Microbial Ecology of Spoilage 385
12.3 GROWTH OF BACTERIA ON VEGETABLES
12.3.1 DECAY OF VEGETABLES BY BACTERIA
Bacteria are a major cause of disease and decay of vegetables in the field as well
as after harvest. The ability of bacteria to degrade plant tissue is attributed to their
ability to degrade pectic substances, which are important components of the middle
lamella and the primary cell walls of the parenchymatous tissue of dicotyledonous
and monocotyledonous plants. (Figure 12.1, Selvendren [59]).
The major constituents of pectic substances are rhamnogalacturonans. Those in
which a proportion of the galacturonic acid residues are present as methyl esters are
designated pectinic acids or pectins; those without methyl ester groups are called
pectic acids.
According to Selvendren [59] there is still debate about the localization of pectins
within the walls of parenchymatous tissues. The middle lamella of parenchymatous
tissue is thought to consist principally of the calcium salts of pectins. The properties
of pectin substances in plant cell walls have been reviewed by Selvendren [59].
The ability of bacteria to cause rotting of plant tissue results mainly from their
production of an array of enzymes that attack the rhamnogalacturonan polymer by
hydrolysis (hydrolases) or by elimination (lyases) [60,61]. The bacteria that are
mainly responsible for rotting of vegetables after harvest are E. carotovora and
certain fluorescent pseudomonads (Pseudomonas marginalis) [3,17,18]. They also
contribute to the spoilage of prepared, ready-to-use, fresh vegetables [34]. Pectic
enzymes are formed by many plant pathogenic bacteria, including species of
Erwinia, Pseudomonas, and Xanthomonas. Other bacteria that form pectic enzymes
and cause softening of plant tissue include A. liquefaciens, associated with the
softening of ripe olives [62,63], Cytophaga spp. [16], Bacillus spp., Clostridium spp.
FIGURE 12.1Pectic polymers in the primary wall of dicotyledonous and monocotyledonous
plants. (From Selvendren, R.R., J. Cell Sci., 2 (Suppl.), 51, 1985. With permission.)