An inhibitory synergistic effect of a nisin-monolaurin combination on Bacillus sp vegetative cells in milk

The inhibitory activities of nisin and monolaurin, used alone or in combina tion, were investigated against four Bacillus species vegetative cells in m ilk at 37 degreesC for 5 days, in the absence of inhibitors, the four strai ns grew and sporulated at the end of the exponential growth step and throug hout the stationary phase. Nisin (100 IU ml(-1)) induced an immediate reduc tion in the population level but transient because regrowth appeared and wa s strain-dependent; cell concentrations reached the control culture level e .g. 6-7 log((10)) as well as the spore load (4-5 log((10))). On the other h and, monolaurin (250 mug ml(-1)) had a durable bacteriostatic effect follow ed by a regrowth level constantly lower than that of the control culture,. sporulation was low (between 13 and 7 x 10(3) spl ml(-1)) and did not occur in the case of B. coagula ns. The use of these inhibitors in combination, induced a synergistic bactericidal effect leading to a total inhibition (0 cfu ml(-1)) until day 5, except in the case of B. cereus where a concentrat ion of 500 cfu ml(-1) was constant till the end of the experiment; conseque ntly, sporulation was absent. (C) 2001 Academic Press.