**1. Introduction**

Antibiotics in the environment represent a growing concern as their presence can promote the selection of antibiotic resistant bacteria (ARB) that pose a serious public health threat. ARB can further spread resistance genes in the environment by the mechanism of horizontal gene transfer through which environmental bacteria can then mediate pathogens to acquire antibiotic resistance genes [1–4]. Among the various sources accounting for the spread of ARB, organic wastes, including wastes of municipal and agricultural origin, have been widely reported to be potent reservoirs of ARB- harbouring genes for multidrug resistance. Previous studies have pointed out that numerous ARB and resistant genes have been detected in sewage sludge from municipal wastewater treatment plants (WWTPs) [4,5]. One predominant antibiotic resistance mechanism is the presence of Extended Spectrum β-Lactamases (ESBLs). ESBLs are of great microbiological and clinical importance in Enterobacteriaceae, especially *Escherichia coli* and *Klebsiella* spp. and other non-fermenting bacteria such as *Acinetobacter* spp. and *Pseudomonas aeruginosa* [6,7]. The presence of ESBL in surface water has been frequently demonstrated all over the world, which leads to the conclusion that if the bacteria in the water are able to host ESBL genes, then there will be ESBL in the population [8–11]. The spread of ESBL is enhanced by the localization of most of the ESBL genes on mobile genetic elements which allow the transmission of resistance genes to strains and species which are better adapted to the surface water environment. As a consequence of this, environmental bacteria can acquire resistance genes from e.g., strains of clinical origin [8–11]. Methicillin resistant *Staphylococcus aureus* (MRSA) originates from the clinical setting, as hospital acquired (HA)-MRSA. Nevertheless, MRSA strains started to spread among the healthy human population (so called community acquired CA-MRSA) and livestock (LA-MRSA) within the last decades like ESBL [12–14]. MRSA detections from environmental reservoirs, including surface water, are very rare compared to multiresistant Gram negative bacteria isolation. Although the population of Staphylococci flushed into the wastewater is high, the survival of Staphylococci in water environment seems to be much lower than that of Gram negative bacilli. Therefore reports of MRSA from this reservoir are mainly restricted to areas of high human influence, e.g., hospital waste water effluent [15–17]. Vancomycin resistant Enterococci (VRE) are one of the first documented antibiotic resistant bacteria with primary origin in animal farming. The rise of VRE was caused by the use of the glycopeptide avoparcin as a growth promoter from 1975 on. Although glycopeptide use was banned in livestock production in the European Union (1996) VRE are still present in animals and can also be found in hospital settings [18–20]. Hence VRE are present in waste and surface water, it seems that they are detected mostly sporadically. Furthermore, the number of studies covering this topic is limited. The aim of the present study was to investigate the presence of multidrug resistant bacteria such as ESBL-producing Enterobacteriaceae, MRSA and VRE in activated sludge in the second largest commercial WWTP in Austria.
