Fundamental Food Microbiology, Third Edition - Fuad Fathir

230 FUNDAMENTAL FOOD MICROBIOLOGY
A. 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, Lab. 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 bacteriocinproducing
strain from the natural environment as well as 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 other antibacterial compound
(a circular clear zone for bacteriocins as opposed to a fuzzy zone for organic acids). 8
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 Gramnegative
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: 8–10
• 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 same or
different bacteriocins (e.g., pediocin AcH is produced by many Ped. acidilactici
strains, but sakacin A and sakacin P are produced by different strains of Lab. sake).
• Strains from different species and genera can produce the same bacteriocins (e.g.,
pediocin AcH is produced by strains of Ped. acidilactici, Ped. pentosaceus, Ped.
parvulus, Lab. plantarum, Bacillus coagulans).
• Strains from different subspecies can produce different bacteriocins (e.g., different
Lac. lactis ssp. lactis produce nisin A and lacticin 481).
• Different species from a genus can produce different bacteriocins (e.g., enterococcin
EFS2 and enterocin 900 are produced by strains of Ent. faecalis and Ent.
faecium, respectively).