Bacterial Infiltration and Internalization in Fruits and Vegetables 445
14.3.3 EXTRACELLULAR POLYSACCHARIDES
The second stage of bacterial adhesion to plant surfaces seems to involve the
production of extracellular polysaccharides. This process occurs within hours or
days after contact between bacteria and plant surfaces. Increases in bacterial popu-
lations that become firmly bound to leaves over time have been reported (Beattie
and Lindlow, 1994; Wilson et al., 1999). Such increases in the extent of bacterial
adhesion may be attributed to colonization of internal leaf sites, especially by plant
pathogens (Wilson et al., 1999), and production of extracellular polysaccharides
(Takahashi and Doke, 1984). Published reports describing amorphous material
around bacteria on leaf surfaces (Beattie and Lindlow, 1999) and around clumps of
bacterial cells detached from leaves (Morris et al., 1997) provide ample evidence in
support of bacterial production of extracellular polysaccharides on leaves. Davies
et al. (1993) reported the induction of some bacterial genes linked to the production
of extracellular polysaccharides via bacterial contact with a solid surface. The pres-
ence of extracellular polysaccharides on surfaces usually results in the formation of
biofilms, which enhance bacterial colonization and survival on leaves (Morris et al.,
1997; Beattie and Lindlow, 1999). Injury of the leaf cuticle and release of juices
from damaged tissue could result in multiplication of bacteria in biofilms and
subsequent spread of bacterial cells to internal leaf tissue. A comprehensive account
of bacterial attachment and biofilms on fruits and vegetables is provided in Chapter
13 of this book.
14.4 INTERNALIZATION OF BACTERIA IN FRUITS
AND VEGETABLES14.4.1 FACTORS THAT FACILITATE INTERNALIZATION
14.4.1.1 Natural Openings
Most microorganisms on the surface of intact fresh produce are prevented from
entering subsurface tissues by the cuticular layer that covers the epidermis of leaves,
stems, and fruits (Nguyen-The and Carlin, 2000). However, natural openings on the
surface of fruits and vegetables can provide channels through which bacteria can
enter these food products. For example, Leben (1972) found bacterial cells inside
more than 60% of the buds of field-grown crops, including certain legumes (soybean
and red clover), turnips, grapes, and cucumbers. Itoh et al. (1998) reported that
E. coli O157:H7 were found in the stomata of radish sprouts grown in contaminated
water. Bartz and Showalter (1981) demonstrated that Serratia marcescens became
internalized in tomatoes that were immersed in cell suspensions of the organism.
Populations of this organism were more concentrated in tissues just below the stem
scar area of tomatoes. Similar studies involving immersion of apples (Buchanan et
al., 1999), oranges, and grapefruit (Merker et al., 1999) in a dye solution indicated
the potential for pathogenic bacteria to infiltrate natural openings and become inter-
nalized in the core areas of these fruits. Major routes of bacterial entry were the
blossom end and stem scar area of apples and citrus fruits, respectively. Bacteria