Subinhibitory concentrations (sub-MICs) of antibiotics, although not able to kill bacteria, can modify their physico-chemical characteristics and the architecture of their outermost surface and may interfere with some bacterial functions. This study investigated the ability of sub-MIC piperacillin/ tazobactam (P/T) to interfere with the bacterial virulence parameters of adhesiveness, cell-surface hydrophobicity, motility, biofilm formation and sensitivity to oxidative stress. Antimicrobial activity against five Pseudomonas aeruginosa clinical isolates, representative of clonal lineages of 96 strains of nosocomial origin, and six control strains (ATCC 27853, PAO1, AK1, MT1562, PT623, PAO1algC) was evaluated in vitro using the NCCLS microdilution method. The effects of sub-MIC on bacterial adhesion and biofilm formation were studied using a modified microtitre plate assay. The relative cell-surface hydrophobicity of P. aeruginosa strains was determined by measuring their ability to adhere to n-hexadecane. P. aeruginosa that had been exposed overnight to P/T and incubated with P/T in the plate were also screened for their ability to swim using flagella and to twitch and for their sensitivity to oxidative stress. The results obtained showed that the impact of sub-MIC P/T on bacterial characteristics was different for the various strains of P. aeruginosa. There was a change in bacterial morphology and hydrophobicity that could explain a significant decrease in adhesion values in all clinical isolates and controls tested, a decrease in biofilm formation, a significant increase in sensitivity to oxidative stress, a significant decrease in flagellum-mediated swimming and a decrease in type IV fimbriae-mediated twitching. The results obtained indicate that sub-MIC P/T interferes with the pathogenic potential of P. aeruginosa.

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Aim:  To investigate the influence of antibiotic‐induced morphological changes on adhesion and motility abilities and surface properties of nosocomial Pseudomonas aeruginosa under different physiological states. Methods and Results:  The effects of subinhibitory concentration (sub‐MIC) of Piperacillin/Tazobactam (P/T) and Imipenem (IMP) were studied on P. aeruginosa adhesion (1 h) using a modified microtitre‐plate assay, on their ability to swim and to twitch, on surface hydrophobicity and on acid–base interactions of P. aeruginosa strains by measuring their ability to adhere to n‐hexadecane, chloroform and ethyl acetate, respectively. Our results show that antibiotic‐induced morphological changes and bacterial physiological state can affect differently surface properties, motility and adhesion abilities of P. aeruginosa. Conclusions:  Under different physiological states P/T induced morphological changes, reduced motility abilities, decreased adhesion to polystyrene and cell surface hydrophobicity (CSH). Moreover, P/T and IMP led to similar changes in exponential population adhesion to chloroform and ethyl acetate. Additionally, IMP induced morphological changes and showed no differences on CSH, adhesion and motility abilities in both growth phases. Significance and Impact of the Study:  Adhesion is an attractive target for new antibacterial strategies, namely by using sub‐MIC antibiotics that induce morphological, motility and surface properties changes, which are dependent of P. aeruginosa phenotype and physiological state.