Effect of sub-inhibitory antibacterial stress on bacterial surface properties and biofilm formation

• Biofilm formation was enhanced under sub-inhibitory stress of norfloxacin.
• xDLVO free energy barrier for bacterial attachment was reduced under stress.
• Force of detachment (AFM) for individual bacterium was increased under stress.
• Initial (transient) attachment is prevented by physicochemical interactions.
• Subsequent detachment is precluded by specific molecular interactions.

Sub-inhibitory concentrations of antibacterial compounds do not effectively interfere with bacterial growth and only impart stress on them. Bacteria may react to external stress by abandoning planktonic life and form biofilms as the latter life-form offers superior resistance. This study on the effect of norfloxacin on biofilm formation by Staphylococcus aureus and Pseudomonas aeruginosa found that biofilm formation is enhanced under sub-inhibitory stress. Studies have reported increase in biofilm formation under the influence of external stress. In addition, the physicochemical properties of bacteria (i.e. zeta potential and contact angle) in the presence of antibacterial compounds were determined and its effect on initial attachment of bacteria to surfaces was examined. Single cell AFM force spectroscopy measurements were carried out to model the detachment force once the transient attachment takes place. The results indicate that once bacteria breach the physiochemical free energy barrier and come in direct contact with a surface, specific molecular interactions occur and detachment demands more energy than physicochemical forces alone can explain, especially at relatively higher separation distances.

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