Abstract
BACTERIOCINS are antibiotics produced by bacteria and which act on strains of the same or closely related species. They are protein in nature. De Klerk and Coetzee1 reported bacteriocin production by homofermentative and heterofermentative species of Lactobacillus. Bacteriocin was detected in the supernatants of 10 day old broth cultures of eleven out of fifty-nine strains of Lactobacillus acidophilus and one out of forty-two strains of L. fermenti. The production of bacteriocins often depends on growth conditions2, and strains which produce bacteriocin on agar may show little or no activity in broth3. Because the methods previously used may not have been optimal2 it was decided to re-investigate the incidence of bacteriocinogeny in L. fermenti strains. One hundred and twenty-one strains of L. fermenti isolated locally from as many different sources of human saliva were tested. Many of the strains showed different lytic patterns when tested with a series of lactobacillus bacteriophages (unpublished observations). The media used were MRS broth and agar4 and cultures were incubated at 37° C in an atmosphere of carbon dioxide. Bacteriocin production was investigated by stabbing single colonies of the 121 strains into fresh plates. After overnight incubation the plates were sterilized with chloroform vapour. They were then layered with soft agar seeded with an indicator strain and re-incubated overnight. All 121 strains were also used as indicators of bacteriocinogeny in different experiments. Twenty-five of the strains showed clear rings of inhibition of indicator organisms 2–3 mm wide. All twenty-five strains inhibited the same forty-four L. fermenti indicators. Sixty-four homofermentative strains of lactobacillus were then also tested for susceptibility. Five strains of L. acidophilus were inhibited by the same twenty-five L. fermenti strains. Subcultures of agar fragments from clear areas of inhibition never showed growth and the inhibitory activity could not be serially transmitted to fresh lawns of the original indicator organisms. Subsequently inhibitory activity in undiluted supernatants of overnight broth cultures of the producer strains was demonstrated by techniques previously used2. This activity was variable and could frequently be discerned only as a hazy spot of inhibition in the indicator lawn. The inhibitory activity of supernatants could be concentrated by precipitation with saturated ammonium sulphate and the agents diffused more than 1 cm from a central hole in agar. This was demonstrated by filling the hole with a concentrated solution of the agent and allowing diffusion to occur overnight. The plates were then covered with soft agar containing the indicator organism. These properties label the inhibitory agents as bacteriocins5. No resistant mutants of indicator strains were obtained and this precluded the use of cross-resistance tests6 to classify these bacteriocins.
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References
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DE KLERK, H. Bacteriocinogeny in Lactobacillus fermenti. Nature 214, 609 (1967). https://doi.org/10.1038/214609a0
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DOI: https://doi.org/10.1038/214609a0
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