520 Produce Degradation: Reaction Pathways and their Prevention
17.3.4.2 Bacteriocins
The biopreservative effect of lactic acid bacteria is well established and has been
exploited in fermentations since early civilization. Of course, the main mode of
enhancing food safety and stability by fermentation is through the production of
organic acids that reduce the pH of the food matrix. As discussed previously, the
generation of antimicrobial oxygen species can also contribute to suppressing micro-
bial growth of spoilage microbes. However, a number of lactic acid bacteria also
produce small peptides that have antibacterial properties and are termed bacteriocins
(Table 17.3). Characteristically, bacteriocins have a narrow spectrum of activity
(compared to antibiotics) centered about homologous (closely related) species. To
date the majority of studies of bacteriocins have focused on antimicrobial properties
against virulent pathogens such as Listeria monocytogenes and Clostridium spp.
(reviewed by Cleveland et al., 2001). The role of bacteriocins in preventing spoilage
of vegetables and fruits has received less attention. This is probably based on the
poor antimicrobial activity of bacteriocins produced by lactic acid bacteria on key
spoilage microbes such as pseudomonads. Although the bacteriocins from lactic acid
bacteria have attracted the most attention, it should be noted that Gram-negative
bacteria also produce antimicrobial peptides. Indeed, the very first bacteriocins
isolated were colicins from E. coli. The mode of action of the majority of antimi-
crobial peptides is via disruption of cell membranes or nucleic acids.
In terms of suppressing plant pathogens attention has been given to a group of
bacteriocins (pyocins) produced by Pseudomoas spp. The capacity to produce
pyocins is widespread in P. aeruginosa species, but they are only produced in
significant amounts when cultures are exposed to mutagenic agents, for example,
UV radiation or mitomycin C (Michel-Briand and Baysse, 2002). In broad terms
there are three types of pyocin produced by P. aeruginosa. The R2 (R-type) and F2
(F-type) pyocins are phage tails, both of which are resistant to nuclease and protease.
S-type pyocins are thought to act as nucleases and kill cells by breaking down the
DNA of susceptible cells. It should be noted that pyocins produced by other
pseudomonads are different from those of P. auruginosa, although the narrow spec-
ificity is a common feature (Parret and de Mot, 2002; Parret et al., 2003).
TABLE 17.3
Selection of Bacteriocins Produced by Bacteria
Bacteriocin Target Organisms Ref.Nisin A Listeria monocytogenes Cutter and Siragusa, 1998
Pisciolin 126 Listeria monocytogenes Jack et al., 1996
Pedicon AcH Listeria monocytogenes Goff et al., 1996
Enterocin Listeria monocytogens Aymerich et al., 2000
Colicins Coliforms Lazdunski, 1998
Pyrocins Pseudomonas aeruginosa Michel-Briand and Baysse, 2002
Agrocin 84 Agrobacterium Penyalver and Lopez, 1999