Growth and proteolytic activity of Lactococcus
lactis strains were
investigated in milks subjected to different heat treatments.
Only 13·7% of the
strains were of proteolytic phenotype. Proteinase-positive
(Prt+) strains exhibited a
proteolytic activity that increased when the level of heat
treatment was reduced and
grew to a higher level than proteinase-negative
(Prt−) strains in low-heat-treated
milk. When grown in autoclaved reconstituted skim milk, both
Prt− and Prt+ strains
had the same specific growth rate. Total DNA from four
Prt+ and eleven Prt− strains
was extracted and used for polymerase chain reaction
amplification of fragments
belonging to genes encoding for cell wall proteinase.
Specific amplification products
were displayed by all the Prt+
strains, and by the Prt−
strain Lc. lactis subsp. lactis
NWC 125. Reverse transcriptase-polymerase chain reaction
was performed to detect the transcript of the proteinase genes.
In addition to the Prt+ strains, the experiment
also detected the specific transcript in Lc.
lactis subsp. lactis NWC 125, suggesting
that other structural or functional deficiencies
occurred in this strain.