Antibiofilm Effects of Epigallocatechin Gallate Against Proteus mirabilis Wild-Type and Ampicillin-Induced Strains.

Proteus mirabilis is an opportunistic pathogen associated with nosocomial infections and foodborne diseases. The resistance and biofilm formation of P. mirabilis have been a great concern. In this study a multidrug-resistant P. mirabilis strain 012 was exposed to a lethal dose of ampicillin (10 mg/mL, 2.5-fold minimal bactericidal concentration) for 24 h at 37°C. After resuscitation and isolation, five variant isolates were selected and subjected to ampicillin induction by repeatedly streaking on ampicillin-containing plates (10 mg/mL) for at least three times. In biofilm formation assays by using crystal violet staining, we found that the variant strains had enhanced biofilm-forming abilities. (-)-epigallocatechin-3-gallate (EGCG) at a minimum inhibitory concentration (MIC) (256 μg/mL) significantly reduced the biofilm formation of all variant strains and the wild-type strain (p < 0.01). Sub-MIC of EGCG (128 μg/mL) suppressed the biofilms of wild-type and two variants. However, it stimulated the biofilms of the other three variants. The antibiofilm effects of EGCG against the wild-type strain were further confirmed by confocal laser scanning microscopy. Scanning electron microscopy revealed that EGCG induced variants to form more fibrous structures. Our results revealed that a lethal dose of antibiotic exposure increased antibiotic resistance and biofilm formation of P. mirabilis. EGCG may be used as a promising antibiofilm agent to prevent the P. mirabilis biofilm formation in the food industry. However, the sub-MIC of EGCG is not effective and will not be applied.