Antibiotic resistance genes in soil microbes

We’ve known for years that antibiotic resistant bacteria (ARB) are increasingly a problem in hospital settings. As the recently published (and excellent) book Superbug describes, ARB are also increasingly common in the community – in sports teams, prisons, and on pig farms, for example.

A recent study finds that antibiotic resistance is on the increase in the very ground we walk on. Microbes in soil samples collected in the Netherlands over a sixty year period were analyzed to see if antibiotic resistance genes (ARG) had become more abundant. Resistance was tested for four classes of antibiotics, including tetracyclines and penicillins.

The study reports: “Results show that ARG from all classes of antibiotics tested have significantly increased since 1940, but especially within the tetracyclines, with some individual ARG being >15 times more abundant now than in the 1970s.” Also: “Seventy-eight percent of detected resistance genes, associated with four classes of antibiotics, showed increasing levels since 1940.”

Gene transfer can happen in the food and water supply

The authors of the study expressed concern for the public health implications of their findings. The concern stems from the ability of bacteria to transfer their genes to other bacteria in their surroundings, not just to their own offspring. Humans pass their genes vertically, from parent to child. But single-celled microbes can pass genetic material to another living microbe. It’s called horizontal (or lateral) gene transfer. In fact, this may be the most common form of gene transfer in single-celled microbes.

So we’re not talking about picking up disease-causing bacteria while walking in the woods, the way athletes contract MRSA from towels in the locker room. The significance of an increase in ARG in soil microbes is the ability to transfer resistance genes to bacteria that make us sick.

As lead researcher David Graham explains: “The genes themselves do not get passed directly to humans per se. The genes get passed from exposed bacteria to bacteria … [that] might ultimately end up in humans, some of which might be pathogenic. An example is on food or in water that has been exposed to resistance bacteria.”

Like food that’s grown in soil. Or water that comes in contact with the earth.

Update:
Just came across an article from the 1990s in the journal Toxic/Hazardous Substances Environmental Engineering that discusses antibiotic resistance gene transfers in drinking water. It looked at enterotoxigenic E. coli, which are the bacteria most commonly associated with traveler’s diarrhea and the leading cause of diarrhea in the developing world. From the abstract: “These strains [of E. coli] also showed transfer of their antibiotic resistances at significant frequencies. Presence of transferable antibiotic resistance, particularly in enterotoxigenic E.coli in drinking water due to the faecal contamination, is a major factor for incidences of various diarrhoeal diseases.”
Anita R. Gautam et al., Incidence of transferable antibiotic resistance among enterotoxigenic Escherichia coli in urban drinking water, August 1993.

Resources:

Photo: Bokashicycle

Charles W. Knapp, Jan Dolfing, Phillip A. I. Ehlert, and David W. Graham, Evidence of Increasing Antibiotic Resistance Gene Abundances in Archived Soils since 1940, Environmental Science & Technology, December 21, 2009 (abstract only)

Evidence of increasing antibiotic resistance of soil microbes, Physorg, May 5, 2010

Michael Ricciardi, Antibiotic Resistant Genes Increasing in Soil Microbes, Eco Localizer, December 31, 2009

Maryn McKenna, Superbug: The Fatal Menace of MRSA