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Abstract
Cheese whey and yoghurt samples were tested for the presence of virulent and temperate dairy bacteriophages. Detection was carried out by the spot test, turbidity test and milk activity test as easy and simple laboratory methods. Virulent bacteriophages specific for Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis and Lactobacillus delbrueckii subsp. bulgaricus were detect by the spot test in two samples out of eighteen samples of cheese whey. In another experiment virulent bacteriophages specific for Lactobacillus sp. and specific for Lactobacillus pentosus were detected in one out of six kareish cheese whey using the turbidity test. Virulent bacteriophages specific for Lactobaci-llus casei subsp. plantarum and Lactobacillus casei subsp. pseudoplantarum were detected by the milk activity test procedures in two out of four whey samples.
Twenty six local yoghurt samples obtained from different regions of Cairo Governorate were examined for the type of bacterial starter and occurrence of virulent bacteriophages. Fermentation of twelve yoghurt samples was due to the lactic acid bacteria and fermentation of fourteen samples was due to Saccharomyces and Bacillus. No virulent bacteriophages specific for lactic acid bacteria were detected in the lactic acid fermented yoghurt samples. The isolated lactic acid bacteria were tested for their lysogenicity after induction by mitomycin-C, detection of temperate bacteriophages in the isolated lactic acid bacterial cultures was done by the spot test and it was found that Streptococcus sp. and Lactobacillus casei subsp. plantarum are lysogenic strains, and the temperate bacteriophages appeared after induction with rate of 1.4 x 10 to 1.5 x 103 pfu/ml.
Purified bacteriophage stock lysate specific for Lactobacillus pentosus was prepared and its U.V absorption properties was measured. The A280/A260 was 1.102 and Amax/Amin was 1.167 indicating that viral particles contain high ratio of both nucleic acid and proteins. The pure Lactobacillus pentosus bacteriophage had very limited host range.
Lactobacillus casei subsp. plantarum mutant resistant to virulent bacteriophage was obtained by single colony isolation from the bacterial lysis zone, and it was found that it had the same fermentation ability as compared to the parental Lactobacillus casei subsp. Plantarum.
The obtained virulent bacteriophage reduced significantly the total count of the main host after infection as well as the total acidity required for fermentation.