Agriculture Reference
In-Depth Information
These results indicate that conventional processes are inadequate to address con-
tamination by pathogens in protected areas. Irradiation, as a penetrating process, holds
more promise in targeting this type of contamination, but the literature on the effi cacy
of this application is not extensive.
Pathogens within Biofi lms
Research has recently begun to assess the ability of irradiation to inactivate internal-
ized or biofi lm-associated pathogens. Irradiation is a penetrating process, but the
effi ciency of irradiation in killing protected pathogens is not well known. The limited
data available suggest that the particular isolate and the biofi lm culture conditions
(growth temperature, medium, time of cultivation, etc.) can infl uence irradiation effi -
cacy. Biofi lm-associated cells of
S
. Stanley and
S
. Enteritidis were signifi cantly more
sensitive to ionizing radiation than respective planktonic cells, although
S
. Anatum
showed no increase in radiation sensitivity for biofi lm-associated cells (Niemira and
Solomon 2005 ). The antimicrobial effi cacy of irradiation against
Salmonella
is there-
fore observed to be preserved or enhanced when treating biofi lm - associated bacteria
based on in vitro evidence. Biofi lm-associated cells of
E. coli
O157:H7 ATCC 43894
grown at 37 °C for 24, 48, or 72 h were signifi cantly more sensitive to irradiation than
respective planktonic cells (Niemira 2007).
E. coli
O157:H7 C9490 biofi lms grown
for 24 h showed a similar increase in sensitivity; the radiation-sensitivity of biofi lm-
associated cells of
E. coli
O157:H7 ATCC 35150 were signifi cantly reduced at 24 h.
The response of biofi lm and planktonic cells were not different for either of these two
E. coli
O157:H7 isolates in older biofi lms (48 and 72h). Biofi lm - associated cells
of
E. coli
O157:H7 were therefore sometimes more sensitive to irradiation and
sometimes less so, with D
10
values that varied as much as 27% above or below the
D
10
values obtained for planktonic cells. The modest amount of information that is
available on the relative sensitivity of planktonic and biofi lm cells to irradiation sug-
gests a complex difference between the two physiological states of these cells (Niemira
2007). Further research in this emerging fi eld is expected to improve our understand-
ing of how biofi lms may alter the effi cacy of irradiation.
Internalized Pathogens
Pathogen internalization in produce and the resulting increase in the risk of FBI is
a subject of ongoing research. Studies with lettuce (Solomon and others 2002a,b),
barley (Kutter and others 2005), and maize (Bernstein and others 2007) have shown
that pathogen internalization can occur when introduced via irrigation water, contami-
nated soil, or other means. However, other studies using tomatoes have found that
internalization via the root system either does not occur, or is extremely ineffi cient
(Jablasone and others 2004, 2005). Additional research will further improve our
understanding.
Bacterial populations within a leaf, fruit, or vegetable are isolated from conven-
tional antimicrobial treatments. A penetrating process such as irradiation may be
suited to addressing this problem, although very few studies have yet investigated this.
The ineffi cient uptake of bacteria via roots and vasculature make microbiological
analysis problematic. Nevertheless, although the body of literature is relatively scant
