Nisin A and Nisin Z are of vegetal origin, two natural variants of the Nisin that are produced by Lactococcus lactis are known. They have a similar structure but differ in a single amino acid residue at position 27; histidine in Nisin A and asparagine in Nisin Z. The Nisin variants were purified to apparent homogeneity, and their biological activities were compared. Identical MICs of Nisin A and Nisin Z were found with all tested indicator strains of six different species of gram-positive bacteria. However, at concentrations above the MICs, with Nisin Z the inhibition zones obtained in agar diffusion assays were invariably larger than those obtained with Nisin A. This was observed with all tested indicator strains. These results suggest that Nisin Z has better diffusion properties than Nisin A in agar. The distribution of the Nisin variants in various lactococcal strains was determined by amplification of the Nisin structural gene by polymerase chain reaction followed by direct sequencing of the amplification product. In this way, it was established that the nisZ gene for Nisin Z production is widely distributed, having been found in 14 of the 26 L. lactis strains analyzed.

The solubility of Nisin A is highest at low pH values and gradually decreases by almost 2 orders of magnitude when the pH of the solution exceeds a value of 7. At low pH, Nisin Z exhibits a decreased solubility relative to that of Nisin A; at neutral and higher pH values, the solubilities of both variants are comparable. Two mutants of Nisin Z, which contain lysyl residues at positions 27 and 31, respectively, instead of Asn-27 and His-31, were produced with the aim of reaching higher solubility at neutral pH. Both mutants were purified to homogeneity, and their structures were confirmed by one-and two-dimensional 1H nuclear magnetic resonance.

Their antimicrobial activities were found to be similar to that of Nisin Z, whereas their solubilities at pH 7 increased by factors of 4 and 7, respectively. The chemical stability of Nisin A was studied in the pH range of 2 to 8 and at a 20, 37, and 75 degrees C. Optimal stability was observed at pH 3.0. Nisin Z showed a behavior similar to that of Nisin A. A mutant containing dehydrobutyrine at position 5 instead of dehydroalanine had lower activity but were significantly more resistant to acid-catalyzed chemical degradation than wild-type Nisin Z.

Sources: HS Rollema, OP Kuipers, P Both, WM de Vos and RJ Siezen Department of Biophysical Chemistry, Netherlands Institute for Dairy Research (NIZO), Ede.