Emerging Issues in Manure Management: Are manures increasing antibiotic resistant bacteria?

Did you know it is the World Health Organizations Antibiotic Awareness Week? In light of this, I decided it was a good week to look at antibiotic resistance bacteria in manures. Now this isn’t a topic I consider myself and expert on so I’m going to be borrowing heavily from my college at Iowa State, Dr. Michelle Soupir.

Antimicrobials, such as antibiotics, are used in the livestock industry at therapeutic levels for disease treatment. Once the antibiotic has been administered, the animal will begin to metabolize it (break it down), but not all of it is metabolized, some is excreted with the feces and urine, ending up mixed with the manure.  For example, in the case of tylosin about ¾ of the mass administered to an animal ends up in the manure.

Why does this matter? Well once these compounds are in the manure they put a selective pressure on the microbes in it to become resistant to that compound. It is sort of like this, if its cold outside you can stand outside and shiver or you can put a jacket on. For microbes it’s sort of the same thing; the presence of that compound might negatively impact most of the microbes, but perhaps a few of them will figure out to put a jacket on (become resistant to that antibiotic). Then when the manure is land applied as a fertilizer, these resistant bacteria enter the environment. If it happens to be a microbe that makes people sick and they come into contact with it there is a chance that our normal antibiotics might not be as effective for this guy, because it already has some built up tolerance to it. Sure, there are lots of ifs in this case – are they in the manure to start with, how long do they last, does the animal and human antibiotic even work in the same way (similar mechanisms) – but that’s what people are trying to find out.

Picture of Enterococcus

So in a recent study Dr. Soupir performed she tested how swine manure application (fall applied swine manure at 168 kg N/ha, or about 150 lb N/acre compared to spring injected UAN) and tillage practices (chisel plowing versus no-till) impacted the persistence and transport of enterococci; both total enterococci and those resistant to tylosin (a type of bacteriosat that was used at the farm the manure was obtained from).

So they measured lots of things in this study but what we are going to focus on is enterococci (and tylosin resistant bacteria) in the manure (at the time of application), in the soil (both in the fall after manure was applied and in the spring), and in tile water over the following growing season. They found that the manure had between 90,000 and 570,000 colony forming units per gram of manure and that between 70-100% of these bacteria were resistant to tylosin. When it came to the soil samples, they tested both in the manure application band, outside the manure application band, and in the control plots that didn’t receive manure. Not surprisingly, enterococci concentrations were the greatest in the manure injection band, and lowest in the control soils that didn’t receive manure. Concentrations of enterococci between the manure bands were similar to the non-manured soils. Over the winter, enterococci concentrations decreased by about 70 and by the time the manure had been in the soil for a year had returned to levels equivalent to soils not receiving manure. No difference in enterococci concentrations in the tile drainage water were found.

So where does this leave use? At least during this study, when weather conditions were drier than normal for Iowa, it doesn’t appear that manure injection changed the risk of to water quality. However, different weather conditions where it is wetter during and after manure application, may impact these results.

A publication detailing this study is available at:
http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1819&context=abe_eng_pubs
Check it out if you are interested in learning more.