newly - discovered antibiotic a10255 factors b , c , e , and f are produced by submerged aerobic fermentation of previously - unknown streptomyces gardneri strain nrrl 15922 , or an a10255 - producing variant , mutant or recombinant thereof . the antibiotics are active against a wide variety of pathogenic bacteria , and also enhance feed - utilization efficiency in chickens , weanling pigs and cattle .