Use of MRSD medium and the hydrophobic grid membrane filter technique to differentiate between pediococci and lactobacilli in fermented meat and starter cultures.

Modifications of MRS medium were made by incorporation of 0.1 M L-arginine-HCl, 0.0025% phenol red, 100 IU polymyxin B sulfate, by deletion of meat extract, use of only 1.2% (w/v) glucose and increase of Mn2+ to 1000 ppm. In addition, adoption of the hydrophobic grid membrane filter (HGMF) system with 0.025% Fast Green FCF dye and adjustment of the agar medium to pH 5.5 gave MRSD (differential) medium. Incubation at 25 degrees C anaerobically under N2 or CO2 followed by a post-growth staining procedure involving use of 0.4% (w/v) bromocresol purple yielded conditions under which pediococci colonies were blue whereas homo- and heterofermentative lactobacilli were green in color. Under these conditions, 7 pediococci, 16 lactobacilli, and 18 commercial meat starter cultures were successfully analyzed by plate count to yield a differential assessment of the lactobacilli and pediococci present without interference from the 9 other genera tested. Streptococcus lactis and Leuconostoc spp. produced blue and green colonies, respectively, at 25 degrees C which might interference but these organisms are not present in significant numbers in fermented meats. Pediococcus parvulus and Streptococcus faecalis produced green and blue colonies, respectively, but their very poor growth at 25 degrees C prevented their interference. Use of the developed MRSD medium was described for enumeration of both pediococci and lactobacilli in starter cultures and in fermenting dry sausages to enable documentation of starter culture performance.