Food Protection Trends - May/June 2024 - 241

cantly greater hydrophobicity, greater extracellular polymeric
substance protein and sugar concentrations, less negative net
charge, and higher point of zero charge than did the E. coli
isolates from stream water. Whether bacterial characteristics
permit these organisms to adhere to sediment particles or
whether contact with sediment induces bacterial characteristics
necessary for adherence has yet to be determined.
Algae and aquatic plants
Similar to sediment, plant surfaces provide nutrients,
sites for microbial attachment, and protection against
damaging radiation and predators (15, 18, 33, 84, 133).
Because of health risks to the public during water recreation,
the association of human pathogens with aquatic plants
has frequently been studied in recreational waters. In Lake
Michigan, high levels of E. coli and enterococci have been
reported in Cladophora, a green alga that grows in dense
strands and mats along the shoreline (15). Blooms of algae
and other select aquatic plant species appear to extend
pathogen survival in water (15, 18, 105, 133).
Mathai et al. (77) studied how E. coli populations in 10
freshwater lakes in Minnesota were affected by Eurasian
watermilfoil, an invasive submerged macrophyte that has
spread across thousands of lakes in North America. E. coli
levels were generally elevated on the plants, peaking in June.
Several potentially pathogenic bacteria, including members
of the Enterobacteriaceae family and Aeromonas, Yersinia, and
Clostridium, were present on the Eurasian watermilfoil at significantly
greater levels than found in water samples. Source
tracking biomarkers indicated the bacteria were predominantly
from waterfowl feces (77). In Florida, aquatic plants in
a freshwater lake contained high levels (8.7 × 104
CFU/g) of
fecal indicator bacteria (FIB), suggesting that the plants may
serve as important reservoirs for these bacteria (105). Sbodio
et al. (101) observed that during periods of elevated temperatures
in the summer in California, increased total coliform
levels coincided with algae blooms in irrigation canals in the
San Joaquin Valley and reservoirs on produce farms on the
central coast.
Because of bacterial attachment to plants, researchers have
studied the use of aquatic plants to remove pathogens contaminating
agricultural water sources. Some plants remove
pathogens from water better than other plants. Clairmont et
al. (21) studied the ability of two species of wetland plants
to remove E. coli, Enterococcus spp., and Salmonella from
water in constructed wetlands and found that E. coli and
Enterococcus removal varied based on the type of plant, but
Salmonella thrived in the rhizoplane (roots and associated
soil particles) of both plants. Other research corroborates
preferential bacterial attachment to roots of a specific aquatic
plant over those of other plants (18, 133). Irrigation ditches
and reservoirs often contain plants, but whether they remove
pathogens from water or protect attached pathogens that
could then contaminate the surrounding water has not been
studied. Most likely, both functions are operating, and the
dominant net effect will depend on multiple codependent or
interacting factors.
Agrochemicals
Many agrochemical (e.g., synthetic, natural inorganic,
organic, and biological) formulations used to enhance soil
health, for crop protection (e.g., pesticides), and as plant
nutrients or biostimulants are sold in a form that requires
mixing and diluting with water for application. When the water
used to mix the formulations is contaminated with human
pathogens, pathogen survival in the solution may be beneficially
or detrimentally affected by the class of agrochemical
or a specific agrochemical. Agrochemicals may directly kill
or injure pathogens (e.g., cause DNA, protein, oxidative, or
membrane damage) or destroy their protective habitat and/
or nutrients or may enhance pathogen survival by killing
their predators or competitors or providing direct growth
nutrients as part of the chemical formulation (97, 106).
Staley et al. (105, 107, 108) reported primarily indirect
effects on FIB and pathogenic E. coli in their studies of pesticides
in agricultural water. Their research on atrazine's effect
on E. coli suggested that the herbicide had no direct effect on
E. coli survival but rather reduced algae in the water column,
which led to redistribution of E. coli into sediment (107). In
studying the effects of the fungicide chlorothalonil on FIB
and E. coli O157:H7 survival, these authors found that chlorothalonil
indirectly elevated E. coli O157:H7 and FIB levels
by reducing levels of predaceous protozoans (108). These
effects diminished over time as the chemical degraded or
microorganisms adopted resistance. Ng et al. (83) evaluated
naturally occurring microorganisms in 10 commercially available
pesticides reconstituted in agricultural water from bore,
dam, and river sources. After 48 h, the majority of pesticides
supported the growth of naturally occurring microorganisms,
including bacteria known to be human pathogens.
Mahovic et al. (73) and Lopez-Velasco et al. (71) investigated
survival of Salmonella in pesticide solutions (23
commercial brands) commonly used on tomatoes grown
on the eastern shore of Virginia and in California, respectively.
Mahovic et al. (73) observed a 5.0-log reduction in
Salmonella in the pesticide solution containing peracetic
acid (PAA) as the active ingredient, but no other pesticide
affected Salmonella populations in solution. Lopez-Velasco
et al. (71) found that Salmonella growth was not hindered
by most pesticides and that specific pesticides even supported
the pathogen's growth in solution. Salmonella growth in
pesticide solutions was also dependent on water composition
and temperature. When the contaminated pesticide
solutions were applied to plants during in-field production,
Salmonella survived up to 15 days in 80% of plant samples.
The researchers concluded that foliar application of pesticides
may elevate the risk of contamination beyond that of
the water source alone.
May/June Food Protection Trends 241
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