Agriculture Reference
In-Depth Information
damaged tomatoes can reportedly raise the core tissue pH to levels more conducive for
Salmonella
growth (Wade and Beuchat 2003a,b; Wade and others 2003). Submerging
25 °C tomatoes in a 320 ppm free chlorine solution at 37 °C for 2 min decreased the core
tissue populations only 1 log, whereas Ibarra-Sánchez and others (2004) reportedly
reduced
S
. Typhimurium populations in tomato core tissue from 2 logs to undetectable
levels using a 15-sec 2% lactic acid spray treatment at 5 or 55 °C.
The salmonellosis outbreaks just mentioned have also raised a number of prehar-
vest food safety issues related to the microbiological safety of irrigation and pesticide
water, animal manure, and sewage sludge (Guan and others 2005), any of which could
contaminate soil and potentially lead to internalization of
Salmonella
or other food-
borne pathogens through the root system. When placed in contact with inoculated soil,
Salmonella
can migrate from the soil directly into the stem scar tissue of green toma-
toes (Guo and others 2002b) as was mentioned earlier in regard to
E. coli
O157
migration into apples (Seeman and others 2002). Uptake of
Salmonella
through the
root system was demonstrated by Guo and others (2002a) using hydroponically grown
tomato plants. When the roots were exposed to a hydroponic solution containing
S.
Montevideo,
S.
Enteritidis, and three other serovars,
S.
Montevideo migrated through
the root system into the hypocotyls and cotyledons, stems, and leaves, whereas
S.
Enteritidis was absent. Stem injection and fl ower brushing of tomato plants with
Salmonella
cocktails also led to recovery of the pathogen both internally and exter-
nally. These fi ndings help explain why Jablasone and others (2004) failed to detect
Salmonella
in the leaves, stems, or fruit of potted tomato plants that were irrigated
with
S
. Enteritidis-inoculated water at 10
5
CFU/ml.
Sprouts
Since 1995 a series of widely publicized
Salmonella
and to a lesser extent
E. coli
O157:H7 outbreaks in the United States and elsewhere were traced to various types
of sprouted seeds that were contaminated prior to germination. Commercially, these
seeds are sprouted at 25 - 35 ° C under very high relative humidity — conditions that
promote rapid microbial growth and subsequent biofi lm formation, as confi rmed on
2-day-old alfalfa sprouts by both direct bacterial enumeration and scanning electron
microscopy as previously shown in Figure 3.2 (Fett 2000).
Following a major 1996 radish sprout outbreak in Japan that was traced to
E. coli
O157:H7, a team of Japanese investigators (Hara-Kudo and others 1997) demonstrated
the ability of this pathogen to grow in experimentally contaminated radish seeds with
populations increasing 3 to 5 logs during germination. Several immunofl uorescence
and scanning electron microscopy images by Itoh and others (1998) provide further
proof that
E. coli
O157:H7 can migrate through surface stomata and become internal-
ized within the inner tissue of sprouts. Several years later, Gandhi and others (2001)
were among the fi rst to study the interaction between
Salmonella
and alfalfa sprouts
during seed germination and sprouting. When alfalfa seeds were immersed in a GFP-
labeled suspension of
Salmonella
Stanley (
7 log CFU/ml) for 5 min at 22 ° C, an initial
population of 3.7 log CFU/g was achieved in the seed. These numbers increased thou-
sandfold during initial germination with the target pathogen penetrating to a depth of
at least 18
∼
m in mature sprouts as determined by CLSM. Subsequently, Barak and
others (2002) demonstrated that
S. enterica
and several plant-associated bacteria were
μ
