Cresol Red Thallium Acetate Sucrose Inulin (CTSI) agar for the selective recovery of Carnobacterium spp.

Carnobacterium spp. are commonly isolated from a variety of foods, especially from meats stored under anaerobic atmospheres at refrigeration temperatures, but the role of these organisms in the spoilage of meat and meat products is yet to be determined. Cresol Red Thallium Acetate Sucrose (CTAS) agar was developed as a selective medium for enumeration of carnobacteria, however problems such as poor recovery of Carnobacterium spp. and interference by other microorganisms have precluded its general use. The aim of this study was to improve CTAS agar by broadening its spectrum of selective recovery for carnobacteria while restricting the ability of interfering species to grow. Ten Carnobacterium spp. (five ATCC cultures and five isolates from fresh pork) and 20 other genera were used in testing the agar. A wider range of Carnobacterium spp. recovery was obtained by modifying concentrations of sucrose, manganese sulphate and thallium acetate. Additions of inulin and thiamine hydrochloride also improved growth response. The additions of vancomycin and Chrisin® (nisin) eliminated interference from other microorganisms. A two-temperature incubation procedure was included to improve the characteristic growth of Carnobacterium spp. on the modified medium, identified as Cresol Red Thallium Sucrose Inulin (CTSI) agar. Lactic acid bacteria and Enterobacteriaceae were unable to grow on CTSI incubated aerobically. Growth of Carnobacterium spp. on CTSI yielded pink colonies, except for Cb. mobile, which formed gray colonies. In some instances, a red precipitate formed in the center of the colony. Yellowing and clearing of the growth medium was also frequently observed. Recovery of carnobacteria using CTSI was identical to that obtained with All Purpose Tween (APT) agar.