Bacteriocin-producing starter cultures have been suggested as natural food preservatives; however, development of resistance in the target organism is a major concern. We investigated the development of resistance in Listeria monocytogenes to the two major bacteriocins pediocin PA-1 and nisin A, with a focus on the variations between strains and the influence of environmental conditions. While considerable strain-specific variations in the frequency of resistance development and associated fitness costs were observed, the influence of environmental stress seemed to be bacteriocin specific. Pediocin resistance frequencies were determined for 20 strains and were in most cases ca. 10 ؊6 . However, two strains with intermediate pediocin sensitivity had 100-foldhigher pediocin resistance frequencies. Nisin resistance frequencies (14 strains) were in the range of 10 ؊7 to 10 ؊2 . Strains with intermediate nisin sensitivity were among those with the highest frequencies. Environmental stress in the form of low temperature (10°C), reduced pH (5.5), or the presence of NaCl (6.5%) did not influence the frequency of pediocin resistance development; in contrast, the nisin resistance frequency was considerably reduced (<5 ؋ 10 ؊8 ). Pediocin resistance in all spontaneous mutants was very stable, but the stability of nisin resistance varied. Pediocin-resistant mutants had fitness costs in the form of reduction down to 44% of the maximum specific growth rate of the wild-type strain. Nisin-resistant mutants had fewer and less-pronounced growth rate reductions. The fitness costs were not increased upon applying environmental stress (5°C, 6.5% NaCl, or pH 5.5), indicating that the bacteriocin-resistant mutants were not more stress sensitive than the wild-type strains. In a saveloy-type meat model at 5°C, however, the growth differences seemed to be negligible. The applicational perspectives of the results are discussed.

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Three years' results of sheep uterine weight bioassays on a range of clover swards are presented. Trifolium pratense L. (var. Grasslands Hamua Broad) and Trifolium subterraneum L. (var. Dinninup, Dwalganup, Tallarook, and Yarloop) showed high activity whenever they were green. Trifolium fragiferum L. (mixed+Palestine), Trifolium repens L. (var. Victorian Irrigation), and Trifolium subterraneum L. (var. Bacchus Marsh, Howard, Marrar, and a breeder's selection) showed variable activity; Medicago sativa L. (var. Hunter River), Medicago littoralis (Rhode), Trifolium alexandrinum L., Trifolium incarnatum L. (var. Dixie), and Trifolium repens L. (var. Grasslands Huia and Ladino) have not shown activity. Spraying two of the clovers with paraquat and dosing the sheep with cobalt sulphate had no effect on activity. There was a strong seasonal trend with highest activity in midwinter and lowest in mid spring. The relationships between chemical assay and bioassay results are discussed. There is reasonable evidence that the activity of Victorian Irrigation white clover is not related to any of the compounds being measured.

In four experiments from 1975 to 1979, wheat or oat crops were grazed to a standard height of about 6 cm (and to 2 cm as well in 1975 and 1976) at various times during winter by sheep, and by cattle also in 1979. Nitrogen was applied to a portion of each plot at the end of grazing. The crops were then allowed to recover for grain production. Herbage and grain yields, grain yield components, grazing days and liveweight gain were recorded. In two years, grazing significantly depressed grain yield relative to that of the ungrazed control, by 25-79% depending on treatment. In all years more herbage and animal production were obtained from a July or August grazing than from a June grazing. The greatest number of sheep grazing days recorded was 3414/ha for hoggets grazing oats for August 1977. lsis wheat was generally inferior to oats for grazing and grain production. Nitrogen increased grain and hay yields in three years; this increase was not profitable for grain, but was profitable for hay in at least one year. The most profitable use of winter cereal crops depends strongly on the relative prices of meat, grain, and hay. Our data suggest relationships between animal, hay, and grain production, which may be useful for farmers wishing to decide probable best options for using grazing-grain crops.