Significance of antibiotics in the environment

Abstract

Pharmaceuticals are designed to stimulate a physiological response in humans, animals, bacteria or other organisms. During the past decade, concern has grown about the adverse effects the use and disposal of pharmaceuticals might potentially have on human and ecological health. Research has shown that after passing through wastewater treatment, pharmaceuticals, amongst other compounds, are released directly into the environment.1 The selection and development of antibiotic-resistant bacteria is one of the greatest concerns with regard to the use of antimicrobials.2–5 In a report by the House of Lords, it is stated that: ‘resistance to antibiotics and other anti-infective agents constitutes a major threat to public health and ought to be recognized as such more widely than it is at present’.4 Therefore, the European Union (EU) recommends the prudent use of antimicrobial agents in human medicine.2 With respect to the causes of resistance, the focus is on the use of antimicrobials in hospitals, by medical practitioners, i.e. in prescriptions2 and in animal husbandry. ‘…Coordination between human, veterinary and environment sectors should be ensured and the magnitude of the relationship between the occurrence of antimicrobial resistant pathogens in humans, animals and the environment should be further clarified…’.2 However, very little is known about their contribution to the level of bacterial resistance in the environment and its significance. Also, surprisingly, little is known about the extent of environmental occurrence, transport, and ultimate fate and effects of pharmaceuticals in general, as well as of antibiotics in particular.1 In 1996, about 10 200 tons of antibiotics were used in the EU, of which approximately 50% was applied in veterinary medicine and as growth promoters.6 According to data supplied by the European Federation of Animal Health (FEDESA),7 in 1999, 13 288 tons of antibiotics were used in the EU and Switzerland, of which 65% was used in human medicine; 29% was used in the veterinary field and 6% as growth promoters.7 Since most growth promoters have now been banned within the EU, only four compounds remain in this group of feed additives. This may explain the decline in the use of antibiotics in animal husbandry compared with human medicine. In the USA, ∼16 200 tons were produced in 20008 of which 70% was used in livestock farming. This is eight times the amount used in human medicine. Wise estimated total antibiotic market consumption world-wide to lie between 100 000 and 200 000 tons.9 Unused therapeutic drugs are sometimes disposed of into the sewage system. If the drugs are not degraded or eliminated during sewage treatment, in soil or in other environmental compartments, they will reach surface water and ground water, and, potentially, drinking water. Unmetabolized antibiotic substances are often passed into the aquatic environment in wastewater. Antibiotics used for veterinary purposes or as growth promoters are excreted by the animals and end up in manure. Manure is used as an agricultural fertilizer; thus, the antibiotics seep through the soil and enter ground water (Figure 1). However, very little is known about the occurrence, fate and risks associated with antibiotics entering the environment after being used in human and veterinary medicine and as growth promoters; 95% community use is reported for the UK.4 The figure for the USA is 75%.9 In Germany, ∼75% of antimicrobials are used in the community and 25% in hospitals. Ciprofloxacin, for example, was found in concentrations of between 0.7 and 124.5 µg/L in hospital effluent.10 Ampicillin was found in concentrations of between 20 and 80 µg/L in the effluent of a large German hospital.11 Antibiotic concentrations calculated and measured in hospital effluents are of the same order of magnitude as the minimum inhibitory concentrations for susceptible pathogenic bacteria.12 The dilution of hospital effluents by municipal sewage will lower the concentration of antibiotics only moderately, because municipal waste water also contains antibiotic substances and disinfectants from households, veterinary sources and to a minor extent from livestock. Antibiotics have been detected in the µg/L range in municipal sewage, in the effluent of sewage treatment plants (STPs), in surface water and in ground water.13–16 These included quinolones such as ciprofloxacin, sulphonamides, roxythromycin, dehydrated erythromycin and others. If antibiotics are used in animal husbandry, they pass into the soil from manure. Tetracyclines have been detected in concentrations of up to 0.2 µg per kg in soil17 whereas others have been found in the sediment under fish farms. Only a few of the compounds were partially biodegraded under test conditions in aquatic systems.18,19 Most were persistent. The genotoxicity of compounds such as quinolones or metronidazole was not removed during these tests.18 Quinolones, for example, adsorb strongly onto sewage sludge, s