(36f) Probiotics As Viable Antimicrobials Inhibiting Pathogens during Biofilm Formation

Biofilms are sessile microbial communities formed on the biotic and abiotic surfaces by the secretion of extracellular polymeric substances that enhance adherence to the surfaces and microbial aggregation. Biofilm bacteria are highly tolerant to exogenous stresses such as antibacterial agents due to mass transfer limitation in biofilm matrix and non-metabolizing nature of cells inside the biofilms. Many pathogens take advantages of living as biofilms, and thus 75%~80% of human infectious diseases are closely involved in the formation of biofilms. Therefore, understanding the biofilm formation is critical to establishing novel strategies of controlling infectious diseases. Although the majority of biofilm studies has examined mono-species cultures, the biofilm formation is intra- and inter-species phenomena that require dynamic interactions between bacteria in mixed biofilm communities. However, little is known to understand molecular interactions between biofilm bacteria. Escherichia coli strain Nissle 1917 (EcN) is a probiotic bacterium that has antagonistic effects on adherence, growth, and biofilm formation of other E. coli strains. In this study, we investigated the ability of EcN to outcompete with the biofilm formation of pathogenic bacteria such as pathogenic E. coli (EHEC), Pseudomonas aeruginosa, Staphylococcus aureus, and S. epidermidis. When dual-species biofilms were formed, EcN inhibited the EHEC biofilm population by 14-fold compared to EHEC single-species biofilms. This figure was 1,100-fold for S. aureus and 8,300-fold for S. epidermidis; however, EcN did not inhibit P. aeruginosa biofilms. In contrast, commensal E. coli did not exhibit any inhibitory effect toward other bacterial biofilms. We identified that EcN secretes DegP, a bifunctional (protease and chaperone) periplasmic protein, outside the cells and controls other biofilms. Although three E. coli strains tested in this study expressed degP, only the EcN strain secreted DegP outside the cells, and purified DegP directly repressed EHEC biofilm formation. Hence, probiotic E. coli outcompetes pathogenic biofilms via extracellular DegP activity during dual-species biofilm formation. In addition, through transposon mutagenesis screening of EcN, we identified that four genes argC, argH, serB, and purL are involved in repressing biofilms of EHEC as well as S. aureus. These results provide insights in applying and developing probiotics as an effective treatment for the biofilm-related infectious diseases.