Environmental concentrations of antibiotics induced polymyxin B tolerance in Aeromonas hydrophila.

Polymyxin B is one of the last lines of defense in infections caused by multidrug-resistant Gram-negative bacteria. Aeromonas hydrophila are important fish pathogens and the occurrence of polymyxin B-resistant A. hydrophila isolates is increasing. While investigating 14 environmental chemical pollutants that may affect bacterial polymyxin B sensitivity in aquatic bacteria, we discovered that tolerance of A. hydrophila to polymyxin B is increased by short-term (90 min) concurrent exposure to tetracyclines, tigecycline or gentamicin at environmentally relevant concentrations (0.0625 μg/mL) and persists as long as the inducer is present. The exposure increased the growth of A. hydrophila at an inhibitory concentration of polymyxin B. The increased polymyxin B tolerance was attributed to changes in gene expression, without alterations in genotype and independent of cell surface charge variations. Such changes are relate to six KEGG pathways, including ribosome, nucleotide metabolism, pyrimidine metabolism, glyoxylate and dicarboxylate metabolism, purine metabolism, starch and sucrose metabolism. The dysregulated genes were involved in broad physiological changes, such as cell motility, flagella biosynthesis, quorum sensing, biofilm formation, and chemotaxis. Furthermore, the up-regulation of genes encoding Mg2+ transport, biotin synthesis, lipoprotein, glycerol phospholipid metabolism, phospholipid transport, and the down-regulation of genes, such as ompK, yidD and ompA related to enhanced cell membrane barrier, may contribute to the increased polymyxin B tolerance in A. hydrophila. In summary, the study results revealed the impact of environmental antibiotics in promoting microbial polymyxin B tolerance. Our findings underscore the role of environmental antibiotics in promoting polymyxin B tolerance and provide insights into the mechanisms of polymyxin B tolerance evolution in A. hydrophila.