140 ◾ Fundamental Food Microbiology
genetic codes for vital functions of a cell (such as a key enzyme in the EMP or HMS pathway
in lactic acid bacteria). Although both plasmid DNA and transposons can carry genetic codes,
they are only for nonvital functions, that is, characteristics that are not absolutely necessary for
the survival of a cell (such as the ability to hydrolyze a large protein). However, having such a
genetic code gives a strain a competitive advantage over other strains that lack it but share the
same environment. Initial research in the early 1970s revealed that many industrially important phenotypes in different lactic acid bacteria are plasmid linked. Since then, because specific
techniques are available, the genetic basis of many plasmid-encoded important phenotypes in
many starter-culture bacteria, particularly in Lactococcus lactis, some Leuconostoc spp., and some
Lactobacillus spp. has been studied. These studies have not only helped identify the locations
of many genes, their structure, and the control systems involved in their expression, but also
enabled researchers to transfer genes into a cell lacking a specific phenotype and to create a new
desirable strain. Characteristics of plasmids and some plasmid-linked traits in starter-culture
bacteria are discussed here. 1
Important Characteristics of Bacterial Plasmids
◾ Plasmids are double-stranded, circular, self-replicating DNA that can vary in size (<1 to
>100 kb).
◾ Plasmids may not be present in all species or all strains in a species.
◾ A strain can have more than one type of plasmid that differs in size and the genetic code it
carries.
◾ A plasmid can be present in more than one copy in a cell (copy number; this is in contrast
to a single copy of chromosome that a cell can carry).
◾ For some plasmids, copy numbers can be reduced (depressed) or increased (amplified) by
manipulating the control systems.
◾ Plasmids can differ in their stability in a cell. A plasmid from a cell can be lost spontaneously
or by manipulation.
◾ Two types of plasmids in a cell may be incompatible, resulting in the loss of one.
◾ Plasmids can be transferred from one cell (donor) to another compatible cell (recipient)
spontaneously or through manipulation.
◾ Plasmid transfer can occur either only between closely related strains (narrow host range) or
between widely related strains from different species or genera (broad host range).
◾ A plasmid can be cryptic (i.e., not known to carry the genetic code for a known trait).
◾ Effective techniques for the isolation, purification, molecular weight determination, and
nucleotide sequence determination of bacterial plasmids have been developed.
◾ Genetic codes from different sources (different prokaryotes and eukaryotes) can be introduced into a plasmid, which then can be transferred into the cell of an unrelated bacterial
species in which the phenotype may be expressed.
Some Characteristics of Small (ca. 10 kb) and
Large (over 10 to ca. 150 kb) Plasmids
◾ Copy number. Small plasmids generally occur in multiple copies (10–40 per cell); large plasmids are generally present in lesser copies (for a very large plasmid even one copy per cell).
◾ Amplification. Many small plasmids can be amplified to a very high copy number; large
plasmids, especially very large ones, cannot be amplified.
genetic codes for vital functions of a cell (such as a key enzyme in the EMP or HMS pathway
in lactic acid bacteria). Although both plasmid DNA and transposons can carry genetic codes,
they are only for nonvital functions, that is, characteristics that are not absolutely necessary for
the survival of a cell (such as the ability to hydrolyze a large protein). However, having such a
genetic code gives a strain a competitive advantage over other strains that lack it but share the
same environment. Initial research in the early 1970s revealed that many industrially important phenotypes in different lactic acid bacteria are plasmid linked. Since then, because specific
techniques are available, the genetic basis of many plasmid-encoded important phenotypes in
many starter-culture bacteria, particularly in Lactococcus lactis, some Leuconostoc spp., and some
Lactobacillus spp. has been studied. These studies have not only helped identify the locations
of many genes, their structure, and the control systems involved in their expression, but also
enabled researchers to transfer genes into a cell lacking a specific phenotype and to create a new
desirable strain. Characteristics of plasmids and some plasmid-linked traits in starter-culture
bacteria are discussed here. 1
Important Characteristics of Bacterial Plasmids
◾ Plasmids are double-stranded, circular, self-replicating DNA that can vary in size (<1 to
>100 kb).
◾ Plasmids may not be present in all species or all strains in a species.
◾ A strain can have more than one type of plasmid that differs in size and the genetic code it
carries.
◾ A plasmid can be present in more than one copy in a cell (copy number; this is in contrast
to a single copy of chromosome that a cell can carry).
◾ For some plasmids, copy numbers can be reduced (depressed) or increased (amplified) by
manipulating the control systems.
◾ Plasmids can differ in their stability in a cell. A plasmid from a cell can be lost spontaneously
or by manipulation.
◾ Two types of plasmids in a cell may be incompatible, resulting in the loss of one.
◾ Plasmids can be transferred from one cell (donor) to another compatible cell (recipient)
spontaneously or through manipulation.
◾ Plasmid transfer can occur either only between closely related strains (narrow host range) or
between widely related strains from different species or genera (broad host range).
◾ A plasmid can be cryptic (i.e., not known to carry the genetic code for a known trait).
◾ Effective techniques for the isolation, purification, molecular weight determination, and
nucleotide sequence determination of bacterial plasmids have been developed.
◾ Genetic codes from different sources (different prokaryotes and eukaryotes) can be introduced into a plasmid, which then can be transferred into the cell of an unrelated bacterial
species in which the phenotype may be expressed.
Some Characteristics of Small (ca. 10 kb) and
Large (over 10 to ca. 150 kb) Plasmids
◾ Copy number. Small plasmids generally occur in multiple copies (10–40 per cell); large plasmids are generally present in lesser copies (for a very large plasmid even one copy per cell).
◾ Amplification. Many small plasmids can be amplified to a very high copy number; large
plasmids, especially very large ones, cannot be amplified.