Abstract
We investigated change in number of the resistance genes blaCTX-M and blaTEM, which are known to be related to ESBL, in activated sludge samples collected at municipal and hospital wastewater treatment plants in Thailand and Japan when the samples were cultured in Luria-Bertani (LB) broth containing a high concentration of antibiotics. The insertion sequences IS26 and ISEcp1, which are expected to contribute to horizontal gene transfer of the resistance genes, were also quantified in the same sludge samples. The tested antibiotics were amoxicillin (AMPC), ampicillin (ABPC), cefepime (CFPM), meropenem (MRPM) and doripenem (DRPM), all of which are beta-lactam antibiotics, as well as ciprofloxacin (CPFX) and tetracycline (TC). Both resistance genes in all of the sludge samples significantly increased during the cultivation under the existence of AMPC and ABPC. These genes also increased in some samples under the pressure of CPFX, which is often used in medical treatment as alternative to beta-lactam antibiotics, demonstrating the existence of CPFX-resistant ESBL generating bacteria. MRPM-resistant bacteria, which maybe carbapenem-resistant Enterobacteriaceae (CRE), were found in the sludge sample collected at a hospital wastewater treatment plant in Thailand, although they did not seem to have the above resistance genes. The insertion sequences IS26 and ISEcp1 increased when either of the resistance genes increased in all of the samples added with AMPC and ABPC. The correlation analysis indicates a highly significant contributions of IS26 and ISEcp1 to the transfers of blaTEM and blaCTX-M in the sludge samples, respectively.
