Agriculture Reference
In-Depth Information
promote the internalization of foodborne pathogens in leafy greens. After immersing
iceberg lettuce leaves for 20 min in an E. coli O157:H7 suspension containing
10 7 CFU/ml, Seo and Frank (1999) reported that the pathogen adhered to the surface,
trichomes, and stomata with a tenfold increase in attachment seen to cut edges. When
these lettuce leaves were incubated in the suspension for 24 h, E. coli O157:H7 infi l-
trated the stomata and cut surfaces to depths of 20 to 100
m with more deeply inter-
nalized cells surviving a 5 min exposure to a 20 ppm chlorine solution. In a similar
follow-up study, Takeuchi and Frank (2000) found that E. coli O157:H7 penetrated
the cut edges of iceberg lettuce to a mean depth of 73
μ
μ
m when stored at 4 ° C com-
pared to 27 to 39
m at 7 to 37 °C, with internalized cells protected during a 5 min
exposure to a 200 ppm chlorine solution. In contrast to chemical sanitizers, gamma
(Fan and others 2008; Niemira 2007) and x-ray irradiation (Jeong and others 2008),
which have been shown to effectively reduce populations of E. coli O157:H7 on and/
or in leafy greens, may provide an industry-viable nonthermal means to better ensure
the safety of fresh produce.
Prior to the 2006 spinach outbreak in the United States, this product attracted
minimal attention compared to lettuce, which is in large part due to the vast difference
in consumption rates between these two products. When Warriner and others (2003a)
assessed the interaction of a bioluminescent lux gene - labeled nonverotoxigenic E. coli
strain with growing spinach plants, E. coli grew around the roots of developing seed-
lings and internalized within the root tissue and hypocotyls. However, infi ltration
occurred only when the plants were grown hydroponically and not in soil. The authors
suggested that competitive microfl ora in the soil may have negated root colonization
by E. coli . When 20-day-old spinach seedlings were grown in soil inoculated with
E. coli at 2 log CFU/g, intact surface - sterilized 32 - and 35 - day - old plants contained
2.16 to 2.91 log E. coli CFU/g. Separate surface-sterilized root and leaf samples from
mature 42-day-old plants yielded average E. coli counts of 3.78 and
μ
1.20 log CFU/g.
Because no infi ltration of the leaves was seen when similar spinach seedlings were
grown hydroponically, this nonpathogenic E. coli strain could not gain entry into
growing spinach plants through the root system. In a follow-up study using a fl uores-
cently labeled strain of E. coli O157:H7, Hora and others (2005) were still unable to
detect this pathogen inside spinach leaves taken from growing plants despite prior
mechanical disruption of the roots, coinoculation of the spinach plants with P. syringae
via vacuum infi ltration, or inoculation with the northern root-knot nematode
Meloidogyne hapala. Although internalization of E. coli O157:H7 through the root
system into the leaves of growing spinach plants currently appears to be unlikely, this
does not preclude direct infi ltration of the spinach leaves through natural openings or
damaged areas.
Among the remaining varieties of leafy greens, parsley and cilantro have attracted
some attention in response to several earlier outbreaks. In limited work with parsley,
Duffy and others (2005) used both surface sanitization (2000 ppm chlorine) and CLSM
to demonstrate the uptake of three GFP-labeled Salmonella strains into parsley that
had been submerged in the inoculum for 3 or 15 min and then stored for 7 or 30 days
at 25 and 4 °C, respectively, with the pathogen growing internally in samples stored
at 25 °C. The following year, Lapidot and others (2006) reported that one S .
Typhimurium strain formed a biofi lm on parsley after dip inoculation. When compared
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