Effect of the interaction of nisin Z with various polysaccharides on its antibacterial activity

The effect of various polysaccharides commonly used as thickening or gelling agents in foods on the antibacterial activity of nisin Z was investigated. Addition of 1 g.L-1 sodium alginate in tryptic soy broth resulted in a four-fold increase of minimal inhibitory (MIC) and bactericidal (MBC) concentrations of nisin Z against Listeria innocua ATCC 33090 and Kocuria rhizophila ATCC 9341, while the addition of citrus pectin or various exopolysaccharides (Leuconostoc mesenteroides P35 exopolysaccharides, low (9-11 kDa) or medium (60-76 kDa) molecular weight dextran from Ln. mesenteroides and 2 kefirans (extracted from kefir grains provided by Crokfun or Kefiralia)) did not increase MIC and MBC of nisin Z against these bacteria. Zeta potential determination at pH 7 of the nisin Z, bacteria and polysaccharides allowed us to observe that only nisin Z had a slightly positive zeta potential, while the bacteria, sodium alginate and highly methoxylated citrus pectin zeta potentials were lower than -20 mV and zeta potentials of all exopolysaccharides were between 0 and - 10 mV. It was thus proposed that anionic sodium alginate, which is the only polysaccharide having a lower zeta potential than both bacteria, forms complexes stabilized by electrostatic interactions with oppositely charged nisin Z, thereby limiting the quantity of "free" nisin Z interacting with nisin-susceptible bacteria. This hypothesis was substantiated by the observation of nisin Z - sodium alginate aggregation at pH 7 and the estimation of the apparent affinity of sodium alginate for "free" nisin Z and immobilized nisin Z by partition experiments and by surface plasmon resonance analysis, respectively. This study thus provides useful information for food formulation regarding the effect of different polysaccharides on nisin Z antibacterial activity and some understanding of how interactions between nisin Z, polysaccharides, and nisin Z - sensitive bacteria are explanatory.