To view the article abstracts from this months research update follow this link: JUNE 2018 RESEARCH UPDATE
ARGHHH!!! Biosolids ARGs
I have two terms for you to learn before you read this blurb and certainly before you read any of the articles. These are:
ARB – - antibiotic resistant bacteria
ARG – antibiotic resistant genes
And for bonus points I would have you learn that a bacteriophage is a like a virus that carries genetic material and invades and conquers bacteria
Lately it seems like all anyone can talk about when they talk about contaminants in biosolids are PFOAs and PFAS. This month, to honor a new publication by our University of Arizona research colleagues Ian Pepper, John Brooks and Chuck Gerba, I decided to take a step back in time, to those long-ago days when we were worried about antibiotic resistance instead of non-stick coatings. You see, we are still worried about antibiotic resistance. The question is whether wastewater treatment, specifically the land application of biosolids, is a significant part of this problem.
The library starts with a bang. The new article by Pepper et al. gets to the heart of the topic. In a generally accessibly written piece, the authors go over concerns about antibiotic resistance including the presence of ARB in soils both with and without the application of biosolids or manures. They make an important point that ARB and ARG are naturally occurring and discuss several examples in the literature that show this to be the case. They do a rough calculation of how many ABR there are in the soil around a home. The short answer is there is a lot. They point out that concentrations of ARB and ARG are not dissimilar (somewhat greater) in biosolids and manures than concentrations found in soil.
I need to make a side note here. The numbers are generally large and sometimes it is tough to realize how big differences are. For example, in Table 1 we see that soils have between 102 - 107 ARB per gram. Class B biosolids have between 104 - 109 and swine manure has 105 - 107. So biosolids have the same to more, with more meaning 1000000000 versus 10000000 ARB per gram. I somehow think that I am not alone in getting a little dizzy in counting the zeros. They calculate that, by applying biosolids or manures to the soil, the potential increase in numbers of ARB and ARG is inconsequential. They also note the difficulty of these ARB in surviving in soil and in transferring genes. In other words, ARB is a problem, but it just really isn’t a problem in manure or biosolids amended soils. One important point in the article is that animals and farmed fish consume 80% of the antibiotics sold in the U.S., with 24 million pounds of antibiotics used in CAFOs annually. So, antibiotic use is a problem, just not our problem alone.
You can stop reading now, but if your curiosity is peaked, please continue move.
The next two articles are reports from field studies. Both include manures and biosolids. The first is a study in Michigan that quantified ARG from different manures, biosolids and soils. Soil concentrations were about 50% lower than either manures or biosolids. Applying manures and biosolids increased ARG at one farm, with increases more pronounced in the manure-amended soils than in the biosolids-amended soils, for the 4 months post-application that the site was sampled. At the other farm, applying biosolids had no effect.
The next study is by Ed Topp and Joseph Ross. Ed is a scientist with Ag Canada whose work with PCPPs has been critical to our understanding of the behavior of these compounds in biosolids amended soils. In this paper he is focused on the potential for bacteriophages (or, phages) to cause ARG transfer in biosolids- and manure-amended soils. This is not my area of expertise and much of the discussion here is tough for me to fully understand. However, I hope that I get the major points right. The authors are looking at relative numbers of copies of different genes in bacteria and in phages. They sampled soil with and without the different amendments up to 30 days after application and measured gene copies of ARG in both bacteria and phages. Raw, digested and dewatered animal manure-amended soils have greater numbers of both bacterial and phage DNA fragments than the control soil or composted manure for up to 30 days post application [yet another reason composting is good]. Differences between the control soil and biosolids-amended soils were less pronounced than with the manure-amended soils. Phage numbers were generally similar between control and biosolids-amended as were ARG from bacteria. Then they added antibiotics to extracts containing bacteria from soils. When antibiotics were added to these extracts they observed gene transfer through phages. This is not in soil; this is with extracts to which antibiotics were added. But, to the authors, this is proof of possibility. But, then again, if you ask Dr. Pepper, this possibility is far from likely, and Pepper would potentially equate this possibility to your possibility of winning the lottery.
The fourth paper is meant to reassure you. As Topp pointed out, the composted manure was much lower in ARG than the otherwise treated manures. In this paper, the authors (including Andy Bary and Craig Cogger from WSU) look at the impact of thermophilic composting on four antibiotics. Short answer, all four were pretty much gone by day 15.
The final paper, also with Topp as a leading author, puts the whole ARB discussion into a greater perspective. The paper Is a summary from a discussion held at an international meeting on antibiotic resistance. The advice of the conferees was to reduce use of antibiotics in aquaponics and agriculture, to place better controls on emissions from manufacturing, and to come up with alternatives to antibiotics that will reduce their usage. That is the big answer to this problem.
The bottom line: ARB and ARG are big problems for the world, generally speaking, but much less so for biosolids-amended soils.
Sally Brown, University of Washington