Morfología y diferenciación de colonias de tres tipos de bacterias lácticas

Abstract

Yogurt is a milky product fermented with lactic cultures that include Streptococcus, thermophilus and Lactobacilus bulgaricus as mixed cultures. Probiotic bacteria, like L. acidophilus, are believed to have thera peutic properties, that is the reason why they may be added to yogurt in order to provide it with additional properties. Yogurt must contain, at least, 106 UFC ml-1 of viable bacteria at the time of its consumption, this is why it is important to state the differentiation of this type of bacteria in fermented products. The aim of this work was to state the difference, with a combination of culture means, of the bacterial groups present in a sample of fermented milk inoculated with S. thermophilus, L. bulgaricus and. L acidophilus. Six different means of culture were assayed: 1) agar with skimmed milk at 20%, 2) agar with skimmed milk at 10%, 3) MRS, 4) MRS with sorbitol at 5%, 5) MRS with sorbitol at 5%, and controlled pH at 5,4, 6) M17 with controlled pH at 7,2; and growth, and the differentiation in the morphology of each one of the three studied bacteria were evaluated. The means used for the differentiation of the colonies were MRS with sorbitol, and pH fit to 5,4 for the L. bulgaricus and L. acidophilus, where a different morphology for each of these microorganisms is observed. L. bulgaricus colonies had, approximately, 1,0 mm in diameter, with a morphology with indefinite edges, flat, and with a trans lucent white, and opaque color. The colonies of L. acidophilus were convex, circular, punctiform and smaller than the previous ones. The S. thermophilus grew in the M17 Agar, and the observed colonies were round, white, smooth and shining; in this mean, growth of the lactobacillus was not observed. The use of MRS with sorbitol at 5% with a controlled pH of 5,4 and M17 is a recommended mean for the differentiation of the studied bacteria.