Microbial Metabolites in Fruits and Vegetables 521
The physiological role of pyocins is unclear but is likely to decrease potential
competition in niches such as the rhizosphere. This has promoted their use as a
potential biocontrol agent for plant pathogens, although studies have only met with
limited success. As with many bacteriocins the narrow specificity range is a key
problem along with the generation of resistant strains. The need for DNA-damaging
conditions to induce pyocin production could be a further limitation. In addition,
because P. auruginosa is an opportunistic pathogen there would be a need to insert
the genes into an alternative host. Nevertheless, the pyocins may offer a novel method
to suppress the activity of spoilage pseudomonads on fresh-cut produce.
17.3.4.3 Mycotoxins
Mycotoxins are a type of antibiotic in terms of antimicrobial activity, but the key
difference is their toxicity to both animals and plants. Indeed, mycotoxins produced
by some molds are well-established safety hazards that cause acute, chronic,
mutagenic, and teratogenic toxicity (Bennett and Klich, 2003). In plants mycotoxins
frequently lead to stunted plant growth and decrease in yields (Masuda et al., 2003).
In basic terms, mycotoxins^ are small, secondary metabolites produced by fila-
mentous fungi. In addition to the toxic effects against humans and animals the
mycotoxins of some molds are also important in plant pathogenesis. Despite the
significance of mycotoxins, their actual physiological role is unclear. It is known
that mycotoxins have a wide range of biological activities, including antibiotic
(antibacterial and antifungal) properties, that may function to suppress competitive
microflora.
In terms of plant degradation the key mycotoxins of concern are fumonisins and
patulin. However, it should be noted that the food safety risks of mycotoxins greatly
outweigh the detrimental effects on crop quality by mold growth. Fumonsins are
made by the plant phytopathogen of Fusarium^ species (Hirooka et al., 1996). Several
fumonisins have so far been isolated, but only FB1, FB2, and FB3 are produced in
the natural environment of the plant. The mold is typically carried on seeds of corn,
and some (e.g., Fusarium moniliforme) can become integrated into the internal
(endophytic) microflora. Interestingly, the fumonisin of F. moniliforme inhibits the
production of aflatoxin by Aspergillus flavus (Hirooka et al., 1996), but this obviously
does not lead to a safer product.
Patulin was initially isolated from Penicillium patulum and received significant
attention due to its potent antibiotic activity against protozoa, bacteria, and viruses.
The^ same secondary metabolite was also isolated from other species and given^ the
names clavacin, claviformin, expansin, mycoin C, and penicidin. However, it soon
became apparent that patulin was also a potent toxin against plants and animals, and
therefore it was reclassified as a mycotoxin. Patulin is commonly associated with
soft fruits (apples, pears) contaminated with Penicillium expansum.
Conditions that stimulate mycotoxin production are typically associated with
high relative humidity in combination with temperatures on the order of 20 to 30°C.
In the field this resembles drought conditions wherein the stress imposed on plants
compromises their defenses, thereby enabling invasion by pathogenic fungi. It fol-
lows that improper storage of grain (especially if mechanically damaged) will be