Epidemiological cut-off values for Yersina ruckeri disc diffusion data generated by a standardised method.

In order to establish the meaning of data generated in antimicrobial agent susceptibility tests, it is necessary to develop internationally harmonised interpretive criteria. Currently, such criteria have not been developed for data generated in studies of the susceptibility of the fish pathogen Yersinia ruckeri. This work generated the data that would be required to set epidemiological cut-off values for the susceptibility data of this species that had been generated using a standardised disc diffusion method that specified the use of Mueller Hinton agar and incubation at 22°C for 24-28 h. Using this method, sets of inhibition zones data for 4 antimicrobial agents were generated by 3 independent laboratories. The data from these laboratories were aggregated and analysed using the statistically based normalised resistance interpretation. For ampicillin, florfenicol, oxytetracycline and trimethoprim-sulfamethoxazole the cut-off values calculated by this analysis were ≥16, ≥23, ≥24 and ≥30 mm, respectively. Evidence is presented demonstrating that the data for these 4 agents was of sufficient quantity and quality that they could be used by the relevant authorities to set internationally harmonised, consensus epidemiological cut-off values for Y. ruckeri.

Novel antimicrobial activity of a dichloromethane extract obtained from red seaweed Ceramium rubrum (Hudson) (Rhodophyta: Florideophyceae) against Yersinia ruckeri and Saprolegnia parasitica, agents that cause diseases in salmonids

Background Enteric red mouth disease and Saprolegniasis, which are caused by the bacteria Yersinia ruckeri and the oomycete Saprolegnia parasitica, respectively, are important illnesses that affect salmonid farming. Sanitary problems in farms are addressed by the prevention of disease outbreaks or by the treatment of diseases with chemicals. Environmental and governmental restrictions, toxicity and high treatment costs limit the use of drugs. Marine organisms, such as algae, sponges and corals, have developed an antimicrobial defense strategy based on the production of bioactive metabolites. Among these organisms, seaweeds offer a particularly rich source of potential new drugs. Hence, many pharmacologically active substances have been isolated from seaweeds. In the Ceramium genus, Ceramium rubrum has been emphasized by several authors for its antimicrobial properties. Based on this background, the present study focused on the antimicrobial activity of a lipophilic extract of C. rubrum on Y. ruckeri and S. parasitica. Results The alga, collected from the Pacific coast of Chile, underwent an ethanol extraction, and the concentrated extract was partitioned between water and dichloromethane. From the dichloromethane extract, fatty acids, fatty acid esters, one hydrocarbon and phytol were identified by Gas Chromatography-Mass Spectrometry (GC/MS) analysis. The antimicrobial study showed that the whole extract was more active than the individual components, which suggests a strong synergistic effect among the components. Conclusions These results may constitute a basis for promising future applied research that could investigate the use of C. rubrum seaweed as a source of antimicrobial compounds against fish pathogens.