Fundamental Food Microbiology, Third Edition - Fuad Fathir

GENETICS OF SOME BENEFICIAL TRAITS 153
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II. PLASMIDS AND PLASMID-LINKED TRAITS IN
STARTER-CULTURE BACTERIA
Starter-culture bacteria, like other bacteria, carry genetic information (genetic code)
in the circular chromosomal DNA, circular plasmids, and transposons. Chromosomal
DNA carries 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, i.e.,
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. 1,2
A. Important Characteristics of Bacterial Plasmids
• Plasmids are double-stranded, circular, self-replicating DNA that can vary in size
(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 differ 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 procaryotes and eucaryotes) 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.