Food Biopreservatives of Microbial Origin, Bacteriocin, and Nanotechnology ◾ 215
Bacteriocins of Lactic Acid Bacteria as Food Preservatives
The term bacteriocin is currently used to refer to a group of bioactive peptides produced by many
bacterial strains from Gram-positive and Gram-negative groups. The bacteriocins produced by
many strains of lactic acid bacteria and some propionic acid bacteria are of special interest in food
microbiology because of their bactericidal effect, normally, to different Gram-positive spoilage and
pathogenic bacteria and, under stressed conditions, to different Gram-negative bacteria important
in food. Chemically, bacteriocin peptides are ribosomally synthesized, cationic, amphipathic, have
α-helical or β-sheet structures, or both, and can have thioethers, disulfide bridges, or free thiol
groups. The presence of an amphipathic α-helical structure with opposing polar and nonpolar
faces along the long axis enables bacteriocins to interact with both the aqueous and lipid phases
when bound to the surface of the membrane of a sensitive bacterial cell, leading to its functional
destabilization and death of the cell. This aspect is discussed later. 9
Bacteriocin-Producing Strains
Many bacteriocin-producing strains from the different genera and species of lactic acid bacteria
have been isolated. Some of the species include Lac. lactis, Streptococcus thermophilus, Lactobacillus
acidophilus, Lab. plantarum, Lab. sake, Lab. curvatus, Leuconostoc mesenteroides, Leu. carnosum,
Leu. gelidium, Pediococcus acidilactici, Ped. pentosaceus, Ped. parvulus, Tetragenococcus halophilus, Carnobacterium piscicola, Enterococcus faecalis, Ent. faecium, and Bifidobacterium bifidum. It
appears now that bacteriocin-producing strains of lactic acid bacteria are quite common in the
food environment. A proper procedure and experience in differentiating between a bacteriocin
producer and a nonproducer (including one that produces other types of antibacterial compounds)
are important to successfully isolate a bacteriocin-producing strain from the natural environment
as well as to identify a strain already present in culture collections. Some important factors to be
considered in the isolation from the natural environment are boosting the number of producer
cells over the associative microorganisms (such as incubating in low-pH broth), selecting sensitive strains of indicators (Gram-positive, preferably from the lactic acid bacterial group and more
than one strain), using a known producer strain as a control, and recognizing the zone of growth
inhibition around a colony on an agar-medium plate produced by a bacteriocin producer and a
producer of another antibacterial compound (a circular clear zone for bacteriocins as opposed to a
fuzzy zone for organic acids). 9
Bacteriocins of lactic acid bacteria are bactericidal to sensitive cells, and death occurs very
rapidly at a low concentration. Normally, Gram-positive bacterial strains are sensitive, the spectrum or range of which can vary greatly. Although Gram-negative bacterial cells are normally
resistant to bacteriocins of lactic acid bacteria, they become sensitive following impairment of the
cell surface lipopolysaccharide structure by physical and chemical stresses. Many bacteriocins are
bactericidal against a few related species and strains, but several are effective against many strains
from different species and genera. Some other general features of bacteriocins of lactic acid bacteria
can be grouped as follows: 9–12
◾ A producer strain is immune to its own bacteriocin but can be sensitive or resistant to other
bacteriocins (e.g., pediocin AcH producing Ped. acidilactici is immune to pediocin AcH,
sensitive to nisin A, and resistant to sakacin A).
◾ A strain can produce more than one bacteriocin (e.g., a Lac. lactis strain produces lactococcin A, B, and M); many strains of the same species can produce the same or different
Bacteriocins of Lactic Acid Bacteria as Food Preservatives
The term bacteriocin is currently used to refer to a group of bioactive peptides produced by many
bacterial strains from Gram-positive and Gram-negative groups. The bacteriocins produced by
many strains of lactic acid bacteria and some propionic acid bacteria are of special interest in food
microbiology because of their bactericidal effect, normally, to different Gram-positive spoilage and
pathogenic bacteria and, under stressed conditions, to different Gram-negative bacteria important
in food. Chemically, bacteriocin peptides are ribosomally synthesized, cationic, amphipathic, have
α-helical or β-sheet structures, or both, and can have thioethers, disulfide bridges, or free thiol
groups. The presence of an amphipathic α-helical structure with opposing polar and nonpolar
faces along the long axis enables bacteriocins to interact with both the aqueous and lipid phases
when bound to the surface of the membrane of a sensitive bacterial cell, leading to its functional
destabilization and death of the cell. This aspect is discussed later. 9
Bacteriocin-Producing Strains
Many bacteriocin-producing strains from the different genera and species of lactic acid bacteria
have been isolated. Some of the species include Lac. lactis, Streptococcus thermophilus, Lactobacillus
acidophilus, Lab. plantarum, Lab. sake, Lab. curvatus, Leuconostoc mesenteroides, Leu. carnosum,
Leu. gelidium, Pediococcus acidilactici, Ped. pentosaceus, Ped. parvulus, Tetragenococcus halophilus, Carnobacterium piscicola, Enterococcus faecalis, Ent. faecium, and Bifidobacterium bifidum. It
appears now that bacteriocin-producing strains of lactic acid bacteria are quite common in the
food environment. A proper procedure and experience in differentiating between a bacteriocin
producer and a nonproducer (including one that produces other types of antibacterial compounds)
are important to successfully isolate a bacteriocin-producing strain from the natural environment
as well as to identify a strain already present in culture collections. Some important factors to be
considered in the isolation from the natural environment are boosting the number of producer
cells over the associative microorganisms (such as incubating in low-pH broth), selecting sensitive strains of indicators (Gram-positive, preferably from the lactic acid bacterial group and more
than one strain), using a known producer strain as a control, and recognizing the zone of growth
inhibition around a colony on an agar-medium plate produced by a bacteriocin producer and a
producer of another antibacterial compound (a circular clear zone for bacteriocins as opposed to a
fuzzy zone for organic acids). 9
Bacteriocins of lactic acid bacteria are bactericidal to sensitive cells, and death occurs very
rapidly at a low concentration. Normally, Gram-positive bacterial strains are sensitive, the spectrum or range of which can vary greatly. Although Gram-negative bacterial cells are normally
resistant to bacteriocins of lactic acid bacteria, they become sensitive following impairment of the
cell surface lipopolysaccharide structure by physical and chemical stresses. Many bacteriocins are
bactericidal against a few related species and strains, but several are effective against many strains
from different species and genera. Some other general features of bacteriocins of lactic acid bacteria
can be grouped as follows: 9–12
◾ A producer strain is immune to its own bacteriocin but can be sensitive or resistant to other
bacteriocins (e.g., pediocin AcH producing Ped. acidilactici is immune to pediocin AcH,
sensitive to nisin A, and resistant to sakacin A).
◾ A strain can produce more than one bacteriocin (e.g., a Lac. lactis strain produces lactococcin A, B, and M); many strains of the same species can produce the same or different