17 August 2014

Soil Connections: Drought, Dust and Antibiotic Resistant Bacteria

Posted by John Freeland

Photo by Alan Stark

Photo by Alan Stark

An unfortunate sequence of events involving drought, depleted water resources, wastewater management, antibiotic resistant bacteria (ABR) and dust storms may pose a real health risk in desert states – and, perhaps, beyond.

Briefly, here’s the sequence of steps, beginning with drought and ending with a respiratory infection:

1. Arid states can’t grow everything they want with what little rainfall they get – so they irrigate.

2. Limited fresh water used to irrigate is depleted faster than it is replenished as described here here and here.

3. Animal feed lot wastewater is available, requires a means of disposal, and is used to irrigate fields instead of fresh water from wells or reservoirs.

4. Antibiotics widely used in animal feeding operations select for antibacterial resistant bacteria (ABR) found in air, water and soil.

5. Severe dust storms (video) affecting populated areas mobilize ABR-laden dust particles that are inhaled downwind by humans. Another video: Arizona dust storm hits a wedding ceremony
6. Inhaled dust sets up a respiratory disease such as “Q Fever”

Soil and the Rhizosphere “Hot Spot”
Land application of animal waste has long been shown to have important soil benefits including replacement of moisture and nutrients, a well as offering safe disposal of manure. The biologically active zone of the soil-plant root interface known as the rhizosphere hosts complex microbial “loops” that cycle nutrients necessary to plant growth.

Recent research by Berg et al. on the colonization of rhizosphere by opportunistic human pathogens suggests that the soil, rather than mitigating harmful ABR, may actually have the potential of enhancing the proliferation of those kinds of bacteria. From Berg et al (2005):

During the last years, the number of human infections caused by opportunistic pathogens has increased dramatically. One natural reservoir of opportunistic pathogens is the rhizosphere, the zone around roots that is influenced by the plant. Due to a high content of nutrients, this habitat is a ‘microbial hot-spot’, where bacterial abundances including those with strong antagonistic traits are enhanced. Various bacterial genera, including Burkholderia, Enterobacter, Herbaspirillum, Ochrobactrum, Pseudomonas, Ralstonia, Staphylococcus and Stenotrophomonas, contain root-associated strains that can encounter bivalent interactions with both plant and human hosts…

Multiple resistances against antibiotics are not only found with clinical strains but also with strains isolated from the rhizosphere. High competition, the occurrence of diverse antibiotics in the rhizosphere, and enhanced horizontal gene transfer rates in this microenvironment appear to contribute to the high levels of natural resistances.

So, it seems at least plausible, if not confirmed, that ABR introduced by animal waste from livestock treated with antibiotics could continue to multiply once incorporated into the soil.

Livestock operations that routinely land-apply manure have a huge stake (and a lot of steaks) in this issue. They don’t want to be blamed for bacteria that may already be present in the soil without the addition of manure.

Background levels of ABR is one subject of Jean McClain’s research at the Water Resources Research Center at the University of Arizona. In the June 2014 issue of CSA News, Dr. McClain states:

…many studies have detected ABR genes in recycled wastewater…This has raised enormous alarm about the use of recycled water. In far too many studies, natural levels of these genes are not assessed. As a result, the findings of the study could be heavily – and perhaps incorrectly – skewed against the use of recycled municipal wastewater.

The same argument, it seems, could be applied to the use of animal waste.

The identification of “natural levels” of ABR may prove particularly difficult, however. USGS scientist Dale Griffin studies the atmospheric migration of microbes on a global scale. It turns out that bacteria “book” transoceanic flights on dust particles, leading to significant consequences to human health.

When characterizing soil microbiology, can scientists distinguish between “naturally occurring” versus “foreign” strains of bacteria introduced by global wind circulation?

Or, will uncertainty over contamination source effectively prevent management decisions designed to protect health and the environment?

Recommended reading: Antibiotic Resistance in Agricultural Systems by Ken Doyle, CSA News, June 2014.