Abstract
Surviving predation is a fitness trait of Escherichia coli O157:H7 (EcO157) that provides ample time for the pathogen to be transported from reservoirs (e.g. dairies and feedlots) to farm produce grown in proximity. Ionophore dietary supplements that inhibit rumen protozoa may provide such a selective advantage for EcO157 to proliferate in lagoons as the pathogen is released along with the undigested supplement as manure washings. This study evaluated the fate of an outbreak strain of EcO157, protozoan and bacterial communities in wastewater treated with monensin. Although total protozoa and native bacteria were unaffected by monensin, the time for 90% decrease in EcO157 increased from 0.8 to 5.1 days. 18S and 16S rRNA gene sequencing of wastewater samples revealed that monensin eliminated almost all colpodean and oligohymenophorean ciliates, probably facilitating the extended survival of EcO157. Total protozoan numbers remained high in treated wastewater as monensin enriched 94% of protozoan sequences undetected with untreated wastewater. Monensin stimulated 30-fold increases in Cyrtohymena citrina, a spirotrichean ciliate, and also biflagellate bicosoecids and cercozoans. Sequences of gram-negative Proteobacteria increased from 1% to 46% with monensin, but gram-positive Firmicutes decreased from 93% to 46%. It is noteworthy that EcO157 numbers increased significantly (P<0.01) in Sonneborn medium containing monensin, probably due to monensin-inhibited growth of Vorticella microstoma (P<0.05), a ciliate isolated from wastewater. We conclude that dietary monensin inhibits ciliate protozoa that feed on EcO157. Feed supplements or other methods that enrich these protozoa in cattle manure could be a novel strategy to control the environmental dissemination of EcO157 from dairies to produce production environments.
Highlights
Consumption of contaminated produce has been responsible for 19% of all Escherichia coli O157:H7 (EcO157) outbreaks during 1998 to 2007 [1]
A dietary feed supplement, extended survival of pathogenic EcO157 in dairy wastewater by inhibiting protozoa that consume EcO157 and by eliminating gram-positive bacteria that compete for nutrients
We have shown that the antibiotic is not inhibitory to EcO157 and gram-negative bacteria and facilitated the growth of members of Proteobacteria undetectable from monensin-free wastewater
Summary
Consumption of contaminated produce has been responsible for 19% of all Escherichia coli O157:H7 (EcO157) outbreaks during 1998 to 2007 [1]. Major outbreaks associated with produce indicate that pre-harvest contamination in the field has occurred often, so it is critical to identify sources of pathogens in the environment and interventions for minimizing them [2]. Controlling pathogens in livestock and agricultural environments to minimize pre-harvest contamination will require an understanding of the biological and environmental factors that regulate the proliferation of pathogens during their transport from animal reservoirs, e.g. livestock operations and wildlife, to produce grown in proximity to point sources. The survival and growth of EcO157 in the environment are important factors in pre-harvest contamination. Environmental factors that cause fluctuations in enteric pathogens provide opportunities to alter the chemical or biological nature of the environment and minimize pre-harvest contamination of fruits and vegetables grown in proximity to dairies. The rapid decline of EcO157 populations in dairy wastewater due to inhibitory chemicals [6] and protozoan predation [7] are examples of fluctuations that could be exploited to decrease the pathogen
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