154 ◾ Fundamental Food Microbiology
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, the loss of genes, the presence of pseudogenes, the function of each gene and its
essential and nonessential nature, and the function 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 byproducts. 18,19
Bacteriophages
The genomes of several important temperate and virulent bacteriophages of lactic acid bacteria (from genera Lactococcus, Lactobacillus, Pediococcus, Enterococcus, 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 the lysis of host cells,
and genes controlling the lysogenic and lytic cycles of a phage. An understanding of these factors
has helped develop phage-resistant 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 14.
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 a 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
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. 21,22
The lac genes, depending on species and strain, can be located either on a chromosome or on
a plasmid. In most Lac. lactis strains, they are plasmid linked, but in Lab. delbrueckii ssp. bulgaricus and Lab. helveticus they are on the chromosomes. Whereas in Lac. lactis they are inducible
(also in Lab. acidophilus), in the two Lactobacillus species, they are constitutive. Limited studies have revealed lac genes to be plasmid linked in Leu. lactis and chromosomally linked in Str.
thermophilus.
In the Lac. lactis strains, lactose fermentation usually includes its transportation by the PEPPST system as lactose phosphate, hydrolysis by phospho-β-galactosidase to glucose and galactose6-phosphate, and conversion of galactose-6-phosphate to tagatose-6-phosphate and then to two
triosephosphates before entering the EMP pathway (see Chapter 12). Glucose is also converted
to triosephosphate in the EMP pathway. Triosephosphates are finally converted to lactic acid. In
Lab. delbrueckii ssp. bulgaricus, lactose is transported (along with a proton) by lactose permease
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, the loss of genes, the presence of pseudogenes, the function of each gene and its
essential and nonessential nature, and the function 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 byproducts. 18,19
Bacteriophages
The genomes of several important temperate and virulent bacteriophages of lactic acid bacteria (from genera Lactococcus, Lactobacillus, Pediococcus, Enterococcus, 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 the lysis of host cells,
and genes controlling the lysogenic and lytic cycles of a phage. An understanding of these factors
has helped develop phage-resistant 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 14.
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 a 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
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. 21,22
The lac genes, depending on species and strain, can be located either on a chromosome or on
a plasmid. In most Lac. lactis strains, they are plasmid linked, but in Lab. delbrueckii ssp. bulgaricus and Lab. helveticus they are on the chromosomes. Whereas in Lac. lactis they are inducible
(also in Lab. acidophilus), in the two Lactobacillus species, they are constitutive. Limited studies have revealed lac genes to be plasmid linked in Leu. lactis and chromosomally linked in Str.
thermophilus.
In the Lac. lactis strains, lactose fermentation usually includes its transportation by the PEPPST system as lactose phosphate, hydrolysis by phospho-β-galactosidase to glucose and galactose6-phosphate, and conversion of galactose-6-phosphate to tagatose-6-phosphate and then to two
triosephosphates before entering the EMP pathway (see Chapter 12). Glucose is also converted
to triosephosphate in the EMP pathway. Triosephosphates are finally converted to lactic acid. In
Lab. delbrueckii ssp. bulgaricus, lactose is transported (along with a proton) by lactose permease