Toxicity of the Colistin Sulfate Antibiotic Used in Animal Farming to Mixed Cultures of Nitrifying Organisms C. R. Bressan & A. Kunz & W. Schmidell & H. M. Soares Received: 23 July 2012 / Accepted: 10 January 2013 # Springer Science+Business Media Dordrecht 2013 Abstract Colistin is a peptide antibiotic widely used as a food additive in animal farming, specially swine and poultry, and also has recently been applied in human medicine to treat infections caused by multi- resistant gram-negative bacteria strains. When orally administered, colistin is eliminated in feces virtually unaltered; thus, it may reach water bodies and waste- water treatment facilities in its active form. Apart from the risks associated with development of antimicrobial resistance and environmental toxicity issues, the pres- ence of antimicrobials in wastewater can, additionally, interfere in biological processes commonly used to treat them. Nitrifying bacteria are among the most sensitive microorganisms to inhibitory compounds, including pharmaceuticals, and are useful as biosen- sors to access contaminant toxicity information in wastewater treatment plants. Therefore, in order to assess the colistin acute toxicity to the microorganisms involved in the nitrification processes, the nitritation and nitratation kinetics were monitored under different colistin concentrations. The results showed that only ammonia-oxidizing bacteria are sensitive to the antibiotic, presenting an IC50 of 10.8 mgL -1 of colis- tin when used as a commercial formulation and 67.0 mgL -1 when used as raw colistin sulfate. For nitrite-oxidizing bacteria, even the highest colistin con- centration used in the assays (316 mgL -1 ) was not sufficient to inhibit the process. According to these results, the colistin concentrations expected in animal farming wastewater, when its dosage is used as a growth promoter, would not be enough to keep nitrifi- cation from taking place. Nevertheless, when used in higher concentrations, such as for therapeutic purposes, it could endanger the maintenance of the process. Keywords Colistin . Polymyxin E . Nitrification . Acute toxicity . Antibiotics 1 Introduction Colistin sulfate is widely used in animal farming as a food additive, especially in intensive pig farms, being one of the strongest antibiotics for treating intestinal infections caused by gram-negative bacteria such as Escherichia coli, Salmonella sp., and Pseudomonas sp. Besides the therapeutic applications, colistin is commonly used in lower concentrations as a growth promoter (prophylactic use) (Casal et al. 2007; Callens et al. 2012). Additionally, colistin use is also reemerging in human medicine as an option for the treatment of multi- resistant gram-negative strains such as Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella Water Air Soil Pollut (2013) 224:1441 DOI 10.1007/s11270-013-1441-4 C. R. Bressan (*) : W. Schmidell : H. M. Soares Chemical Engineering and Food Engineering Department, Federal University of Santa Catarina—UFSC, P.O. Box 476, CEP 88040-900 Florianópolis, SC, Brazil e-mail: cleorb@gmail.com A. Kunz Embrapa Swine and Poultry, P.O. Box 21, 89700-000 Concórdia, SC, Brazil