The autolysin, N-acetyl muramidase (AcmA), of six commercial
Lactococcus lactis subsp. cremoris starter strains and eight Lc. lactis subsp. cremoris
derivatives or plasmid-free strains was shown by renaturing SDS-PAGE (zymogram
analysis) to be degraded by the cell envelope proteinase (lactocepin; EC 3.4.21.96)
after growth of strains in milk at 30 °C for 72 h. Degradation of AcmA was
less in starter strains and derivatives producing lactocepin I/III (intermediate
specificity) than in strains producing lactocepin I. This supports previous
observations on AcmA degradation in derivatives of the laboratory strain Lc. lactis
subsp. cremoris MG1363 (Buist et al. Journal of Bacteriology180 5947–5953 1998). In
contrast to the MG1363 derivatives, however, the extent of autolysis in milk of the
commercial Lc. lactis subsp. cremoris starter strains in this study did not always
correlate with lactocepin specificity and AcmA degradation. The distribution of
autolysins within the cell envelope of Lc. lactis subsp. cremoris starter strains and
derivatives harvested during growth in milk was compared by zymogram analysis.
AcmA was found associated with cell membranes as well as cell walls and some
cleavage of AcmA occurred independently of lactocepin activity. An AcmA product
intermediate in size between precursor (46 kDa) and mature (41 kDa) forms of AcmA
was clearly visible on zymograms, even in the absence of lactocepin I activity. These
results show that autolysis of commercial Lc. lactis subsp. cremoris starter strains is
not primarily determined by AcmA activity in relation to lactocepin specificity and
that proteolytic cleavage of AcmA in vivo is not fully defined.
Diversity of cell envelope proteinase specificity among strains of Lactococcus lactis and its relationship to charge characteristics of the substrate-binding region.
