br Materials and methods br Results br Discussion

Materials and methods


In the present study, the MAR susceptibility profiles of S. Typhimurium clinical isolates from pigs were determined using a new ciprofloxacin breakpoint, which was recently revised to reduce technical difficulties and to increase the reliability of clinical laboratories (Humphries et al., 2012). MAR showed the highest potency against all isolates, including ACSSuT (36.4%) (Table 1). The MAR resistance rate in isolates was similar to that of previous reports in Korea and lower than those in other countries (Lim et al., 2009; Kuang et al., 2015; de Jong et al., 2012). Regarding the relatively small exposure to MAR than is used for other drugs in veterinary use in Korea (QIA, 2014), the quantity is based on the strategy for treatment with MAR to prevent the emergence of resistant S. Typhimurium strains.
Dosing above the MPC has been suggested to minimize the selection of mutants during antibiotic treatment (Drlica and Zhao, 2007). Recently, a one-dose regimen for MAR based on the MPC was reported to result in no emergence of bacterial resistance in clinical susceptible pathogens (Vallé et al., 2012). Furthermore, our laboratory suggested that adjusting the AUC24h/MPC ratio of MAR using an in vitro dynamic model for a 3-d treatment could prevent the selection of resistant S. Typhimurium (Lee et al., 2016). Therefore, to reduce chance for the development of MAR resistance by a single or repeated exposure to FQ, an optimized dosage based on the MPC concept will be required. Overall, MPC-based approaches were used in this study, suggesting a higher efficacy of MAR in preventing the selection of single-step mutants of S. Typhimurium induced by molecular resistance mechanisms, target mutation, and overexpression of multi-drug efflux pumps.

Our findings suggest that a single-exposure to a sub-MPC of MAR was sufficient to reduce susceptibility in Salmonella clinical isolates without leading to high-level FQ resistance. Our results confirmed that gyrA mutations play a critical role in the development of FQ resistance in both resistant isolates and single-step mutants. In addition, the acrAB-tolC-mediated efflux mechanism was found to contribute to decreased susceptibility to MAR in the presence or absence of gyrA mutations. This mechanism was associated with the (RS)-CPP Supplier of global regulators (marA/soxS/ramA) and partially associated with a local regulator (acrR). Therefore, to reduce chance for the development of MAR resistance, an optimized dosage based on the MPC concept and continuous monitoring of S. Typhimurium resistance on farms will be required.

Conflicts of interest

Ethical approval

In general, avian influenza A viruses (AIVs) have host-species barriers (Yoon et al., 2014). While the wild birds are the natural host of AIVs (Choi et al., 2017; Jiang et al., 2017; Webster et al., 1992), AIVs can also infect mammals including tigers, leopards, dogs, cats, plateau pikas, rhesus macaques, and humans (Cheng et al., 2014b; Gao et al., 2013; Keawcharoen et al., 2004; Shinya et al., 2012; Songserm et al., 2006; Subbarao et al., 1998; Yu et al., 2014; Zhang et al., 2013a, 2013b). Considering AIVs are reported to be capable of infection with mammals and transmission between mammals (Belser et al., 2013; Gao et al., 2009; Kimble et al., 2011; Sang et al., 2015b; Wu et al., 2010; Zhang et al., 2013c), AIVs may possess pandemic potential.
H3N2 AIVs circulate widely in areas including North America (Corzo et al., 2012; Guo et al., 2015; Pasick et al., 2010), Europe (Campitelli et al., 2002; Jonassen and Handeland, 2007), Asia (Choi et al., 2012; Gerloff et al., 2016; Li et al., 2016; Yang et al., 2015), and Oceania (Peroulis and O\’Riley, 2004). Transmission of H3N2 AIVs to mammals has been reported on previous studies (Lee et al., 2009; Lei et al., 2012; Song et al., 2008; Su et al., 2012). In 1968, 1957 H2N2 pandemic influenza virus was replaced by 1968 H3N2 pandemic influenza virus that possessed an H3 HA gene and a PB1 gene of H3 AIVs origin (Neumann et al., 2009). Additionally, avian-origin H3N2 swine influenza strains were isolated from pigs in Italy in 1985 that appeared to result from reassortment between the genes encoding the HA and NA proteins of the human-like H3N2 swine influenza virus and the internal proteins of the avian-origin H1N1 swine viruses (Castrucci et al., 1993). From then on, avian-origin H3N2 swine influenza viruses continue to circulate in swine in the world (Campitelli et al., 1997). A previous report found that dogs inoculated with an avian-origin H3N2 virus were capable of transmitting the virus to naïve animals in direct-contact models (Song et al., 2009a). Considering reports of mammals’ infection with H3N2 AIVs and a lack of pre-existing immunity against H3N2 AIVs in mammals and humans, H3N2 AIVs may pose a pandemic threat. Therefore, the pathogenesis involved in the mammalian adaptation of H3N2 AIVs should be further studied.