Agriculture Reference
In-Depth Information
Postharvest
Immersing warm produce in cooler water during fl uming or washing is another
common means for bacterial internalization. The decrease in produce temperature that
follows allows the atmospheric and hydrostatic forces on the immersed produce to
equilibrate with the internal pressure, thus facilitating ingress of water along with
uptake of any bacteria present (Bartz and Showalter 1981; Buchanan and others 1999).
Such internalization of
E. coli
O157:H7 has been shown for apples, with much of the
penetration occurring through the blossom end when the aqueous bacterial suspension
was cooler than the fruit (Buchanan and others 1999; Burnett and others 2000). Given
its close association with the plant vascular system, stem scar tissue is also highly
prone to infi ltration by endophytic bacteria as well as
E. coli
O157:H7 and
Salmonella
(Bartz and Showalter 1981; Eblen and others 2004). The extent of internalization is
infl uenced by age, with fresh stem scars on tomatoes more vulnerable to infi ltration
than older stem scars, and green and pink tomatoes more susceptible to water uptake
than red fruit (Bartz and Showalter 1981).
Bacterial Movement in Plants
As early as the 1980s, internalized plant endophytes such as growth-promoting rhi-
zobacteria and various plant pathogens were shown to migrate through the plant
vascular system (Hallmann and others 1997b; Kluepfel 1993; Lamb and others 1996;
Yan and others 2003). Lamb and others (1996) detected
Rhizobacterium
spp.
and
Pseudomonas aureofaciens
in interior aerial tissue from 16 types of monocoty-
ledonous (e.g., corn, wheat) and dicotyledonous plants (e.g., broccoli) grown from
inoculated seed with direct vascular transport from the roots suggested. However,
passive uptake of
E. asburiae
was not reported in cotton plants (Quadt-Hallmann
and others 1997).
Human pathogens that infi ltrate plant tissue can either remain localized or move
systemically (Solomon and others 2002b; Wachtel and others 2002a).
Pseudomonas
aeruginosa
strain PA14, a human opportunistic pathogen that infects
Arabidopsis
,
reportedly moved long distances through the vascular parenchyma (Plotnikova and
others 2000 ).
Salmonella enterica
and
E. coli
O157:H7 were also shown to internalize
and move long distances through the
Arabidopsis
vascular system, resulting in whole
plant contamination in the absence of microbial competitors (Cooley and others 2003).
However, both pathogen populations signifi cantly decreased when
Arabidopsis
was
grown in nonsterile soil containing
E. asburiae,
with invasion through the lateral root
junctions seen using fl uorescently labeled strains of
S
.
enterica
and
E. coli
O157:H7.
Movement was eliminated and invasion decreased when nonmotile mutants of
S.
enterica
were used (Cooley and others 2003), suggesting that internalization is an
active process based partially on fl agellar motility.
Methods for Examining Bacterial Internalization
Proper methodology is critical in confi rming bacterial internalization. Hallmann and
others (1997b) provided a thorough review of methods to study bacterial endophytes.
The key contents from this review paper and some newly developed methods reported
from literature are now summarized below.
