Lytr, a phage-derived amidase is most effective in induced lysis of Lactococcus lactis compared with other lactococcal amidases and glucosaminidases

In the genome of Lactococcus lactis IL1403 five genes encoding peptidoglycan hydrolases are present: four glucosaminidases (acmA, acmB, acmC and acmD) and an endopeptidase (yjgB). Genes for six prophage lysins have also been identified. The genes acmB, acmC, acmD, yjgB and the lysin lytR of prophage rl t and the lytic gene of the prophage bIL309 were cloned downstream of the nisin-inducible promoter in L. lactis NZ9000. Upon nisin induction, increased lysis of the expressing strain was obtained in all cases. Most lysis was obtained with AcmC and the two prophage lysins. Moreover, lysis by AcmB, AcmD and YjgB depends on the presence of the autolysin AcmA. The genes lytR, acmD and acmD in an operon-like structure with acmA were expressed in a foodgrade manner in a L. lactis strain that was used in conjunction with a starter to make Gouda-type cheese. Overexpression of lytR or the combination acmA/acmD resulted in increased cellular lysis in the cheese compared to the reference culture. LytR induced lysis was delayed but eventually resulted in increased lysis in trans of the acidifying L. lactis strain 13 M. (C) 2007 Elsevier Ltd. All rights reserved.