Development of Similar Broth Microdilution Methods to Determine the Antimicrobial Susceptibility of Flavobacterium columnare and F. psychrophilum.

Flavobacterium columnare and F. psychrophilum are major fish pathogens that cause diseases that may require antimicrobial therapy. Choice of appropriate treatment is dependent upon determining the antimicrobial susceptibility of isolates. Therefore we optimized methods for broth microdilution testing of F. columnare and F. psychrophilum to facilitate standardizing an antimicrobial susceptibility test. We developed adaptations to make reproducible broth inoculums and confirmed the proper incubation time and media composition. We tested the stability of potential quality-control bacteria and compared test results between different operators. Log phase occurred at 48 h for F. columnare and 72-96 h for F. psychrophilum, confirming the test should be incubated at 28°C for approximately 48 h and at 18°C for approximately 96 h, respectively. The most consistent susceptibility results were achieved with plain, 4-g/L, dilute Mueller-Hinton broth supplemented with dilute calcium and magnesium. Supplementing the broth with horse serum did not improve growth. The quality-control strains, Escherichia coli ATCC 25922 and Aeromonas salmonicida subsp. salmonicida ATCC 33658, yielded stable minimal inhibitory concentrations (MIC) against all seven antimicrobials tested after 30 passes at 28°C and 15 passes at 18°C. In comparison tests, most MICs of the isolates agreed 100% within one drug dilution for ampicillin, florfenicol, and oxytetracycline. The agreement was lower with the ormetoprim-sulfdimethoxine combination, but there was at least 75% agreement for all but one isolate. These experiments have provided methods to help standardize antimicrobial susceptibility testing of these nutritionally fastidious aquatic bacteria. Received June 24, 2015; accepted October 2, 2015.

Ciprofloxacin resistance in Campylobacter jejuni evolves rapidly in chickens treated with fluoroquinolones.

Fluoroquinolones are commonly used to treat gastroenteritis caused by Campylobacter species. Domestically acquired fluoroquinolone-resistant Campylobacter infection has been documented recently in the United States. It has been proposed that the increase in resistance is due, in part, to the use of fluoroquinolones in poultry. In separate experiments, the effects of sarafloxacin and enrofloxacin treatment of Campylobacter jejuni-infected chickens on the development of ciprofloxacin resistance were measured. Fecal samples were collected before and after treatment and were cultured for C. jejuni. When enrofloxacin or sarafloxacin was used at US Food and Drug Administration-approved doses in broiler chickens, resistance developed rapidly and persisted in C. jejuni. MICs of ciprofloxacin increased from a base of 0.25 microg/mL to 32 microg/mL within the 5-day treatment time frame. These results show that the use of these drugs in chickens rapidly selects for resistant Campylobacter organisms and may result in less effective fluoroquinolone therapy for cases of human campylobacteriosis acquired from exposure to contaminated chicken.