Influence of selected plant extracts on bacterial motility, aggregation, hydrophobicity, exopolysaccharide production and quorum sensing during biofilm formation of enterohaemorrhagic Escherichia coli O157:H7

Diarrhoeagenic Escherichia coli is a serious threat to human and animal health, with antimicrobial drug resistance contributing to the problem. Alternative mechanisms of prevention and treatment of E. coli-induced diarrhoea include those targeting biofilm production and quorum sensing, properties that have been associated with some investigated plant extracts. Plants were selected from various genera, including Vachellia, Senegalia, Morus, Leucaena, Salix, Grewia, Ziziphus, Searsia, Dichrostachys and Ceratonia, based on their known antimicrobial activity. The antibacterial effects of leaf extracts against multidrug-resistant E. coli O157:H7 were investigated using a broth microdilution method, and cytotoxicity to normal mammalian cell lines was studied using a tetrazolium colorimetric assay. Subsequently, antibiofilm activity and inhibition of extracellular polymeric substance (EPS) production by plant extracts against E. coli O157:H7 was investigated using crystal violet as a staining dye after 0 and 24 h incubation and carbohydrate estimation 5 % phenol-sulfuric assay. The biosensor strain Chromobacterium violaceum ATCC 12472 was used to study the anti-quorum sensing potential of the selected plant extracts. Cell motility, cell surface hydrophobicity (CSH), and aggregation were also investigated. All plant extracts had weak antibacterial activity (MIC > 0.62 mg/ml) with relatively low cytotoxicity. The acetone extracts of Salix babylonica and Leucaena leucocephala prevented bacterial cell attachment (0 h) by 81.21 % and 89.36 %, respectively. Approximately 28 % of plant extracts eradicated established biofilms by more than 50 %, however, they were unable to inhibit EPS production above 30 %. The half-maximal concentration of extracts generally inhibited violacein production (ranging from 0.01 to 0.02 mg/ml), with V. erioloba acetone extract being the most effective at quorum quenching. Some plant extracts exhibited a significant difference (p<0.05) in motility, aggregation and hydrophobicity compared to untreated cells. This study highlights the potential of selected plant extracts to act via different mechanisms of action to decrease virulence of enteric E. coli, motivating further investigation of the bioactive compounds in the leaves.