Altered Protozoan and Bacterial Communities and Survival of <em>Escherichia coli</em> O157:H7 in Monensin-Treated Wastewater from a Dairy Lagoon

<div><p>Surviving predation is a fitness trait of <em>Escherichia coli</em> 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 <em>Cyrtohymena citrina</em>, a spirotrichean ciliate, and also biflagellate bicosoecids and cercozoans. Sequences of gram-negative <em>Proteobacteria</em> increased from 1% to 46% with monensin, but gram-positive <em>Firmicutes</em> decreased from 93% to 46%. It is noteworthy that EcO157 numbers increased significantly (<em>P<</em>0.01) in Sonneborn medium containing monensin, probably due to monensin-inhibited growth of <em>Vorticella microstoma (P<</em>0.05<em>),</em> 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.</p>