Abstract
Aeromonas hydrophila is a well-known bacterial pathogen associated with mass mortalities in aquaculture. Yet, few reports are available on whiteleg shrimp-pathogenic A. hydrophila. In the present study, a virulent isolate WS05 was confirmed as a causative agent of diseased freshwater-cultured whiteleg shrimp and showed a mean lethal dose (LD50) value of 4.8 × 104 CFU mL−1. It was identified phenotypically and molecularly as an A. hydrophila strain, and exhibited susceptibility to several veterinary antibiotics extensively used in aquaculture, including cotrimoxazole, doxycycline, florfenicol, neomycin, and tetracycline. In view of the strongest inhibition zone of florfenicol against isolate WS05, the synergistic effect of the combinations of florfenicol and herb extracts was further evaluated, and the result indicated that Punica granatum extract was a potential synergist of florfenicol against isolate WS05 and the fractional inhibitory concentration index (FICI) for the florfenicol-P. granatum extract was calculated as 0.31. When combined with 7.81 mg mL−1 P. granatum extract, the minimum inhibitory concentration (MIC) of florfenicol against isolate WS05 was reduced from 0.50 to 0.03 mg L−1, and its activity against isolate WS05 was also enhanced with a significant reduction of ≥3.61 log in cell density after 24 h of treatment compared with that in the single drug treatment. In addition, the protective effect was potentiated by the combination of florfenicol and P. granatum extract, with a cumulative mortality of 36.66% (p < 0.05) and 33.33% (p < 0.05) lower than that in the single treatment with florfenicol and P. granatum extract after the challenge with isolate WS05 for seven days. As far as we know, this is the first study to describe whiteleg shrimp-pathogenic A. hydrophila and suggest P. granatum extract as a potential synergist of florfenicol against the A. hydrophila pathogen.
Highlights
The whiteleg shrimp Litopenaeus vannamei is one of the most important commercial shrimp species and is extensively cultivated in Central and South America, the United States of America (USA), East and South-East Asia, the Middle East, and Africa [1], which accounts for 75% of the global shrimp products [2]
This industry has been seriously affected by bacterial diseases [5], which are caused by several bacterial pathogens, such as Vibrio parahaemolyticus, Vibrio harveyi, Vibrio cholerae, Proteus penneri, Aeromonas schubertii, and Shewanella algae [6,7,8,9,10,11]
In order to limit the use of antibiotics, the use of herb extracts in combination with conventional antibiotics is commonly suggested for the treatment of bacterial pathogens in aquaculture [22], which could significantly reduce the dosage of antibiotics against bacterial pathogens [23]
Summary
The whiteleg shrimp Litopenaeus vannamei is one of the most important commercial shrimp species and is extensively cultivated in Central and South America, the United States of America (USA), East and South-East Asia, the Middle East, and Africa [1], which accounts for 75% of the global shrimp products [2]. In China, with the rapid development of farming techniques, the whiteleg shrimp has been successfully cultured in freshwater since 2000 [3], with an annual freshwater aquaculture production of over 591,000 tons in 2017 [4] This industry has been seriously affected by bacterial diseases [5], which are caused by several bacterial pathogens, such as Vibrio parahaemolyticus, Vibrio harveyi, Vibrio cholerae, Proteus penneri, Aeromonas schubertii, and Shewanella algae [6,7,8,9,10,11]. In view of the strongest inhibition zone of florfenicol against the isolate, the control of the isolate with combinations of florfenicol and herb extracts was further evaluated To our knowledge, this is the first report of A. hydrophila as a bacterial pathogen of freshwater-cultured whiteleg shrimp, and the findings of this study can be used as a reference for disease control and health management in shrimp aquaculture
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