Genetics of Some Beneficial Traits ◾ 141
◾ Conjugal transfer. Small plasmids are nonconjugative (but can be transferred along with a
conjugative plasmid); large plasmids are generally conjugative.
◾ Stability. Small plasmids are usually unstable; large plasmids are usually stable.
◾ Genetic code. A large plasmid can encode many phenotypes; a small plasmid can encode only
one or a few phenotypes. Plasmids can also have the operon system.
Presence of Plasmids in Some Starter-Culture Bacteria
◾ Lactococcus. Many strains from both subspecies and the biovar have been analyzed. Most
strains have 2–10 or more types of plasmids, both small and large.
◾ Streptococcus thermophilus. Among the strains examined, a few strains carry plasmids, generally only one to three types, mostly small and not very large.
◾ Leuconostoc. Many species and strains carry plasmids of different sizes, ranging from 1 to 10
or more types.
◾ Pediococcus. Limited studies show that the strains either have none or have two to three
plasmids, both small and large types.
◾ Lactobacillus. Only a limited number of species and strains have been tested. Some species rarely carry plasmids, such as Lactobacillus acidophilus. Some carry usually a few, such
as Lactobacillus casei. Some carry a large number (two to seven) of various sizes, such as
Lactobacillus plantarum.
◾ Bifidobacterium. Limited studies reveal that the species harbor two to five different types of
plasmids.
◾ Propionibacterium. Limited studies show that some species contain only a few plasmids.
Phenotype Assignment to a Plasmid
Following the understanding of plasmid characteristics and the revelation that many starter-culture
bacteria carry plasmids, studies were conducted to determine whether a particular plasmid in a
strain carries genetic codes necessary for the expression of a specific phenotype or to determine
whether a particular phenotype is linked to a specific plasmid in a strain. The loss of lactosefermenting ability (Lac + phenotype) among Lac. lactis strains was suspected to be because of the
loss of a plasmid that encodes genes necessary for lactose hydrolysis (also for lactose transport). The
possible linkage of the Lac + phenotype to a plasmid in a Lac. lactis strain was first studied according to the following protocol:
A Lac + strain can become Lac – (with the inability to hydrolyze lactose) spontaneously or when
grown in the presence of a chemical curing agent (e.g., acriflavin) or a physical curing agent (e.g.,
high temperature).
Analysis of the plasmid profile, that is, types of plasmids present as determined from their
molecular size (kb), showed that a 53 kb plasmid present in the Lac + wild strain was missing in the
Lac – cured variant. Thus, loss of this plasmid was correlated with the loss of the Lac + phenotype
in this strain.
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 13.1). Following
curing, the transconjugants were converted to Lac – , and an analysis showed that these variants no
longer had the 53 kb plasmid.
◾ Conjugal transfer. Small plasmids are nonconjugative (but can be transferred along with a
conjugative plasmid); large plasmids are generally conjugative.
◾ Stability. Small plasmids are usually unstable; large plasmids are usually stable.
◾ Genetic code. A large plasmid can encode many phenotypes; a small plasmid can encode only
one or a few phenotypes. Plasmids can also have the operon system.
Presence of Plasmids in Some Starter-Culture Bacteria
◾ Lactococcus. Many strains from both subspecies and the biovar have been analyzed. Most
strains have 2–10 or more types of plasmids, both small and large.
◾ Streptococcus thermophilus. Among the strains examined, a few strains carry plasmids, generally only one to three types, mostly small and not very large.
◾ Leuconostoc. Many species and strains carry plasmids of different sizes, ranging from 1 to 10
or more types.
◾ Pediococcus. Limited studies show that the strains either have none or have two to three
plasmids, both small and large types.
◾ Lactobacillus. Only a limited number of species and strains have been tested. Some species rarely carry plasmids, such as Lactobacillus acidophilus. Some carry usually a few, such
as Lactobacillus casei. Some carry a large number (two to seven) of various sizes, such as
Lactobacillus plantarum.
◾ Bifidobacterium. Limited studies reveal that the species harbor two to five different types of
plasmids.
◾ Propionibacterium. Limited studies show that some species contain only a few plasmids.
Phenotype Assignment to a Plasmid
Following the understanding of plasmid characteristics and the revelation that many starter-culture
bacteria carry plasmids, studies were conducted to determine whether a particular plasmid in a
strain carries genetic codes necessary for the expression of a specific phenotype or to determine
whether a particular phenotype is linked to a specific plasmid in a strain. The loss of lactosefermenting ability (Lac + phenotype) among Lac. lactis strains was suspected to be because of the
loss of a plasmid that encodes genes necessary for lactose hydrolysis (also for lactose transport). The
possible linkage of the Lac + phenotype to a plasmid in a Lac. lactis strain was first studied according to the following protocol:
A Lac + strain can become Lac – (with the inability to hydrolyze lactose) spontaneously or when
grown in the presence of a chemical curing agent (e.g., acriflavin) or a physical curing agent (e.g.,
high temperature).
Analysis of the plasmid profile, that is, types of plasmids present as determined from their
molecular size (kb), showed that a 53 kb plasmid present in the Lac + wild strain was missing in the
Lac – cured variant. Thus, loss of this plasmid was correlated with the loss of the Lac + phenotype
in this strain.
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 13.1). Following
curing, the transconjugants were converted to Lac – , and an analysis showed that these variants no
longer had the 53 kb plasmid.