Fundamental Food Microbiology, Third Edition - Fuad Fathir

GENETICS OF SOME BENEFICIAL TRAITS 155
Figure 12.1 Plasmid profiles of Lactococcus lactis ssp. lactis. Lane A, plasmid standards of
different molecular weights (Mda); lane B, donor strain with lac-plasmid (35.5 MDa
or 53 kb; Lac + Sm s ); Lane C: plasmidless recipient strain (Lac – Sm r ); Lanes D and
E: transconjugants showing lac-plasmid (Lac + Sm r ).
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2. Analysis of the plasmid profile, i.e., types of plasmids present as determined from
their molecular weight (kb), showed that a 53-kb plasmid present in the Lac + wild
strain was missing in Lac – cured variant. Thus, loss of this plasmid was correlated
with the loss of Lac + phenotype in this strain.
3. To determine further that the 53-kb plasmid actually encoded the Lac + phenotype,
the wild Lac + strain was conjugally mated with a plasmidless Lac – strain, and
several Lac + transconjugants were obtained. (This aspect is discussed later in
mechanisms of DNA transfer.) When these transconjugants were analyzed, all
were found to contain the 53-kb plasmid (Figure 12.1). Following curing, the
transconjugants were converted to Lac – , and an analysis showed that these variants
no longer had the 53-kb plasmid.
4. From this series of experiments, it was determined that the 53-kb plasmid in the
specific Lac. lactis strain used encodes Lac + phenotype.
Similar studies were conducted to determine plasmid linkage of several other traits
in starter-culture bacteria.
E. Plasmid-Linked Traits in Starter-Culture Bacteria
Many starter-culture bacteria, especially Lac. lactis strains, have been examined for
the plasmid linkage of different phenotypes. The studies have revealed that in these
bacteria, many commercially important traits are plasmid-encoded. The following
are some traits:
• Lac. lactis
Lac + , lactose hydrolysis (also lactose transport trait)
Pro + , proteinase activity
Cit + , citrate hydrolysis (also citrate transport trait)