Los Microorganismos del Suelo y su Creciente Resistencia a Antibióticos y Herbicidas

Conforme los suelos son expuestos a herbicidas y antibióticos, sus microorganismos se tornan paulatinamente más resistentes a estos microbicidas. Más aun, las parcelas de cultivo o pastos que han sufrido previamente tratamientos con estos últimos, degradan antes los nuevos productos de la misma índole que las “control” previamente no expuestas. Si nos centramos en los antibióticos, que entran en el pack, con los que se trata a los animales en la ganadería industrial, los mecanismos de resistencia pueden ser de diversa naturaleza. El genoma de algunos microorganismos mutan de tal forma que consiguen expulsar el agente toxico al exterior. En otras ocasiones su metabolismo lo modifica para que no les resulte tóxico. La noticia de hoy nos informa que se ha detectado un nuevo mecanismo. Al parecer ciertos actinomicetos, como Microbacterium (detectado tanto en el suelo como en las plantas depuradoras de aguas residuales) logran romper la molécula de los antibióticos utilizando parte de sus fragmentos como fuente para su alimentación en formas de carbono y nitrógeno (dicho de otro modo que entran a engrosar los nutrimentos de su dieta).  Los investigadores responsables del estudio, cuya nota de prensa apareció en Sciencedaily, aplicaron varios antibióticos de uso veterinario en parcelas vírgenes frente a otras que habían recibido previamente este tipo de tóxicos. La razón estriba en que, en muchas regiones, resulta altamente frecuente que los excrementos del ganado tratado se apliquen como fertilizantes orgánicos, al menos una vez al año (“toxic organic farmings?” or “smart microbial communities?). Y efectivamente constataron que, tras once años, las parcelas que habían recibido este abono tóxico con anterioridad lo “metabolizaban”, como en el caso de los herbicidas, logrando alcanzar cepas microbianas de resistencia a los nuevos antibióticos mucho antes que los suelos que no habían sido expuestos con anterioridad. Una de las conclusiones de estos investigadores, a ser corroborada,  estriba en que las bacterias del suelo podrían intercambiar genes (el típico flujo horizontal que aparece en los procariotas) con vistas a romper las moléculas de los antibióticos, más o menos simultáneamente.

De todo ello (ya en parte conocido por investigaciones previas que os hemos mostrado en post precedentes), podemos concluir que (i) la aplicación continuada de herbicidas y antibióticos, genera que las comunidades microbianas del suelo se encuentren más preparadas para metabolizar otros nuevos, generando una retroalimentación positiva que va en contra de las expectativas del agricultor y la vida media de estos productos en él marcado; (ii) que como en el caso de los organismos humanos, aplicar (ingerir) tales sustancias continuadamente daña la salud del receptor (les hace insensibles al tratamiento); (iii) la detección de Microbacterium en las depuradoras de aguas residuales sugiere que los mecanismos aplicados en las mismas pueden verse afectados a la hora de degradar los compuestos orgánicos tóxicos, o crear bacterias resistentes a los mismos (ya sea para bien o para mal de los intereses humanos) y (iv) que los sistemas utilizados por la ganadería industrial son tanto insustentables como dañinos para la salud ciudadana.

ganaderia-industrial-resistencia-a-antibioticos

Ganadería Industrial: generando resistencia a los antibióticos en las comunidades microbianas del suelo. Fuente: Science Now

Juan José Ibáñez

Antibiotic-Eating Bug Unearthed in Soil: Newly Discovered Bacterium Degrades an Antibiotic Both to Protect Itself and Get Nutrition

 Sciencedaily Dec. 7, 2012 It’s well known how bacteria exposed to antibiotics for long periods will find ways to resist the drugs — by quickly pumping them out of their cells, for instance, or modifying the compounds so they’re no longer toxic.

 Now new research has uncovered another possible mechanism of antibiotic «resistance» in soil. In a paper published on Dec. 6 in the Journal of Environmental Quality, a group of Canadian and French scientists report on a soil bacterium that breaks down the common veterinary antibiotic, sulfamethazine, and uses it for growth.

Certain soil bacteria are already known to live off, or «eat,» agricultural pesticides and herbicides, says the study’s leader, Ed Topp, a soil microbiologist with Agriculture and Agri-Food Canada in London, Ontario. In fact, the microbes’ presence in farm fields can cause these agrichemicals to fail.

But to Topp’s knowledge, this is the first report of a soil microorganism that degrades an antibiotic both to protect itself and get nutrition.

«I think it’s kind of a game changer in terms of how we think about our environment and antibiotic resistance,» he says.

Concerns about widespread antibiotic resistance are what led Topp and his collaborators to set up an experiment 14 years ago, in which they dosed soils annually with environmentally relevant concentrations of three veterinary antibiotics: sulfamethazine, tylosin, and chlortetracycline. Commonly fed to pigs and other livestock, antibiotics are thought to keep animals healthier. But they’re also excreted in manure, which is then spread once a year as fertilizer in countless North American farm fields.

The researchers first wanted to know whether these yearly applications were promoting higher levels of antibiotic resistance in soil bacteria. But a few years ago, they also decided to compare the persistence of the drugs in soil plots that had been repeatedly dosed, versus fresh soils where antibiotics were never applied.

They did this experiment, Topp explains, because of previous work indicating that pesticides often break down more quickly in soils with a long history of exposure, indicating that pesticide-degrading microbes have been selected for over time.

Still, it came as a surprise when they saw antibiotics also degrading much faster in long-term, treated plots than in fresh, control soils, he says. In particular, sulfamethazine — a member of the antibiotic class called sulfonamides — disappeared up to five times faster.

The researchers subsequently cultured from the treated plots a new strain of Microbacterium, an actinomycete that uses sulfamethazine as a nitrogen and carbon source. Extremely common in soil, actinomycete bacteria are known to degrade a wide range of organic compounds. And now at least two other sulfanomide-degrading Microbacterium strains have been reported, Topp says: one from soil and another from a sewage treatment plant.

Taken together, the findings suggest that the capability to break down sulfanomides could be widespread. And if it’s indeed true that «the microbiology in the environment is learning to break these drugs down more rapidly when exposed to them, this would effectively reduce the amount of time that the environment is exposed to these drugs and therefore possibly attenuate the impacts,» Topp says.

Not that negative impacts aren’t still occurring, he cautions. In particular, long-term exposure to antibiotics puts significant pressure on soil bacteria to evolve resistance, which they typically do by giving and receiving genes that let them detoxify drugs, or keep the compounds out of their cells.

What the new research suggests, though, is that soil bacteria could be swapping genes for breaking down antibiotics at the same time.

«My guess is that’s probably what’s happening, but it remains to be determined,» Topp says. «It’s actually extremely fascinating.»

The work was funded by Agriculture and Agri-Food Canada.

Story Source: The above story is reprinted from materials provided by American Society of Agronomy. Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference: Edward Topp, Ralph Chapman, Marion Devers-Lamrani, Alain Hartmann, Romain Marti, Fabrice Martin-Laurent, Lyne Sabourin, Andrew Scott, Mark Sumarah. Accelerated Biodegradation of Veterinary Antibiotics in Agricultural Soil following Long-Term Exposure, and Isolation of a Sulfamethazine-degrading sp.. Journal of Environment Quality, 2012; DOI: 10.2134/jeq2012.0162

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