Abstract
To investigate tetracycline resistance and resistant genotype in Riemerella anatipestifer, the tetracycline susceptibility of 212 R. anatipestifer isolates from China between 2011 and 2017 was tested. The results showed that 192 of 212 (90.6%) R. anatipestifer isolates exhibited resistance to tetracycline (the MICs ranged from 4 to 256 μg/ml). The results of PCR detection showed that, 170 of 212 (80.2%) R. anatipestifer isolates possessed the tet(X) gene. Other genes, including tet(A), tet(M), tet(Q), tet(O), tet(B), and tet(O/W/32/O), were found at frequencies of 20.8, 4.7, 1.4, 0.9, 0.9, and 0.5%, respectively. However, tet(C), tet(E), tet(G), tet(K), and tet(W) were not detected in any isolate. In these tet gene positive strains, 31 (14.6%), 2 (0.9%), 5 (2.4%), 1 (0.5%), 3 (1.4%) were detected containing tet(A)/tet(X), tet(M)/tet(O), tet(M)/tet(X), tet(O)/tet(X), and tet(Q)/tet(X) simultaneously, respectively. One isolates, R131, unexpectedly contained three tet genes, i.e., tet(M), tet(O), and tet(X). Sequence analysis of the tet gene ORFs cloned from R. anatipestifer isolates confirmed that tet(A), tet(B), tet(M), tet(O), tet(Q) and an unusual mosaic tet gene tet(O/W/32/O) were present in R. anatipestifer. The MIC results of R. anatipestifer ATCC 11845 transconjugants carrying tet(A), tet(B), tet(M), tet(O), tet(O/W/32/O), tet(Q), and tet(X) genes exhibited tetracycline resistance with MIC values ranging from 4 to 64 μg/ml. Additionally, the tet(X) gene could transfer into susceptible strain via natural transformation (transformation frequencies of ~10−6). In conclusion, the tet(A), tet(B), tet(M), tet(O), tet(O/W/32/O), tet(Q), and tet(X) genes were found and conferred tetracycline resistance in R. anatipestifer isolates. Moreover, the tet(X) is the main mechanism of tetracycline resistance in R. anatipestifer isolates. To our knowledge, this is the first report of tet(A), tet(B), tet(M), tet(O), tet(Q), and mosaic gene tet(O/W/32/O) in R. anatipestifer.
Highlights
Tetracyclines are one of the cheapest broad-spectrum antibacterial agents, with activity against a wide range of host, including aerobic, anaerobic, gram-positive and gram-negative bacteria (Chopra and Roberts, 2001; Roberts, 2003)
R. anatipestifer strains were cultured at 37◦C in GC broth (GCB) or on GC agar (GCA) plates (Liu et al, 2017) when prepared for natural transformation, and were grown at 37◦C in tryptic soybean broth (TSB; Oxoid Ltd, Basingstoke, Hampshire, England) or on TSA plates when prepared for susceptibility testing
We investigated the tetracycline resistance and resistant genotypes in R. anatipestifer isolates in China
Summary
Tetracyclines are one of the cheapest broad-spectrum antibacterial agents, with activity against a wide range of host, including aerobic, anaerobic, gram-positive and gram-negative bacteria (Chopra and Roberts, 2001; Roberts, 2003). The antibiotic activity of tetracycline exerts through targeting the 30S ribosome subunit, resulting in inhibition of protein synthesis (Walsh, 2003). Due to overuse of tetracycline antibiotics in clinics, tetracyclineresistant strains and tetracycline resistance genes (tet) have occurred in bacteria with increasing frequency (Chopra and Roberts, 2001; Zhang et al, 2012). The major mechanisms of tetracycline resistance include: active efflux pumps, ribosomal protection and enzymatic modification (Thaker et al, 2010). The predominant mechanisms of tetracycline resistance are active efflux pumps and ribosomal protection proteins. There are 59 different genes found in various genera of bacteria conferring tetracycline resistance, including: efflux genes (n = 33), ribosomal protection genes (n = 12) and enzymatic inactivation genes (n = 13) An unknown gene tet(U) was reported to be located on the pKq10 plasmid in Enterococcus faecium (Ridenhour et al, 1996)
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