The survival after chlorine stress of the biofilm form of Salmonella typhimurium both alone and in association with the biofilm form of Pseudomo fluorescens was investigated. P. fluorescens showed the best adhesion and more extended growth than S. typhimurium when the two strains were cocultured. The presence of P. fluorescens resulted in an increased resistance of S. typhimurium to chlorine. This phenomenon, which was already seen in 1-day biofilms, increased in 4-day biofilms. Viable but nonculturable cells were observed only in 4-day single-species S. typhimurium biofilms subjected to chlorine stress; only 50% of substrate-responsive bacteria (SRB) were culturable. When daily cycles of disinfection, neutralization, and culture medium supply were performed with S. typhimurium biofilms for 4 days, only 20% of the SRB remained culturable. The chlorine consumption of such biofilms was more than twice that of 4-day single-species S. typhimurium biofilms.

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