Antimicrobial susceptibility and genetic characterisation of oxytetracycline‐resistant Edwardsiella tarda isolated from diseased eels

Abstract

Edwardsiellosis is one of the most important bacterial diseases in eels. Edwardsiella tarda (E. tarda) isolates (n=94) from diseased eels were screened against the eight most commonly used antimicrobial agents in aquaculture in Taiwan. These isolates were highly susceptible to ampicillin, amoxicillin, florfenicol, oxolinic acid and flumequine. In total, 20 of the 94 (21.3 per cent) isolates tested were resistant to oxytetracycline (OTC). Among the 20 tetracycline-resistant E. tarda isolates, tet(A) was the predominant resistance determinant and was detected in 100 per cent of the isolates, whereas 90 per cent of these isolates carried the tet(M) determinant. The efflux pump inhibitor omeprazole reduced the minimum inhibitory concentrations (MICs) of OTC for these isolates by twofold to eightfold, suggesting that an intact efflux pump, presumably encoded by tet(A), is required for high-level tetracycline resistance. Real-time PCR experiments showed that increased expression levels of tet(A) and tet(R) could contribute to OTC resistance. Southern blot hybridisation also confirmed that the 20 OTC-resistant E. tarda isolates all carried the TetA determinant on a plasmid that is approximately 50 or 70 kb in size, and restriction fragment length polymorphisms (RFLP) showed that the tet(A) gene was located on an 8-10 kb EcoRI fragment in all of these plasmids. The same plasmid type and RFLP patterns were detected across different farms in the same region, but differences in their pulsed-field gel electrophoresis (PFGE) patterns were observed. This suggests a possible role for horizontal spreading and local transmission of the plasmid in the OTC-resistant E. tarda population of eels from two different geographic origins.

Occurrence of multiple antibiotic resistance and genotypic characterization inEdwardsiella tardaisolated from cage‐cultured hybrid red tilapia (Oreochromissp.) in the Ping River, Northern Thailand

“…Numerous researchers have also reported that E. tarda isolates in fish in various countries are highly resistant to ampicillin and amoxicillin (Lee & Wendy, ; Ogbonne, Ukazu, & Egbe, ; Shetty et al, ; Wimalasena et al, ). Previous studies have demonstrated that E. tarda is sensitive to oxytetracycline and sulpha‐trimethoprim (Castro, Toranzo, Barja, Nunez, & Magarinos, ; Shetty et al, ; Stock & Wiedemann, ), while the appearance in red tilapia, olive flounder, red sea bream and eels of E. tarda which is resistant to oxytetracycline and sulpha‐trimethoprim has been frequently reported (Lee & Wendy, ; Lo et al, ; Wimalasena et al, ). These observations suggest that there is a general trend of increasing resistance to oxytetracycline and sulpha‐trimethoprim of E. tarda isolates in aquaculture.…”

“…E. tarda is also a zoonotic pathogen with a broad range of hosts (Xu & Zhang, ). Antibiotics have been the primary measure for controlling and treating bacterial infection in aquaculture (Lo, Lee, Wang, & Kuo, ). However, the overuse and misuse of antibiotics have promoted the emergence of antibiotic‐resistant bacteria, which has significant potential for developing into a crisis in the aquaculture industry, in the environment and in humans (Qiao, Ying, Singer, & Zhu, ).…”

“…Tetracycline‐resistant determinant genes such as tetA , tetD , tetM and tetG have been reported in E. tarda in previous studies. The tetA gene was the most widespread determinant in E. tarda isolated from eel and flounder (Jin Jun et al, ; Lo et al, ). Interestingly, almost all the oxytetracycline‐resistant isolates (96%, 26/27) carried the tetA gene, while two oxytetracycline‐sensitive isolates were negative for tetA .…”

A Spontaneous Outbreak of Systemic Edwardsiella piscicida Infection in Largemouth Bass Micropterus salmoides (Lacépède, 1802) in California, USA

“…Extralabel administration of florfenicol (Aquaflor, Merck Animal Health, Madison, NJ, USA) medicated feed was initiated by veterinary feed directive (VFD) in accordance with CPG 615.115 Extralabel Use of Medicated Feeds for Minor Species (FDA, ). Although antibiotic breakpoints are not available for E. piscicida , florfenicol was chosen based on its low MIC and efficacy towards other Edwardsiella species (Lo, Lee, Wang, & Kuo, ; McGinnis, Gaunt, Santucci, Simmons, & Endris, ; Reichley et al, ). The two ponds were fed once daily at a rate of 15 mg of active ingredient per kg of fish body weight per day for 10 days followed by a 42‐day withdrawal period.…”

“…Free and phagocytized bacterial rods were observed within the lumens of lamellar capillaries (Figure 3d). Mild-to-moderate granulomatous infiltrates, with and without observable bacteria, partially enveloped the olfactory nerves (Figure 3e) E. piscicida, florfenicol was chosen based on its low MIC and efficacy towards other Edwardsiella species (Lo, Lee, Wang, & Kuo, 2014;McGinnis, Gaunt, Santucci, Simmons, & Endris, 2003;Reichley et al, 2018). The two ponds were fed once daily at a rate of 15 mg of active ingredient per kg of fish body weight per day for 10 days followed by a 42-day withdrawal period.…”