The β-Lactamase Activity at the Community Level Confers β-Lactam Resistance to Bloom-Forming <i>Microcystis aeruginosa</i> Cells

초록

Many freshwater cyanobacteria, including Microcystis aeruginosa, lack several known antibiotic resistance genes; however, both axenic and xenic M. aeruginosa strains exhibited high antibiotic resistance against many antibiotics under our tested concentrations, including colistin, trimethoprim, and kanamycin. Interestingly, axenic PCC7806, although not the xenic NIBR18 and NIBR452 strains, displayed susceptibility to ampicillin and amoxicillin, indicating that the associated bacteria in the phycosphere could confer such antibiotic resistance to xenic strains. Fluorescence and scanning electron microscopic observations revealed their tight association, leading to possible community-level beta-lactamase activity. Combinatory treatment of ampicillin with a beta-lactamase inhibitor, sulbactam, abolished the ampicillin resistance in the xenic stains. The nitrocefin-based assay confirmed the presence of significant community-level beta-lactamase activity. Our tested low ampicillin concentration and high beta-lactamase activity could potentially balance the competitive advantage of these dominant species and provide opportunities for the less competitive species, thereby resulting in higher bacterial diversity under ampicillin treatment conditions. Non-PCR-based metagenome data from xenic NIBR18 cultures revealed the dominance of bla(OXA)-related antibiotic resistance genes followed by other class A beta-lactamase genes (AST-1 and FAR-1). Alleviation of ampicillin toxicity could be observed only in axenic PCC7806, which had been cocultured with beta-lactamase from other freshwater bacteria. Our study suggested M. aeruginosa develops resistance to old-class beta-lactam antibiotics through altruism, where associated bacteria protect axenic M. aeruginosa cells.

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