Abstract: Methods for increasing the resistance to rumen inactivation of a cultured Gram positive bacteria strain useful for gastrointestinal delivery of bioactive compounds to ruminants, which includes the steps of growing a culture of the bacteria strain through at least one passage in a growth medium containing an amount of lysozyme effective to induce the growth of bacterial cell walls resistant to protozoal predation; and recovering the bacteria strain from the lysozyme-containing medium. Rumen-bypass feed supplements produced by the inventive method are also disclosed, as well as methods for supplementing the diet of a ruminant with the rumen bypass feed supplements and an in vitro method for evaluating the resistance of Gram positive bacteria strains to rumen inactivation in vivo.