CULTURE MEDIA FOR THE SELECTIVE ENUMERATION AND DIFFERENTIATION OF YOGURT STARTER CULTURES

FORNAGUERA M.J.; MARTOS G; FONT DE VALDEZ G.

Tucumán. Argentina

Simposio; III Simposio Internacional de Bacterias Lácticas. II Encuentro Red BAL Argentina; 2009

CERELA.

Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are the starter cultures for yogurt fermentation. The ratio of these bacteria in dairy products is believed to be important in product quality. Quality assurance programs associated with research, development, production and validation of health benefits of these bacteria require their relevant isolation, counting and identification. Several media to differentiate L. bulgaricus and St. thermophilus with respect to morphology and color of the colonies have been described. The aim of this study was to test several agar media to differentiate and selectively enumerate both microorganisms from commercial yogurts.  Pure cultures of both species were also used as standard for reference. Strains of L. bulgaricus (30) and St. thermophilus (20), yogurt starter strains (CRL, CERELA´s culture collection) and several yogurt brand were grown in different agar media such as MRS, LATPg, yogurt lactic agar (YLA), lactic agar supplemented with 0.1 % Tween 80 and 2,3,5-triphenyltetrazolium (50 mg/ml, final concentration) (LAt), skim milk agar (SM) and St. thermophilus agar (ST). All strains were able to grow in the tested media displaying similar viability (ca. 8 Log CFU/ml). ST agar was used as control medium for St. thermophilus strains as L. bulgaricus failed to grow on it. A differential enumeration of the yogurt bacteria on the basis of colony morphology criterium was not successful for all media, e.g., MRS or LAPTg agar. However, in SM agar, colonies from St. thermophilus were circular, white with well-defined edges while L. bulgaricus grew as irregular, translucent colonies with non-defined edges. Random results were obtained in YLA medium, where streptococci gave  small, white colonies, devoid of a zone of cloudiness around them, and lactobacilli  appeared as smooth, white and slightly raised colonies. However, in some cases  lactobacilli colonies also appeared  surrounded by halos so differentiation was not always clear. In LAt, St. thermophilus formed small red colonies with defined edges whereas L. bulgaricus appeared as large, white, smooth colonies, irregular edges with a red point in the middle Successful differential enumeration of rods and cocci in laboratory-derived cultures as well as in yogurt samples is valuable since their relative amounts are believed important in product quality. Differential enumeration was dependent on the strain and the appropriate dilutions in the majority of media tested. In this study, only LAt allowed a suitable and reproducible differentiation between St. thermophilus and L. bulgaricus regardless the assayed strain (pure cultures or yoghurt samples) thus, this medium provides a significant advantage for control yogurt production.