Antibiograms and the estimation of epidemiological cut off values for Vibrio ichthyoenteri isolated from larval olive flounder, Paralichthys olivaceus

In this study, thirty-five strains of Vibrio ichthyoenteri, which is a causative agent of bacterial enteritis, were isolated from diseased larval olive flounder (Paralichthys olivaceus) (n = 35) taken from a hatchery located in Jeju Island. The minimum inhibitory concentration (MIC) of nine antimicrobials was determined for each strain using the Etest or the CLSI broth microdilution method. The MIC distributions were used to generate epidemiological cut off values using normalized resistance interpretation (NRI). The wild type cut off values (COwt) for OTC, ERY, FLU, NA, OXA and CFX were ≤ 2.0, 16.0, 1.5, 3.0, 0.5 and 0.5 mg/L, respectively. Using these COwt values, only isolates from 2007 were treated as non wild-type (NWT) for FLU, NA, OXA and CFX. Most isolates retrieved in 2009 were categorized as wild type (WT) for tetracycline and erythromycin. However, NRI analysis could not be applied to three of the antimicrobials (ampicillin, amoxicillin and trimethoprim/sulfamethoxazole); thus, visual examinations were conducted to determine COwt. We used a limit of 2 mg/L for both ampicillin and amoxicillin, and most strains were classified as NWT for the antibiotics, indicating that beta-lactam antibiotics are not very efficient for the treatment of this disease. Based on the MIC values, V. ichthyoenteri seemed to be sensitive to trimethoprim/sulfamethoxazole. The results of this study showed that the NRI-derived COwt values for the six agents (tetracycline, erythromycin, FLU, NA, OXA and CFX) can be used as provisional epidemiological cut off values.

► We determined MIC for 35 strains of Vibrio ichthyoenteri to nine antimicrobials.
► Wild type cut off (COwt) values for six antibiotics were calculated using NRI method.
► NRI-derived COwt values in this study can be used as epidemiological cut off values.
► Repeated doses of some antibiotics appeared to induce high occurrences of resistance.