SUMMARY
Evaluation of the effect of probiotic cultures on two different yogurt brands over a known population of Staphylococcus aureus and the production of thermonuclease.
The effect of probiotic cultures over known populations of Staphylococcus aureus inoculated in yogurt was studied; also the production and stability of its thermonuclease during yogurt storage was evaluated. In three different occasions, two different yogurt brands, one with additional probiotic cultures (Lactobacillus casei and L. acidophilus), were inoculated with known populations of S. aureus in high and low concentration (10⁹ CFU/g and 10⁷ CFU/g), respectively. These samples were stored for 28 days at 5ºC. Every four days the count of lactic bacteria, S. aureus and pH were evaluated, according to the methodology described in the Compendium of Methods for the Microbiological Examination of Foods, Vanderzant & Splittstoesser. The presence of thermonuclease was determined using petrifilm for S. aureus from 3M company ®. The pH and lactic bacteria population were constant during the testing period. Yogurt with additional probiotic cultures (high and low concentration) lowered the population of S. aureus to non detectable levels in 8 days; but, S. aureus could be cultured from yogurt without probiotics even after 24 days of incubation. Same time, the presence of thermonuclease was positive in all tests; it was not affected by probiotics. The presence of thermonuclease is related to the production of S. aureus enterotoxin. This work emphasizes again the beneficial effects of probiotic cultures in yogurt over bacteria and the importance of keeping hygienic practices in order to avoid the contamination of food with S. aureus and the eventual production of its enterotoxin, since it is not affected by probiotics.

Key words: Yogurt, probiotics, Staphylococcus aureus, thermonuclease.



Recibido: 16/12/2002
Aceptado: 14/06/2004

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