Rcwiiwl I 7 Decmibsr 1973 ; revised 2 I February I 974) S U M M A R Y 111 some nisin-producing strains of Streptococcus lactis, clones which were permanently incapable of producing nisin (Nis-) appeared spontaneously with a frequency of 0.03 to 0.82 : / ;. Treatment with proflavin (Pro), ethidium bromide (EB) or at a temperature of 40 "C increased the number of such clones. In one strain 78 :{ of cells became Nis-after treatment with Pro. Treatment with nitrosoguanidine did not reverse the effect of Pro or EB. There was no evidence that Pro or EB selected spontaneously-occurring Nis-clones. These observations suggest that the gene(s) responsible for nisin production in some S. lactis strains may be located on a plasmid, but further investigation is required to confirm this.
Eighty-seven strains of lactic streptococci (46 of Streptococcus lactis, 24 of S. diacetilactis, and 17 of S. cremoris) were tested for lysogeny; 12 S. lactis strains produced nisin. Lysogeny was found in five S. lactis strains (two of them were nisin producers) and in two S. diacetilactis strains. Four S. lactis and two S. diacetilactis lysogens liberated phages both spontaneously and after ultraviolet treatment, and one S. lactis strain liberated phages spontaneously only. No lysogens were found among the S. cremoris strains tested. An initial characterization of the lysogens and their phages was made. The lytic spectrum of some of the examined phages was very narrow (homospecific), whereas that of others was wide, including strains of the three investigated species.
SUMMARYEscherichia coli F+ donor bacteria were treated with U.V. radiation, mitomycin C, nitrous acid, hydroxylamine or caf€eine, and then incubated for various periods in broth at 37" before conjugation. U.V. radiation, mitomycin C and nitrous acid increased the fertility of F+ populations, but hydroxylamine and caffeine did not. The maximal effect of u.v. irradiation was obtained after using a dose 50-100 ergs/Ima2 and after 45 min. incubation in broth of irradiated bacteria before conjugation. The maximal effect of mitomycin C occurred after treatment with 1-3 yg./ml. and after I I O min. incubation in broth. Depending on the strain used, the fertility of the population increased 8-20 times after U.V. irradiation and 5-21 times after mitomycin C. The effect of u.v. irradiation could be photoreactivated up to 50 %. Following treatment of strain w 1655 with nitrous acid (0.01 M), a three-to four-fold increase in the yield of conjugation was obtained, although irregularly. Caffeine (0.01 %) enhanced the effect of U.V. irradiation about two-fold. Chloramphenicol blocked the expression of the effect of U.V. irradiation and mitomycin C. The effect of chloramphenicol was not permanent, because after its removal followed by further incubation the fertility of the F+ population increased to the same degree as in the samples in which the bacteria, previously exposed to U.V. radiation or mitomycin C, were incubated in broth without chloramphenicol.
Eighty-seven strains of lactic streptococci (46 of
Streptococcus lactis
, 24 of
S. diacetilactis
, and 17 of
S. cremoris
) were tested for lysogeny; 12
S. lactis
strains produced nisin. Lysogeny was found in five
S. lactis
strains (two of them were nisin producers) and in two
S. diacetilactis
strains. Four
S. lactis
and two
S. diacetilactis
lysogens liberated phages both spontaneously and after ultraviolet treatment, and one
S. lactis
strain liberated phages spontaneously only. No lysogens were found among the
S. cremoris
strains tested. An initial characterization of the lysogens and their phages was made. The lytic spectrum of some of the examined phages was very narrow (homospecific), whereas that of others was wide, including strains of the three investigated species.
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