Agriculture Reference
In-Depth Information
Internalization of Pathogens in Produce
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Elliot T. Ryser, Jianjun Hao, and Zhinong Yan
Introduction
Washing fresh produce in various sanitizers is only marginally effective in reducing
populations of pathogenic and spoilage microorganisms, with complete elimination
impossible without compromising product quality (Aruscavage and others 2006;
Beuchat 2004, 2006; Beuchat and Ryu 1997). This failure is attributed to a wide range
of topographical surface characteristics inherent to fresh produce, including 1) water-
repelling waxes (e.g., apples, green peppers), cracks or crevices (e.g., cantaloupe,
various seeds), and natural openings (e.g., stomata, hydathodes, nectarthodes, lenti-
cels, stem scars, calyx) (Beuchat 2002; Takeuchi and Frank 2000); 2) biofi lm forma-
tion on leaves and roots (Warriner and others 2003b; Lapidot and others 2006); and
3) bacterial infi ltration through cut edges or wounds on produce (Beuchat 2002; Seo
and Frank 1999; Mendonca 2005), all of which minimize pathogen exposure to sani-
tizers (Beuchat 2002, 2006; Aruscavage and others 2006).
The more than 600 outbreaks traced to fresh produce since 1990 (DeWaal and
others 2006) have collectively increased the attention now given to internalization of
bacterial foodborne pathogens, particularly Salmonella and Escherichia coli O157:H7,
in fresh produce. Internalization of endophytic bacteria important in plant pathology
and microbial ecology has been recognized since the 1870s and well studied since
1940 (Hallmann and others 1997; Sturz and others 2000; Rosenblueth and Martínez-
Romero 2006; Ryan and others 2008) with several authoritative reviews having been
written on internalization of various foodborne pathogens, including Salmonella and
E. coli O157:H7 into fruits and vegetables by various routes and their ability to persist
internally (Aruscavage and others 2006 ; Beuchat 2006 ; Brandl 2006 ; Mendonca 2005 ;
Solomon and others 2006 ; Stone and others 2000 ; Warriner 2005 ). This internalization
as well as the aforementioned factors leads to enhanced survival of foodborne patho-
gens after treatment with chemical sanitizers. An understanding of the plant rhizo-
sphere, phyllosphere, and plant-microbe interactions is key to the development of
potential strategies to minimize internalization of human foodborne pathogens in
plants. These topics are discussed in greater detail, beginning with the defi nition of
internalization.
Bacterial Endophytes
Bacteria residing internally in plant tissue are known as endophytes, with these organ-
isms found in almost all plant species. Endophytic bacteria were fi rst recognized
during the 1870s when Pasteur and others identifi ed bacteria within asymptomatic
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