Fundamental Food Microbiology, Third Edition - Fuad Fathir

158 FUNDAMENTAL FOOD MICROBIOLOGY
A. 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 13). When induced, the phage DNA separates
out from the bacterial DNA and, on some occasions, can also carry a portion of the
bacterial DNA encoding genes in it. When the phage-carrying 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 – Lac. 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
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.
B. 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 effect this transfer.
If the transferred DNA encodes a phenotype, the transconjugant will have the
phenotype. To make DNA transfer possible, the donor cells should have several
other genes, such as clumping factor (for a physical contact through clumping) and
a mobilizing factor (to enable the DNA to move from a donor to a recipient). The
process consists of selecting the right donor and recipient strains, mixing the two
cell types in a donor:recipient ratio of 2:1 to 10:1 in several different ways for DNA
transfer to occur, and then identify the transconjugants by appropriate selection
techniques.
This technique has been used successfully to transfer several plasmid-linked
phenotypes in some lactic acid bacteria. The plasmid-linked Lac + phenotype was
transferred conjugally between two Lac. lactis species. The transconjugant was Lac +
and had the specific plasmid, the loss of which resulted in its phenotype becoming
Lac – . Subsequently, the Lac + phenotype located in different plasmids in many Lac.
lactis subspecies and strains were conjugally transferred to Lac – strains of the same
species. Conjugal transfer of different plasmid-linked traits has also been reported
in other lactic acid bacteria, such as the diacetyl production trait in Lac. lactis ssp.
lactis biovar diacetilactis.