Viability and dose-response studies on the effects of the immunoenhancing lactic acid bacterium Lactobacillus rhamnosus in mice

Previous studies have indicated that the lactic acid bacterium Lactobacillus rhamnosus HN001 can enhance immune function in mice, following oral delivery. However, the influence of bacterial cell viability on immunoenhancement, and the optimum dose of HN001 required for this effect, have not been determined. In the present study, both live and heat-killed preparations of L. rhamnosus HN001 were shown to enhance the phagocytic activity of blood and peritoneal leucocytes in mice, at a dose of 10(9) micro-organisms daily. In contrast, only live HN001 enhanced gut mucosal antibody responses to cholera toxin vaccine. Feeding mice with 10(7) viable HN001/d for 14 d was shown to enhance the phagocytic capacity of blood leucocytes, with incremental enhancement observed at 10(9) and 10(11) daily doses. In contrast, a minimum dose of 10(9) viable HN001/d was required to enhance the phagocytic activity of peritoneal leucocytes, and no further increment was observed with 10(11) daily. This study demonstrates that L. rhamnosus HN001 exhibits dose-dependent effects on the phagocytic defence system of mice, and suggests that while the innate cellular immune system is responsive to killed forms of food-borne bacteria, specific gut mucosal immunity may only be stimulated by live forms.