Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2008
Publication Date: 12/22/2008
Citation: Darwish, A.M., Farmer, B.D., Hawke, J.P. 2008. Improved methodology for determining antibiotic susceptibility of Flavobacterium columnare isolates by broth microdilution. Journal of Aquatic Animal Health. 20:185-191. Interpretive Summary: Flavobacterium columnare the causative agent of columnaris disease costs the United States aquaculture industry $ 40-50 million annually. The disease has been reported to cause mortalities greater than 90 % in tank-held channel catfish and in salmonid and eel populations. The use of antibiotics to curb columnaris mortalities is indispensable. For successful antibiotic use it is essential to asses the sensitivity of the treated pathogens to the antibiotic to be applied. The current study developed an accurate, reliable and reproducible method to test the sensitivity of F. columnare to antibiotics. The developed method adhered closely to the published standards of the Clinical and Laboratory Standards Institute (CLSI), formally the National Committee for Clinical Laboratory Standards, and could be adopted by CLSI upon further national and international laboratory testing.
Technical Abstract: A simple, accurate and reliable microdilution method has been developed to test the susceptibility of Flavobacterium columnare to antibiotics. The method has been used to determine the minimum inhibitory concentration (MIC) of 23 F. columnare isolates. The developed method conducted at 28 °C used a standardized inoculum, Mueller Hinton broth at 1/5 of the full strength (4 g/l), a reference isolate, positive and negative control wells and standardized custom made microtitre plates, Sensititre® Susceptibility Plates for Aquaculture (Trek Diagnostic Systems, Inc.). Escherichia coli ATCC25922 was used as the reference isolate and produced MIC values within the range published by the Clinical and Laboratory Standards Institute (CLSI), formally the National Committee for Clinical Laboratory Standards (NCCLS). Mueller Hinton broth at 1/5 of the full strength (4 g/l) was found to support significantly better growth for Escherichia coli ATCC25922 and F. columnare ATCC23463, the species type strain, than Mueller Hinton broth at 1/7 of the full strength.