Survival of Planktonic and Sessile Cells of Lactobacillus rhamnosus and Lactobacillus reuteri upon Exposure to Simulated Fasting-State Gastrointestinal Conditions

Kyle B. Klopper, Elanna Bester, Shelly M. Deane, Gideon M. Wolfaardt, Leon M. T. Dicks

In this study, we report on the formation and resilience of Lactobacillus reuteri HFI-LD5 and Lactobacillus rhamnosus HFI-K2 biofilms cultivated in a CO2 evolution measurement system (CEMS) and exposed to biologically relevant, fasting-state gastrointestinal fluids under continuous flow conditions. For comparative purposes, planktonic and sessile populations of L. reuteri HFI-LD5 and L. rhamnosus HFI-K2 were each exposed to fasting-state gastric fluid (FSGF, pH 2.0) for 2 h, fasting-state intestinal fluid (FSIF, pH 7.5) for 6 h, and simulated colonic fluid (SCoF, pH 7.0) for 24 h. Planktonic cell numbers of L. reuteri HFI-LD5 declined from 6.6 log10 CFU/mL to 3.2 log10 CFU/mL and L. rhamnosus HFI-K2 from 6.6 log10 CFU/mL to undetectable levels after exposure to FSGF. Limited loss in viability was observed when free-floating cells were exposed to FSIF and SCoF. Sessile populations of both strains survived and recovered from the sequential exposure to all three gastric fluids despite observed detachment of biofilm biomass and a temporary decrease in metabolic activity to below detection limits, as recorded by changes in whole-biofilm CO2 production rates. The planktonic cell-focused gut microbiome-related research has most likely caused an underestimation in the overall survival ability of microorganisms in the gastrointestinal tract. Sessile cells of L. reuteri HFI-LD5 were metabolically inactive when exposed to gastric (FSGF) and intestinal (FSIF) fluids, suggesting that biofilms are formed in the small intestinal tract as survival mechanism. In the case of L. rhamnosus HFI-K2, cells were released from biofilms when suddenly exposed to pH 2.0.