Attachment of Bacterial Human Pathogens on Fruit and Vegetable Surfaces 423
associations of shigellosis with lettuce, scallions, and parsley; cholera with straw-
berries; parasitic diseases with raspberries, basil, and apple cider; hepatitis A virus
with lettuce, raspberries, and frozen strawberries; and Norwalk/Norwalk-like virus
with melon, salad, and celery have been made [21,22]. A better understanding of
bacterial adhesion to fruits and vegetables is needed for the development of more
effective washing treatments to control microorganisms on fresh-cut produce.
13.3 SURFACES OF FRUITS AND VEGETABLES
13.3.1 SURFACE CHARACTERISTICS
The surfaces of fruits and vegetables show a large diversity in structure and com-
position; the epidermis is covered by an epicuticular wax on aerial organs (leaves,
stem, flowers, and fruits) or periderm on roots and tubers. Stomata, lenticels, broken
trichomes, and scars from detached organs represent natural ways of entry for
microorganisms. Cracks in the surface of vegetables and fruits may occur in certain
growing conditions [23,24], and postharvest handling may cause injuries and bruising.
Unlike fruits with smooth surfaces, the outer surface (rind) of a cantaloupe
presents a variety of surfaces to which a bacterium may bind. The epidermal cell
surface is ruptured with a meshwork of raised tissue (the net). This net consists of
lenticels and phellum (cork) cells. These cells have hydrophobic suberized walls to
reduce water loss and protect against pathogen ingress. Also imparting a hydrophobic
nature to the outer surface of cantaloupe is the cuticle composed of waxes and cutin
that covers the epidermal cells [25]. Hydrophilic components of plant cell walls and
middle lamella may also be exposed to bacterial invasion due to cuticular cracks
and injuries to the epidermal surface.
13.3.2 NATIVE MICROFLORA OF FRUITS AND VEGETABLES
There is great variation in the number and type of native microflora among fruits
and vegetables due in part to their surface structures, chemical composition, type of
fruits or vegetables, growth and harvesting condition, including processing treat-
ments before, during, or after storage. It has been reported that 40 to 70% of the
total flora of peas, snap beans, and corn consist of leuconostocs, streptococci, and
corynebacteria, a Gram-positive rod [26]. Others are groups of bacteria that cause
soft rot. Fruits contain less water and more carbohydrates and therefore would
support growth of bacteria, yeast, and mold. However, the pH of fruits is below the
level that generally favors bacterial growth [27].
13.3.3 TYPES OF SPOILAGE MICROFLORA
Microorganisms responsible for postharvest diseases are not necessarily dominant
on the surface of sound vegetables. On cabbage leaves Botrytis cinerea represented
less than 0.1% of the total microflora [28], and on witloof chicory only 3% of the
epiphytic isolates caused spoilage upon inoculation [29]. Some spoilage microor-
ganisms may be specific to a few vegetable species, whereas others, such as Botrytis