First of all I thank Johan Bengtsson for initiating a lively and much needed discussion on which pollutant we should precisely be targeting, antibiotics or antibiotic resistance genes (ARGs), in our important war against the spread of antibiotic resistance. As Bengtsson correctly alludes, my perspective comes from that of environmental science and engineering. At the core of these disciplines is defining and predicting the fate of pollutants in the environment, as well as designing appropriate means for their control. For these purposes, the definition of the pollutant of interest is of central importance. In general they may be defined as “undesired or harmful constituents within an environmental matrix, usually of human origin.” Pollutants may be classified in all shapes and sizes, including conservative (i.e., not subject to degradation or growth), non-conservative, biotic, abiotic, dissolved, and suspended (i.e., not dissolved). Thus, the first point, regarding the nature by which ARGs are spread disqualifying them from being considered as pollutants, is inaccurate.
At the same time, I recognize and agree that ARGs are indeed a natural and important aspect of the natural ecosystem. I commend recent work revealing the vast “antibiotic-resistome” in ancient environments (D’Costa et al. 2011; Allen et al. 2009), as it provides an essential understanding of the baseline antibiotic resistance in the pre-antibiotic era, which may serve as contrast for observations in the current antibiotic era. Thus, I agree that not all ARGs are pollutants, rather, anthropogenic sources of ARGs are the agents of interest. Perhaps I and others are guilty of not making this distinction more clear. It should also be pointed out that likewise, the vast majority of antibiotics in use today are derived from natural compounds, yet I agree that they can also serve as important environmental pollutants of concern. Thus, it is not necessarily whether the constituent is naturally occurring that defines the pollutant, rather its magnitude and distribution, as influenced by human activities.
It is agreed that viewing ARGs as contaminants does pose technical challenges. They may amplify within a host, or attenuate due to degradation or diminished selection pressure. However, with appropriate understanding of the mechanisms of transport and persistence, accurate models may be developed. I do contend that the jury is still out regarding the relative importance of extracellular and intracellular ARGs. The pool of extracellular DNA remains vastly uncharacterized, and some studies suggest that it is more extensive than previously thought (Wu et al. 2009; Corinaldesi et al. 2005). Other studies have specifically demonstrated the capability of extracellular ARGs to persist under certain environmental conditions and maintain its integrity for host uptake (Cai et al. 2007). While focusing attention on individual resistant strains of bacteria has merit in some instances, this approach is also greatly limited by the unculturability of the vast majority of environmental microbes. As we have now entered the metagenomic era, we now have the tools to tackle the complexity of resistance elements in the environment and precisely define the human influence. Distribution of ARGs may also be considered in parallel with key genetic elements driving their horizontal gene transfer, such as plasmids, transposons, and integrons.
Regarding the antibiotics themselves, clearly they are important. The direct relationship between clinical use and increasing rates of antibiotic resistance is well-documented and certainly continued vigilance in promoting their appropriate use and disposal is called for. What remains much foggier is the exact role of environmental antibiotics in enabling selection once released into the environment. There is good evidence that even sub-inhibitory levels of antibiotics can stimulate various functions in the cell, especially horizontal gene transfer, as reviewed recently by Aminov (2011). However, environmentally-relevant concentrations driving selection of resistant strains are largely unknown. Further, at what point along a discharge pathway from wastewater treatment plant or livestock lagoon do ARGs persist independently of ambient antibiotic conditions? Indeed, some studies have noted correlations between antibiotics and ARGs in environmental matrices while others have noted an absence of such a correlation. In either case, it appears that ARGs persist and are transported further along pathways than antibiotics, suggesting distinct factors governing transport (McKinney et al. 2010; Peak et al. 2007). Research is needed to better understand the mechanisms at play, such as antibiotics other selectors (e.g. metals and other toxins), in leaving a human foot-print on environmental reservoirs of resistance. Nonetheless, a reasonable approach for mitigating risk seems to be focusing attention on developing appropriate technologies for eliminating both antibiotics and genetic material from wastestreams.
Thanks again for opening this discussion- I hope to meet you at a conference sometime in the future!
1. Allen, H.K., Moe, L.A., Rodbumrer, J., Gaarder, A., & Handelsman, J., 2009. Functional metagenomics reveals diverse b-lactamases in a remote Alaskan soil. ISME 3, pp. 243-251.
2. Aminov, R.I., 2011. Horizontal gene exchange in environmental microbiota. Front. Microbiol. 2,158 doi:10.3389/fmicb.2011.00158.
3. Corinaldesi, C., Danovaro, R. & Dell‘Anno, A., 2005. Simultaneous recovery of intracellular and extracellular DNA suitable for molecular studies from marine sediments. Appl. Environ. Microbiol. 71, pp. 46-50.
4. D’Costa, V.M., McGrann, K.M., Hughes, D.W., & Wright, G.D., 2006. Sampling the antibiotic resistome. Science 311, pp. 374-377.
5. McKinney, C.W., Loftin, K.A., Meyer, M.T., Davis, J.G., & Pruden, A., 2010. tet and sul antibiotic resistance genes in livestock lagoons of various operation type, configuration, and antibiotic occurrence. Environ. Sci. Technol. 44 (16), pp. 6102-6109.
6. Peak, N., C.W. Knapp; R.K. Yang; M.M. Hanfelt; M.S. Smith, D.S. Aga, & Graham, D. W., 2007. Abundance of six tetracycline resistance genes in wastewater lagoons at cattle feedlots with different antibiotic use strategies. Environ. Microbiol. 9 (1), pp. 143–151.
7. Wu, J. F. & Xi, C. W., 2009. Evaluation of different methods for extracting extracellular DNA from the biofilm matrix. Appl. Environ. Microbiol. 75, pp. 5390-5395.