Agar Medium for Differential Enumeration of Lactic Streptococci

Abstract

An agar medium containing arginine and calcium citrate as specific substrates, diffusible (K 2 HPO 4 ) and undiffusible (CaCO 3 ) buffer systems, and bromocresol purple as the p H indicator was developed to differentiate among lactic streptococci in pure and mixed cultures. Milk was added as the sole source of carbohydrate (lactose) and to provide growth-stimulating factors. Production of acid from lactose caused developing bacterial colonies to seem yellow. Subsequent arginine utilization by Streptococcus lactis and S. diacetilactis liberated ammonia, resulting in a localized p H shift back toward neutrality and a return of the original purple indicator hue. The effects of production of acid from lactose and ammonia were fixed around individual colonies by the buffering capacity of CaCO 3 . After 36 hr at 32 C in a candle oats jar, colonies of S. cremoris were yellow, whereas colonies of S. lactis and S. diacetilactis were white. S. diacetilactis , on further incubation, utilized suspended calcium citrate, and, after 6 days, the citrate-degrading colonies exhibited clear zoning against a turbid background, making them easily distinguishable from the colonies of the other two species. The medium proved suitable for quantitative differential enumeration when compared with another widely used general agar medium for lactic streptococci.

Flavor Enhancement of Reduced Fat Cheddar Cheese Using an Integrated Culturing System

“…Colony-forming units per gram of cheese at 1, 14, 30, 90, and 150 days were determined for the various cultures using the following: Reddy's differential agar medium (Reddy et al, 1972) for S and C, Kempler-McKay medium (Kempler and McKay, 1980) for J and Rogosa acid acetate agar for the enumeration of Lactobacillus species (Sandine et al, 1976) for L.…”

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“…Such an organism has been categorized as Lactose positive (Lac + ve). It was further confirmed by plating on the modified differential agar of Reddy et al [1,2], to check for the lactose positive character. The composition of the modified differential agar is presented in Table 1.…”

“…To answer this question, we have conducted series of experiments using the beneficial probiotic organism, Lactococcus lactis var. lactis (according to old terminology it is also called Streptococcus lactis), which is extensively used to manufacture the dairy products [1,2]. This particular organism produces an abundant amount of lactic acid by utilizing lactose sugar, and also produces bacteriocin Nisin, which inhibits several pathogenic bacteria etc., in addition to producing several therapeutic peptides.…”

Comparison of growth and survival of single strains of Lactococcus lactis and Lactococcus cremoris during Cheddar cheese manufacture

“…One milliliter aliquots were then serially diluted with 9 mL of 1% sterile peptone dilution blanks. One hundred microliters of sample of each dilution was plated on both M17 and Reddy's agar, and aerobically incubated at 32°C for 24 h. The L. cremoris strains were counted within a 30-to 40-h window whereas L. lactis strains were counted within 36 to 48 h (Reddy et al, 1972). Total bacterial cell numbers were calculated by multiplying colony-forming units by average chain length.…”