To develop a nisin-producing cheese starter, Lactococcus lactis subsp. cremoris SK110 was conjugated with transposon Tn5276-NI, which codes for nisin immunity but not for nisin production. Cheese made with transconjugant SK110::Tn5276-NI as the starter was bitter. The muropeptide of the transconjugant contained a significantly greater amount of tetrapeptides than the muropeptide of strain SK110, which could have decreased the susceptibility of the cells to lysis and thereby the release of intracellular debittering enzymes.