Growth-Phase-Dependent Modulation of Quorum Sensing and Virulence Factors in Pseudomonas aeruginosa ATCC 27853 by Sub-MICs of Antibiotics

<b>Background</b>: Antibiotics at sub-inhibitory concentrations can rewire bacterial regulatory networks, impacting virulence. <b>Objective</b>: The way that exposure to selected antibiotics (ciprofloxacin, amikacin, azithromycin, ceftazidime, and meropenem) below their minimum inhibitory concentration (sub-MIC) modulates the physiology of <i>Pseudomonas aeruginosa</i> is examined in this study using growth-phase-resolved analysis. <b>Methods</b>: Standard <i>P. aeruginosa</i> strain cultures were exposed to ¼ and ½ MIC to determine the growth kinetics under antibiotic stress. The study measured protease and pyocyanin production and the expression level of important quorum sensing and virulence genes (<i>lasI</i>/<i>R</i>, <i>rhlI</i>/<i>R</i>, <i>pqsR</i>/<i>A</i>, and <i>phzA</i>) at different growth phases. <b>Results</b>: Meropenem produced the most noticeable growth suppression at ½ MIC. Sub-MIC antibiotics did not completely stop growth, but caused distinct, dose-dependent changes. Azithromycin eliminated protease activity in all phases and had a biphasic effect on pyocyanin. Ciprofloxacin consistently inhibited both pyocyanin and protease in all phases. The effects of amikacin varied by phase and dose, while β-lactams markedly increased pyocyanin production during the log phase. In contrast to the plateau phase, when expression was often downregulated or unchanged, most quorum-sensing- and virulence-associated genes showed significant upregulation during the death phase under sub-MIC exposure. <b>Conclusions</b>: These findings indicate that sub-MIC antibiotics act as biochemical signal modulators, preserving stress-adapted sub-populations that, in late growth phases, activate quorum sensing and stress tolerance pathways.