Antibiotic Cocktails Harmful to Freshwater Bacteria
16 December 2010 Antibiotic cocktails harmful to freshwater bacteria
New research has indicated that, at its current levels in the environment, a commonly used antibiotic (chlortetracycline) is toxic to freshwater bacteria. Moreover, when combined with four other antibiotics, the collective effect is greater than the sum of the individual effects. replaced Antibiotics are used extensively to treat bacterial infections in both human and veterinary medicine. However, the active substances in the drugs do not completely disappear when ingested but are excreted and passed into sewage systems. Even when the waste water has been treated, small amounts of antibiotics persist and are present in both groundwater and freshwater. Antibiotics are designed to target bacteria in humans and animals, but little is known about their effect on freshwater bacteria.
The study measured the toxicity of five well-known antibiotics (streptomycin, chlortetracycline, chloramphenicol, fusidic acid and rifampicin) to samples of freshwater bacteria extracted from a shallow lake in Sweden. These particular antibiotics all work by inhibiting the synthesis of proteins in bacteria and all of them affected the freshwater bacteria but with different levels of toxicity.
Chlortetracylcine was the most toxic antibiotic and streptomycin was the least toxic. Streptomycin required a concentration that was over 500 times greater (79.1 micromoles (µmol) per litre) than chlortetracycline (0.138 µmol per litre) to produce the same toxic effects that harmed 50 per cent of bacteria.
Researchers created curve diagrams to illustrate the relationship between antibiotic concentration and their toxic effects. On the whole, these curves were quite flat, i.e. toxicity did not increase by very much if concentration levels increased. This could be due to the differing sensitivities of individual bacterial species within the sample.
Guidelines such as those produced by the European Chemicals Agency (ECHA), often suggest using relatively steep curves to predict toxic effects. According to the results of the study, this could underestimate the effects of low concentrations. For example, the ECHA guidelines suggest that the concentration/toxicity curve follows a gradient of 1:10, but if this was applied to the antibiotic rifampicin, then the point at which the ECHA curve predicts no more toxic effects, would actually still be toxic to a third of the bacteria.
The research also examined the toxic effects of a mixture of all five antibiotics in a set ratio. The results were compared with the effects predicted by two popular methods to estimate mixture toxicity: Concentration Addition (CA) and Independent Action (IA). CA assumes that the individual substances of a mixture behave as if they were dilutions of one another, whereas IA assumes that all substances in a mixture have effects that are independent of one another. There are arguments both for and against using either method.
When compared to the actual results, CA slightly underestimated the observed toxic effects of the mixture by a factor of 1.5, whereas IA slightly overestimated the toxicity by a factor of 0.3. This is contrary to usual patterns where CA tends to predict a higher toxicity than IA in most situations and suggests that the toxicity of this antibiotic mixture might be an exception, at least to freshwater bacteria. The choice of whether to use CA to estimate toxicity of antibiotic mixtures will depend on the risk in underestimating the mixture hazard.
Source: Brosche, S. & Backhaus, T. (2010) Toxicity of five protein synthesis inhibiting antibiotics and their mixture to limnic bacterial communities. Aquatic Toxicology. 99: 457-465.
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Theme(s): Chemicals, Risk assessment
Opinions expressed in this News Alert do not necessarily reflect those of the European Commission To cite this article/service: "Science for Environment Policy": European Commission DG Environment News Alert Service, edited by SCU, The University of the West of England, Bristol.
1 European Commission DG ENV News Alert Issue 222 December 2010