Bacterial Infiltration and Internalization in Fruits and Vegetables 443
such as Salmonella and enterohemorrhagic Escherichia coli may come from the
feces of birds; Wallace et al. (1997) isolated vero cytotoxin-producing Escherichia
coli O157:H7 from wild birds.
Compared to epiphytic bacteria, certain human enteric bacteria can survive and
grow in a more limited set of conditions created by bruises and wounds on fruits
and vegetables. Several reports have highlighted the ability of Salmonella and E. coli
O157:H7 to survive on wounded or seemingly undamaged fresh produce (Beuchat,
1996; Kakiomenou et al., 1998; Porto and Eiora, 2001). A variety of vegetables and
relatively low-acid fruits can support rapid proliferation of Salmonella and E. coli
O157:H7 within the temperature range of 15 to 25°C that is usually encountered
during handling of fresh produce. Postharvest proliferation of these pathogens on
fresh produce, particularly on various types of lettuce, can occur under temperature
and relative humidity conditions that permit bacterial growth, thus increasing the
risk of foodborne disease (Abdul Raouf et al., 1993; Diaz and Hotchkiss, 1996).
The potential for contamination of fresh produce with pathogenic bacteria from the
environment and the limited efficacy of mere washing to remove pathogens pose a
food safety risk if leafy vegetables are eaten without cooking. This risk can be further
increased when postharvest processing conditions permit infiltration of pathogenic
bacteria in fresh produce and render them inaccessible to the killing effects of
chemical sanitizers applied to the surface of fruits and vegetables.
14.3 BACTERIAL ADHESION TO FRUITS AND VEGETABLES
14.3.1 BACTERIAL ADHESION
Bacterial adhesion to or contact with damaged or intact plant surfaces precedes entry
of these organisms into fresh produce and is facilitated by several factors, including
cell surface charge (Fletcher and Loeb, 1979; Ukuku and Fett, 2002), hydrophobicity
(Van Loosdrecht et al., 1987; Van der Mei et al., 1991; Ukuku and Fett, 2002), and
extracellular polysaccharides (Fletcher and Floodgate, 1973; Frank, 2000). As found
in some fungi, the process of bacterial adhesion to plant surfaces may occur in two
stages: a rapid stage that occurs soon after bacteria contact the plant surface and a
slower stage that increases during colonization of the surface. The concept of a rapid
stage of bacterial adhesion is supported by Hass and Rotem (1976). Those research-
ers demonstrated that washing of cucumber leaves immediately after inoculating
them with Pseudomonas lachrymans only removed about 17% of this organism.
This percentage of P. lachrymans cells removed by washing remained relatively
constant irrespective of whether a low (10^4 cells/mL) or high (10^11 cells/mL) level
of inoculum was used.
Few studies have addressed the effects of differences in types of foodborne
bacteria on the extent of bacterial adhesion to the surface of fresh and minimally
processed vegetables. Some foodborne bacteria such as Pseudomonas fluorescens
have been shown to preferentially adhere to intact leaf surfaces compared to cut
surfaces (Seo and Frank, 1999; Takeuchi et al., 2000). Seo and Frank (1999) reported
that viable E. coli O157:H7 cells did not adhere to intact surfaces of lettuce leaves
but were found attached to the interior surfaces of stomata following chlorine