An inhibitory synergistic effect of a nisin–monolaurin combination on Bacillus sp. vegetative cells in milk Original Research Article

The inhibitory activities of nisin and monolaurin, used alone or in combination, were investigated against four Bacillus species vegetative cells in milk at 37°C for 5 days. In the absence of inhibitors, the four strains grew and sporulated at the end of the exponential growth step and throughout the stationary phase. Nisin (100 IU ml−1) induced an immediate reduction in the population level but transient because regrowth appeared and was 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 hand, monolaurin (250 μ g ml−1) had a durable bacteriostatic effect followed by a regrowth level constantly lower than that of the control culture; sporulation was low (between 13 and 7×103spl ml−1) and did not occur in the case of B. coagulans. 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 concentration of 500 cfu ml−1was constant till the end of the experiment; consequently, sporulation was absent.