Fundamental Food Microbiology, Third Edition - Fuad Fathir

168 FUNDAMENTAL FOOD MICROBIOLOGY
purines and pyrimidines, complete PTS-sugar transport systems, and partial components
of aerobic respiration. The genome appears to have a fluid structure and is
able to undergo changes through point mutation, DNA rearrangements, and horizontal
gene transfer. The genome organization has resulted from the evolutionary
pressure to enable the species to grow optimally in a nutritionally rich medium. 18,19
As the genome sequence of different species and strains of lactic acid bacteria
become available, the information can be used to compare and understand various
important characteristics of lactic acid bacteria, such as determining the evolutionary
relationship among the species and strains in various genera, functions of each gene
and its essential and nonessential nature, and functions of the noncoding region on
the regulation and expression of a gene or an operon system. This information can
then be used to modulate gene expression and efficiently conduct metabolic engineering
to develop new strains for use in producing novel fermented products and
important by-products. 16–19
B. Bacteriophages
The genomes of 20 important temperate and virulent bacteriophages of lactic acid
bacteria (from genera Lactococcus, Lactobacillus, and Streptococcus) have been
completely sequenced. Most have genomes ranging from 20 to 55 kb. The information
has helped locate and understand the functions of many ORFs and regulatory
regions. Some of them are the genes involved in packaging phage DNA in the heads,
genes associated with lysis of host cells, and genes controlling the lysogenic and
lytic cycles of a phage. An understanding of these factors has helped develop phageresistant
lactic acid bacterial strains for use in food fermentation and to improve
acceptable characteristics of some fermented foods (e.g., accelerated cheese ripening)
and transferring and improving expression of genes in lactic acid bacteria. 20
Several methods of developing phage-resistant starter-culture bacteria are listed in
Chapter 13.
Analyses of genomes of lactic acid bacteria reveal the presence of many prophages.
Although they pose a metabolic burden to host cells and if induced may lyse
the host cells, it is surprising that the cells carry them. However, the cells carrying
the prophage are resistant to attack by temperate phage and will be thus dominant
in the population. In addition, a mutation in the prophage induction system will
enable the cell to maintain resistance without being lysed. This could lead to development
of phage-resistant lactic acid bacterial strains. 20
C. The lac and las Genes
The genes associated with metabolism of lactose to lactic acid in different lactic
acid bacteria are located in at least two operon systems, and are grouped as lac
genes and las genes. The lac genes are associated with the transport and hydrolysis
of lactose to two hexoses, and partial metabolism of some hexoses, whereas the las
(lactic acid synthesis) genes are involved in the production of lactic acid. 17,21
The lac genes, depending on a species and strain, can be located either on a
chromosome or on a plasmid. In most Lac. lactis strains, they are plasmid linked,