bacteria have attracted tremendous attention in the microbiology community, the resistant bacteria that persist in natural environments have been overlooked for a longtime. We previously proposed a new species Paramesorhizobium desertii, isolated from the soil of the Taklimakan Desert in China that is highly resistant to most β-lactam antibiotics. To identify potential β-lactamase(s) in this bacteria, we first confirmed the carbapenemase activity in the freeze-thawed supernatant of a P. desertii A-3-E culture using the modified Hodge assay. We then identified a novel chromosome-encoded carbapenemase (PAD-1) in strain A-3-E, using a shotgun proteomic analysis of the supernatant and genomic information. The bioinformatics analysis indicated that PAD-1 is a class A carbapenemase. Subsequent enzyme kinetic assays with purified PAD-1 confirmed its carbapenemase activity, which is similar to that of clinically significant class A carbapenemases, including BKC-1 and KPC-2. Because the location in which A-3-E was isolated is not affected by human activity, PAD-1 is unlikely to be associated with the selection pressures exerted by modern antibiotics. This study confirmed the diversity of antibiotic-resistant determinants in the environmental resistome.