Antibacterial Nature of Molds Isolated from Aged Beef

Abstract

The antibacterial and competitive nature of the molds Tham-nidium elegans, Mucor mucedo and Chaetostylum fresenii, commonly isolated from beef aging at refrigeration temperature for extended periods, were examined to determine if their presence on meat would inhibit bacterial growth. An agar plate method revealed no growth inhibition of six strains of meat spoilage bacteria by the molds examined. Eye round (semiten-dinosus) roasts were inoculated with spore suspensions containing nine strains of the three fungal species and incubated for 48 h at 4 or 18°C. Initial aerobic bacterial counts (35°C) on the meat samples were 2.4 x 10 4 CFU/cm 2. Mold treatment had no significant effect (P>0.05) on reduction of aerobic bacterial counts (35 and 7°C), coliforms or fecal streptococci when compared with uninoculated controls. No surface fungal growth was evident after 48 h at 4°C, the normal storage temperature for meat, whereas appreciable mycelial development was apparent after 48 h at 18°C. T. elegans, M. mucedo and C. fresenii were not capable of reducing or controlling bacterial growth on meat surfaces at refrigeration or elevated temperatures. The molds Thamnidium elegans, Mucor mucedo and Chaetostylum fresenii have been isolated from beef held in cold storage (2,7). Several U.S. patents suggest these organisms sprayed on carcasses are effective in rapid aging of beef (9-77). In addition to claims stating that the organisms aid in tenderizing and imparting "aged" flavor to meat, inferences were made concerning bacteriostatic and/or bactericidal capabilities. It is well-documented that certain fungi produce antibiotic substances that inhibit growth of other microorganisms. It has also been established that repression of bacterial growth in food systems can be attributed to depletion of essential nutrients by competitive microorganisms. The antimicrobial properties of T. elegans, M. mucedo and C. fresenii have not been documented in the scien-' University of Connecticut. 2 U.S. Department of Agriculture.-Mention of brand names does not imply endorsement by the United States Government. tific literature. Confirmation of the claims stated in the aforementioned patents could provide industry with a natural means of inhibiting surface bacterial growth during aging of carcasses or primal or subprimal cuts. If effective , the procedure could also be utilized in conjunction with high temperature conditioning. The purpose of this study was to determine the antibacterial potential and competitive nature of T. elegans, M. mucedo and C. fre-senii on synthetic media and meat substrates. MATERIALS AND METHODS Antibacterial and competitive nature assay A modified disc assay (6) was used to detect antibiotic production by the molds T. elegans, M. mucedo and C. fresenii. Six pure bacterial cultures and an inoculum mixture of the six cultures were utilized as test organisms. Stock cultures of Es-cherichia coli, Micrococcus varians, Pseudomonas fragi, Pseudomonas fluorescens, Staphylococcus aureus and Strep-tococcus faecalis were maintained on nutrient agar (Difco) 3 slants. Working bacterial cultures were prepared in 5 ml of nutrient broth (Difco) at 25°C for 24 h before use. Working mold cultures were prepared by streaking spores from stock cultures, maintained on potato dextrose agar (PDA) slants, to prepoured plates of antibiotic medium #1 (Difco) and incubated at 23±1°C for 7 to 10 d before use. The strains of molds evaluated were North Eastern Regional Research Center T. ele-gans 1613, 2467, 2468 and 2469; M. mucedo 3634 and 3635; C. fresenii 2305 and 5847; Chaetostylum herbarum 2175; As-pergillus oryzae 1988 and 1989; and American Type Culture Collection T. elegans 11808. Disc assay Petri dishes (150x15 mm) containing 40 ml of antibiotic medium #1 were prepared 24 h before use. An agar overlay containing test bacteria was prepared by inoculating separate 240-ml quantities of sterile antibiotic medium, tempered at 50°C, with 1 ml of a 1:10,000 dilution of each bacterial culture. Twelve plates per bacterial culture were overlayed and allowed to solidify. Agar "discs" (9 mm diam.) were aseptically removed from the sporulated mold cultures using a sterile cork borer. Each mold strain was assigned a number and positioned equidistant from one another on the overlayed antibiotic medium, three per plate, following a 3 x 4 rectangular lattice experimental design (3). This arrangement was utilized to as