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 Lactobacillu s rhamnosus HN001 can enhance immune function in mice, following oral deliv ery. However, the influence of bacterial cell viability on immunoenhancemen t, and the optimum dose of HN001 required for this effect, have not been de termined. In the present study, both live and heat-killed preparations of L . rhamnosus HN001 were shown to enhance the phagocytic activity of blood an d peritoneal leucocytes in mice, at a dose of 10(9) micro-organisms daily. In contrast, only live HN001 enhanced gut mucosal antibody responses to cho lera toxin vaccine. Feeding mice with 10(7) viable HN001/d for 14 d was sho wn to enhance the phagocytic capacity of blood leucocytes, with incremental enhancement observed at 10(9) and 10(11) daily doses. In contrast, a minim um dose of 10(9) viable HN001/d was required to enhance the phagocytic acti vity of peritoneal leucocytes, and no further increment was observed with 1 0(11) daily. This study demonstrates that L. rhamnosus HN001 exhibits dose- dependent effects on the phagocytic defence system of mice, and suggests th at 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.