Microbial Metabolites in Fruits and Vegetables 507
This will encompass describing the significant enzymes associated with cell survival
and degradation. The impact of primary and secondary metabolites on produce
quality will be covered. Finally, aspects of regulation of microbial metabolite pro-
duction will be discussed with specific emphasis on the role of quorum sensing.
17.2 ROLE AND FUNCTION OF MICROBIAL ENZYMES
ASSOCIATED WITH FRUITS AND VEGETABLESThe enzymatic activity of microbes can have adverse effects on vegetables’ or fruits’
visual appearance and sensory properties. However, it must be noted that many of
the spoilage enzymes in microbes are also endogenous to plants. For example,
oxidative enzymes such as polyphenoloxidase and peroxidase in unprocessed veg-
etable and fruit products cause browning or other color changes. Taste and flavor
taints are caused by lipid oxidation due to the action of the plant enzyme lipoxyge-
nase. Hydrolytic enzymes that cause softening (i.e., pectinesterase and cellulase
enzymes) and amylases that degrade starch are common in plant tissue. With this
background activity it can be difficult to assess the significance of microbial enzymes
in degradation reactions. However, microbial enzymes do play a key role in sur-
mounting plant defenses and/or gaining access to nutrients.
17.2.1 SUPEROXIDE DISMUTASE, CATALASE, AND GLUTATHIONE
The early Earth was anaerobic with an atmosphere similar to that of Mars and Venus.
However, through microbial evolution the first photosynthetic bacteria arose that
could harbor the energy emitted by sunlight and further evolve into the complex
eukaryotic cells. Critically, the side product of the photosynthetic reaction was
oxygen, which accumulated in the atmosphere and enabled the evolution of aerobic
life forms. The rise in oxygen concentration proved detrimental to obligate anaer-
obes, and these had to retreat to specialized, oxygen-free niches within the environ-
ment. The success of aerobic and facultative anaerobic microbes can be principally
attributed to the energy-rich rewards of utilizing oxygen as an electron acceptor.
However, early evolution had to find methods to minimize the toxic side products
of oxygen, specifically hydrogen peroxide (H 2 O 2 ) and oxygen-free radicals. Like all
organisms, plants have to contend with the toxic effects oxygen radicals. Environ-
mental stresses such as drought, salinity, UV radiation, and ozone cause production
of high levels of oxygen-free radicals as a result of increased activity in the mito-
chondria and chloroplasts (Dat et al., 2003). However, relevant to the theme of this
chapter, the plant utilizes the antimicrobial action of hydrogen peroxide as a potent
defense mechanism against plant pathogen invasion. Essentially, the early interaction
of pathogens with plants stimulates a plasma membrane NADPH oxidase to produce
hydrogen peroxide (Breusegem et al., 2002). The main effect is to inactivate the
invading pathogen in place by degrading lipid membranes, enzymes, and DNA. The
plant is not immune to hydrogen peroxide and localized necrosis occurs, in addition
to cross-linking of cell membranes. However, the latter effects also provide a means
of localizing the sites of infection, thereby protecting the healthy plant tissue.