400 Produce Degradation: Reaction Pathways and their Prevention
developed highly specific iron-chelating systems, and often they produce siderophores,
which are iron-chelators secreted by the cell. Upon binding of iron the siderophore-
iron complex is taken up by the microbial cells and iron is liberated internally [127].
In particular, pseudomonads are prominent producers of siderophores with high iron
binding constants. The iron-chelating ability has been of particular interest in the
rhizosphere, where the use of pseudomonads as biocontrol agents against fungal dis-
eases has been attributed in part to their competitive advantage vis-à-vis iron [126,128].
Bacteriocinogenic strains of Pediococcus and Enterococcus have recently been shown
to control the growth of L.monocytogenes on mung bean sprouts [129].
Several antagonistic microorganisms have been discovered to reduce postharvest
fungal decay of apples and other pome fruits. Strains of Pseudomonas syringae Va n
Hall are effective in controlling blue mold and gray mold, caused by Penicillium
spp. and Botrytis spp., respectively, of citrus and pome fruits [130,131]. Other
bacteria such as Pseudomonas cepacia Burkholder [132], Pseudomonas gladioli
Severini [133], Bacillus pumilus, and Bacillus amyloliquefaciens [134] also have
been reported to reduce blue mold and/or gray mold on apples and pears. The yeast
Candida oleophila (AspireTM) is effective for the control of blue mold and gray mold
on citrus and pome fruits [135,136]. Species of Cryptococcus, including C. laurentii,
C. albidus, and C. flavus, reduced gray mold in pears [137]. Cryptococcus laurentii
and Rhodotorula glutinis were effective in controlling blue mold on apples and pears
[75]. The yeasts Picia anomala and Candida sake have controlled both blue mold
and gray mold on apples and pears [73,138]. P. syringae [139] are also effective in
controlling blue mold and gray mold on apples. Besides the control of blue mold
and gray mold, these P. syringae isolates have been effective for control of peach
brown rot (M. fructicola) and rhizopus rot (Rhizopus spp.) [139] and apple scab
(V. inaequalis) [140]. However, these P. syringae isolates performed poorly when
tested against M. fructicola on sweet cherries.
12.4.2 METABIOSIS
Innumerable interdependencies exists between different organisms. The term meta-
biosis describes the reliance by an organism on another to produce a favorable
environment. This can be the removal of oxygen by Gram-negative microflora,
allowing anaerobic organisms such as Clostridium botulinum to grow [141], or it
can be situations where one organism provides nutrients that enhance the growth of
another. Several studies have shown that despite the inhibitory activity of
pseudomonads describe above, their presence may also enhance growth of some
microorganisms. Preinoculation with different Gram-negative psychrotrophic bacte-
ria subsequently yielded higher growth and more acid from lactic acid bacteria [142],
and growth of Staphylococcus aureus may also be stimulated by Pseudomonas spp.
[143]. Such nutrient interdependency may also play a role in food spoilage [144,145].
12.4.3 ACYLATED HOMOSERINE LACTONE (AHL)-BASED COMMUNICATION
In recent years, it has been recognized that bacteria can behave not only as individual
cells but, under appropriate conditions and when their numbers reach a critical level,
they can modify their behavior to act as a multicellular group. This is achieved by