149 strains of bacteria, mostly brewery contaminants able to spoil wort or beer, and 12 brewing strains of yeast (8 ale and 4 lager strains) have been screened using a well-test assay for sensitivity to the food preservative, Nisin (E234), Nisin inhibited growth of 92% of the gram-positive strains, predominantly lactic acid bacteria of the genera Lactobacillus and Pediococcus. In contrast, all 32 gram-negative strains tested, except 3 Flavobacter strains, were Nisin-resistant; in addition none of the brewing yeasts showed Nisin-sensitivity. Therefore. Nisin has potential applications in preventing spoilage of worts or beers by lactic acid bacteria.

Nisin, an internationally accepted food preservative has been shown to inhibit the growth of almost all Gram-positive beer-spoilage bacteria investigated. The initial effects on these bac teria appear to be upon the cell membrane. Nisin has little effect on most Gram-negative bacteria and has no effect upon the growth and fermentation properties of brewing yeasts. Nisin activity survives kieselguhr filtration, fining and pasteurisation and has no effect on beer shelf life. Nisin has potential applications in preventing spoilage of worts or beers by Gram-positive bacteria (particularly lactic acid bacteria).

Within one minute of adding nisin to suspensions of Lactobacilli clumping of cells was observed. This happened with all the strains assayed, irrespective of their sensitivity or resistance to nisin. There was, however, no evidence of cell lysis. Two different assays for cell viability were used to show that the vast majority of cells of sensitive strains were killed within one minute of contact with nisin. The time needed to kill all the cells depended on the concentrations of both nisin and cells. One strain was more resistant and only 50-60% of the cells died within one minute of nisin addition.Using an ATP-bioluminescence assay it was shown that the addition of nisin to both Lactobacilli and Pediococci caused a rapid fall in intracellular ATP levels, which was reflected in a simultaneous appear ance of ATP in the extracellular medium. Actively growing cells lost ATP more rapidly than did those in stationary phase. The amount of intracellular ATP lost was affected both by the concentration of nisin used and the sensitivity of the bacteria. It appears that an initial effect of nisin is to make the cell membrane 'leaky' as happens with many antibiotics, yeast killer factors (zymocins), colicins. and other gram-positive bacteriocins.

Depending on the amount used and the strain of bacteria involved, nisin either kills lactic acid bacteria or inhibits their growth. In medium inoculated with approximately 10Bcells ml'1 of a sensitive strain of Lactobacillus (BSO 375) nisin. added at levels recommended for commercial use (100 International Units ml-1, killed all the cells in less than 6 h. In the absence of nisin this inoculum grew to a concentration of 1010cells ml"1 in about 50 h. Lower nisin concentrations killed fewer cells but inhibited the growth of those still viable. For the more resistant strain Lactobacillus (BSO 343) growth was only inhibited at the higher nisin concentrations. Nisin maintained its activity against lactic acid bacteria in brewing fermen tations. It had no effect on the growth and fermentative performance of the 9 brewing yeast strains tested, and, in a pilot brewery fermentation, had no deleterious affect on the taste of the beer produced. Nisin could be used either as a preventative measure by regular addition to fermentations, or as a remedial measure once contamination by lactic acid bacteria had been detected.

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