Department of Biological Sciences, California State University, Hayward,
Studies in our laboratory have shown that bacterial resistance to
commonly used antibiotics is an emerging problem in the ornamental fish industry. In a recently
completed study, isolates of Aeromonas spp. from tropical fish imported from Singapore
showed multiple resistances to tetracycline, sulfa drugs, erythromycin, and occasionally to
quinolone antibacterials (Dixon et al. 1990). In addition to resistance in ornamental fish
populations, antimicrobial resistance was also documented in bacteria isolated from food fish
(Bullock 1984; MacMillan 1985; Ganzhorn 1987; Hastings and McKay 1987; Taylor 1987; Stamm
1989). These reports document aeromonad resistance to Terramycin and Romet®, the only two
antibiotics approved for use on foodfish in this country.
Due to the increasing number of reports, many of the commonly used
antimicrobials may no longer prove efficacious for the treatment of this gram negative fish
pathogen. It may be necessary at this time to begin screening antimicrobial compounds for
efficacy against fish pathogens. This study was undertaken to determine the in vitro
efficacy of Naxcel® for clinical isolates of Aeromonas spp. isolated from
Naxcel®, a sodium salt of ceftiofur, is a broad spectrum B-lactamase
resistant cephalosporin. As a third generation cephalosporin, Naxcel® provides
bactericidal activity resulting from inhibition of cell wall synthesis. Naxcel® is
currently marketed by The Upjohn Co. for treatment of bovine respiratory disease (pneumonia,
shipping fever) associated with Pasteurella hemolytica and P. multocida
infections. Ceftiofur has also demonstrated excellent in vitro activity against gram negative
bacterial pathogens such as Escherichia coli, Salmonella typhimurium and Haemophilus
pleurogneumonia (Yancy et al 1987).
This study was undertaken to determine the in vitro effectiveness of
Naxcel® for Aeromonas spp. isolated from ornamental fish.
Materials and Methods
Lesion and/or kidney samples from ornamental fish were plated on
Rimle-Shotts medium for primary isolation of Aeromonas spp., and incubated at 30C.
Isolated colonies were identified according to Fish Health Section Bluebook guidelines (Amos
1985). Oxidase positive isolates were tested for motility, sensitivity to novobiocin (5mcg) and
0/129 (0.1%), growth in salt free nutrient gelatin and fermentation of glucose on OF medium.
Colonies identifying to the genus were inoculated into the Nonfermenter Test strip (NFT) system
(Anatylab Products, Plainview, NY) for species identification. Once identified, colony isolates
were plated onto Mueller-Hinton agar (Remel, Sacramento, CA) for determination of in
vitro antibiotic sensitivity. The standard Kirby-Bauer disk diffusion method was used to
determine antibiotic sensitivity to commonly used antibiotics and Naxcel R. Following
incubation at 30C for 24 hours, inhibition zone sizes were measured. The following
susceptibility disks were used: erythromycin 0.015mg, nalidixic acid 0.03mg, neomycin 0.03mg,
sulfamethoxazole 25mcg/trimethoprim 25mcg obtained from General Diagnostics (Morris Plains,
NJ), nitrofurantoin 0.3mg, triple sulfa 0.3mg, tetracycline 0.03mg, trimethoprim 5mcg and
ampicillin 10mcg obtained from Difco Laboratories (Detroit, MI). Oxolinic acid 2mcg and
ormetoprim 1.2mcg/ sulfadimethoxine 23.8mcg (Romet R) supplied by Baltimore Biological
Laboratories (Cockneysville, MD), and Naxcel R 3Omcg supplied by Upjohn (Kalamazoo, MI).
Minimum inhibitory concentrations (MIC) were determined by the Veterinary Diagnostic Laboratory
System, University of California, Davis, CA.
Of the 31 isolates identified, 19 were identified as A. sobria, 11
as A. hydrophila and 1 as A. sp., by the Nonfermenter Test strip system. None of
the isolates was resistant to Naxcel® as determined by zone of inhibition size
measurements. MIC determinations ranged from <= 0.25µg/ml to 1.0µg/ml, well within
the susceptible range. However, all of the isolates were resistant to ampicillin, 71% were
resistant to tetracycline. Resistance to all the other antimicrobials was less than 29%.
Naxcel R appears to be highly effective in vitro against
Aeromonas spp. isolated from ornamental fish. Further in vivo study is needed to
determine the potential for Naxcel® as an efficacious treatment for aeromonad infections
Funding for this study was provided by The Upjohn Company, Kalamazoo,
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identification of certain fish pathogens. Fish Health Section, American Fisheries Society
2. Bullock, G. 1984. Enteric red mouth disease of salmonids. U.S.
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3. Dixon, B.A., J. Yamashita and F. Evelyn. 1990. Antibiotic
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