Cross resistance between oxytetracycline and oxolinic acid in Aeromonas salmonicida associated with alterations in outer membrane proteins

Isolation and antimicrobial susceptibility of Aeromonas salmonicida in rainbow trout (Oncorhynchus mykiss) in turkey hatchery farms

Three Aeromonas salmonicida strains were isolated from the livers of 265 rainbow trouts sampled. The antibiotic susceptibility test results showed that A. salmonicida strains were susceptible to streptomycine and ciprofloxacin. However, they were resistant to amoxycilline + clavulanic acid, penicillin, erythromycine, oxytetracycline and cefuroxime sodium.

Type II topoisomerase quinolone resistance-determining regions of Aeromonas caviae, A. hydrophila, and A. sobria complexes and mutations associated with quinolone resistance

Most Aeromonas strains isolated from two European rivers were previously found to be resistant to nalidixic acid. In order to elucidate the mechanism of this resistance, 20 strains of Aeromonas caviae (n = 10), A. hydrophila (n = 5), and A. sobria (n = 5) complexes, including 3 reference strains and 17 environmental isolates, were investigated. Fragments of the gyrA, gyrB, parC, and parE genes encompassing the quinolone resistance-determining regions (QRDRs) were amplified by PCR and sequenced. Results obtained for the six sensitive strains showed that the GyrA, GyrB, ParC, and ParE QRDR fragments of Aeromonas spp. were highly conserved (> or =96.1% identity), despite some genetic polymorphism; they were most closely related to those of Vibrio spp., Pseudomonas spp., and members of the family Enterobacteriaceae (72.4 to 97.1% homology). All 14 environmental resistant strains carried a point mutation in the GyrA QRDR at codon 83, leading to the substitution Ser-83-->Ile (10 strains) or Ser-83-->Arg. In addition, seven strains harbored a mutation in the ParC QRDR either at position 80 (five strains), generating a Ser-80-->Ile (three strains) or Ser-80-->Arg change, or at position 84, yielding a Glu-84-->Lys modification. No amino acid alterations were discovered in the GyrB and ParE QRDRs. Double gyrA-parC missense mutations were associated with higher levels of quinolone resistance compared with the levels associated with single gyrA mutations. The most resistant strains probably had an additional mechanism(s) of resistance, such as decreased accumulation of the drugs. Our data suggest that, in mesophilic Aeromonas spp., as in other gram-negative bacteria, gyrase and topoisomerase IV are the primary and secondary targets for quinolones, respectively.

Occurrence of antimicrobial resistance in fish-pathogenic and environmental bacteria associated with four danish rainbow trout farms

Surveillance of bacterial susceptibility to five antimicrobial agents was performed during a 1-year period in and around four freshwater fish farms situated along a stream in western Denmark. Besides assessing the levels of antibiotic resistance among the culturable fraction of microorganisms in fish, water, and sediment samples, two major fish pathogens (88 Flavobacterium psychrophilum isolates and 134 Yersinia ruckeri isolates) and 313 motile Aeromonas isolates, representing a group of ubiquitous aquatic bacteria, were isolated from the same samples. MICs were obtained applying a standardized agar dilution method. A markedly decreased susceptibility of F. psychrophilum isolates to most antimicrobial agents presently available for use in Danish aquaculture was detected, while the collected Y. ruckeri isolates remained largely sensitive to all therapeutic substances. Comparing the inlet and outlet samples, the increase of the antibiotic-resistant proportions observed among the culturable microflora was more pronounced and statistically significant among the motile aeromonads. High levels of individual and multiple antimicrobial resistances were demonstrated within the collected flavobacteria and aeromonads, thus indicating a substantial impact of fish farming on several groups of bacteria associated with aquacultural environments.

gyrA mutations in quinolone-resistant isolates of the fish pathogen Aeromonas salmonicida

gyrA mutations in quinolone-resistant isolates of Aeromonas salmonicida have been detected by using PCR to amplify the quinolone resistance-determining region of gyrA and subsequent cloning and sequencing of PCR products. Comparison of nucleotide and derived amino acid sequences of PCR products from quinolone-susceptible and -resistant bacteria revealed a serine 83-to-isoleucine substitution in the gyrase A protein of resistant isolates. One of the resistant isolates differed from the other by a two- to fourfold-higher MIC of the fluoroquinolone enrofloxacin and carried an additional alanine 67-to-glycine substitution, which may contribute to the higher level of resistance.