144 ◾ Fundamental Food Microbiology
large, is not determined. To locate the genes expressing a phenotype, different fragments are cloned
in a suitable vector, and afterward, transformation host cells are examined for the expression of the
phenotype. The fragment that expresses the phenotype is then sequenced to determine the locations
of the genes and the regulatory mechanisms. This process has been simplified in recent years by first
purifying the protein coded by a gene and sequencing five or more N-terminal amino acids. From
this information, nucleotide probes are constructed, and by PCR techniques, the location of the
genes in the plasmid is determined; this helps sequence the gene by the several procedures available.
Gene transfer Methods in Starter-Culture Bacteria
Once the genetic basis of a phenotype in bacteria was understood, studies were conducted to
develop means to transfer the genetic materials from one bacterial cell to another. It is recognized
that exchange of genetic materials occurs among bacteria naturally but at a slower pace. However,
if a process of introducing genetic materials can be developed under laboratory conditions, the
process of genetic exchange can not only be expedited but can also help develop desirable strains.
In starter-culture bacteria, this will help to develop a strain for a specific fermentation process that
carries many desirable phenotypes and the least number of undesirable phenotypes. 2
Results of the studies conducted since the 1970s, initially in Lac. lactis subspecies and later in
other lactic acid bacteria, revealed that genetic materials can be introduced into bacterial cells by
several different mechanisms, some of which are discussed here.
Transduction
In this process, a transducing bacteriophage mediates the DNA exchange from one bacterial
cell (donor) to another cell (recipient). DNA of some phages (designated as temperate bacteriophages) following infection of a cell can integrate with bacterial DNA and remain dormant (see
Chapter 14). When induced, the phage DNA separates out from the bacterial DNA and, on some
occasions, also carries a portion of the bacterial DNA-encoding genes in it. When the phagecarrying portion of a bacterial DNA infects a bacterial cell and integrates its DNA with bacterial
DNA, the phenotype of that gene is expressed by the recipient cell. Initially, the Lac + phenotype
from a lactose- hydrolyzing Lac. lactis strain carrying a temperate phage was transduced to a Lac –
Lactococcus lactis strain to obtain a Lac + transductant. This method has been successfully used to
transduce Lac + phenotype and several other phenotypes (such as Pro + ) in different strains of Lac.
lactis subspecies. Investigations show that both chromosomal- and plasmid-encoded genes from
bacteria can be transduced. Transduction has been conducted successfully in some strains of Str.
thermophilus and in strains of several Lactobacillus species.
The transduction process in starter-culture bacteria is important to determine the location of a
gene on the DNA for genetic mapping and to study its characteristics. A temperate phage can be
induced spontaneously, resulting in lysis of bacterial cells; thus, it is not very useful in commercial
fermentation. Also, this method cannot be applied in species that do not have bacteriophages, such
as some Pediococcus species.
Conjugation
In this process, a donor bacterial cell transfers a replica of a portion of its DNA to a recipient cell.
The two cells have to be in physical contact to affect this transfer. If the transferred DNA encodes
large, is not determined. To locate the genes expressing a phenotype, different fragments are cloned
in a suitable vector, and afterward, transformation host cells are examined for the expression of the
phenotype. The fragment that expresses the phenotype is then sequenced to determine the locations
of the genes and the regulatory mechanisms. This process has been simplified in recent years by first
purifying the protein coded by a gene and sequencing five or more N-terminal amino acids. From
this information, nucleotide probes are constructed, and by PCR techniques, the location of the
genes in the plasmid is determined; this helps sequence the gene by the several procedures available.
Gene transfer Methods in Starter-Culture Bacteria
Once the genetic basis of a phenotype in bacteria was understood, studies were conducted to
develop means to transfer the genetic materials from one bacterial cell to another. It is recognized
that exchange of genetic materials occurs among bacteria naturally but at a slower pace. However,
if a process of introducing genetic materials can be developed under laboratory conditions, the
process of genetic exchange can not only be expedited but can also help develop desirable strains.
In starter-culture bacteria, this will help to develop a strain for a specific fermentation process that
carries many desirable phenotypes and the least number of undesirable phenotypes. 2
Results of the studies conducted since the 1970s, initially in Lac. lactis subspecies and later in
other lactic acid bacteria, revealed that genetic materials can be introduced into bacterial cells by
several different mechanisms, some of which are discussed here.
Transduction
In this process, a transducing bacteriophage mediates the DNA exchange from one bacterial
cell (donor) to another cell (recipient). DNA of some phages (designated as temperate bacteriophages) following infection of a cell can integrate with bacterial DNA and remain dormant (see
Chapter 14). When induced, the phage DNA separates out from the bacterial DNA and, on some
occasions, also carries a portion of the bacterial DNA-encoding genes in it. When the phagecarrying portion of a bacterial DNA infects a bacterial cell and integrates its DNA with bacterial
DNA, the phenotype of that gene is expressed by the recipient cell. Initially, the Lac + phenotype
from a lactose- hydrolyzing Lac. lactis strain carrying a temperate phage was transduced to a Lac –
Lactococcus lactis strain to obtain a Lac + transductant. This method has been successfully used to
transduce Lac + phenotype and several other phenotypes (such as Pro + ) in different strains of Lac.
lactis subspecies. Investigations show that both chromosomal- and plasmid-encoded genes from
bacteria can be transduced. Transduction has been conducted successfully in some strains of Str.
thermophilus and in strains of several Lactobacillus species.
The transduction process in starter-culture bacteria is important to determine the location of a
gene on the DNA for genetic mapping and to study its characteristics. A temperate phage can be
induced spontaneously, resulting in lysis of bacterial cells; thus, it is not very useful in commercial
fermentation. Also, this method cannot be applied in species that do not have bacteriophages, such
as some Pediococcus species.
Conjugation
In this process, a donor bacterial cell transfers a replica of a portion of its DNA to a recipient cell.
The two cells have to be in physical contact to affect this transfer. If the transferred DNA encodes