Antimicrobial activity of phenyllactic acid against Klebsiella pneumoniae and its effect on cell wall membrane and genomic DNA

As Klebsiella pneumoniae (KP) has acquired high levels of resistance to multiple antibiotics, it is considered a worldwide pathogen of concern, and substitutes for traditional antibiotics are urgently needed. 3-Phenyllactic acid (PLA) has been reported to have antimicrobial activity against food-borne bacteria. However, there was no experiment evidence for the exact antibacterial effect and mechanism of PLA kills pathogenic KP. In this study, the Oxford cup method indicated that PLA is effective to KP with a minimum inhibitory concentration of 2.5 mg/mL. Furthermore, PLA inhibited the growth and biofilm formation of in a time- and concentration-dependent manner. In vivo, PLA could significantly increase the survival rate of infected mice and reduce the pathological tissue damage. The antibacterial mode of PLA against KP was further explored. Firstly, scanning electron microscopy illustrated the disruption of cellular ultrastructure caused by PLA. Secondly, measurement of leaked alkaline phosphatase demonstrated that PLA disrupted the cell wall integrity of KP and flow cytometry analysis with propidium iodide staining suggested that PLA damaged the cell membrane integrity. Finally, the results of fluorescence spectroscopy and agarose gel electrophoresis demonstrated that PLA bound to genomic DNA and initiated its degradation. The anti-KP mode of action of PLA was attributed to the destruction of the cell wall, membrane, and genomic DNA binding. These findings suggest that PLA has great potential applications as antibiotic substitutes in feed additives against KP infection in animals.