| Streptococcus agalactiae（S.agalactiae）,also known as group B streptococcus,exists in nature widely.In recent years,with the rapid and large-scale development of aquaculture,a variety of aquaculture fish have exploded S.agalactiae disease,causing huge economic losses.The tilapia aquaculture suffered the most.Antibiotics are the main and effective approach of treatment infections for tilapia caused by S.agalactiae.But S.agalactiae is a kind of pathogenic bacteria that can parasitize in the brain through the blood-brain barrier,and the blocking of drugs by the blood-brain barrier greatly reduces the efficacy.In order to solve these problems,this paper took tilapia as the target animal,S.agalactiae as the infection strain,enroflo xacin as the experimental drug.In this study,the formulation of enrofloxacin was improved,and brain targeted enrofloxacin was used to control S.agalactiae in tilapia.The results are as follows:1.In order to solve the problem of low solubility and bioavailability,low permeability of blood-brain barrier and multiple large dose administration in aquaculture disease control of enrofloxacin,the formulation of enrofloxacin was improved to enrofloxacin nanoemulsion.The quality,stability and pharmacodynamics of enrofloxacin nanoemulsion againsting diseases of aquatic animals were investigated.The contents of enrofloxacin was determined by Ultra performance liquid chromatography.The particle size,morphology and structure of enrofloxacin nanoemulsion were characterized.Its antibacterial effect of tilapia S.agalactiae was reseached.The results showed that:The content of enrofloxacin nanoemulsion was5.13%.The appearance was clear and transparent,without stratification,demulsification and precipitation after centrifugation.Enrofloxacin nanoemulsion was spherical,distributed evenly under transmission electron microscope.The average part icle size of enrofloxacin nanoemulsion was（18.23±5.02）nm nm,PDI was 0.129.MIC of enrofloxacin nanoemulsion against S.agalactiae was 1 mg/L and MBC was 2 mg/L,which decreased to 1/2 of enrofloxacin.2.To investigate the pharmacokinetics and distribution of enrofloxacin nanoemulsion and its metabolic residue ciprofloxacin in tilapia.Enrofloxacin nanoemulsion was given at the dosages of 10 mg/kg.bw orally to tilapia at water temperature of（30±1）℃.The samples at 0.5、1、2、4、6、8、12、18、24、48、72、96、120、144、168 h were determined using ultra-high performance liquid chromatography after sample preparation.The Tmax and Cmax of enrofloxacin in plasma were 2 h and 4.38 mg/L.The Tmax and Cmax of ciprofloxacin in plasma were 8 h and 0.1mg/L.The Tmax,Cmax and equation of elimination curve of enrofloxacin in gill,liver,brain,muscle and kidney are 0.5 h,13.29 mg/kg,C=4.54e-0.0293t and 1 h,12.37 mg/kg,C=3.96e-0.0232t and 2 h,4.31 mg/kg,C=7.52e-0.0253t and 4 h,6.22 mg/kg,C=6.04e-0.015tand 6 h,7.82 mg/kg,C=2.73e-0.0205t.The Tmax,Cmax and equation of elimination curve of ciprofloxacin in gill,liver,muscle and kidney were 0.5 h,0.4 mg/kg,C=0.21e-0.0155tand 4 h,0.39 mg/kg,C=0.11e-0.0186t and 6 h,0.18 mg/kg,C=0.29e-0.0184tand 6 h,0.23mg/kg,C=0.17e-0.0132t.The result showed that enrofloxacin nanoemulsion was absorbed rapidly in tilapia,and most of them were metabolized with enrofloxacin.Only 3.22%～7.96%enrofloxacin was metabolized into ciprofloxacin.The content of ciprofloxacin was low,but it was more difficult to eliminate in tilapia than enrofloxacin.Oral administration of enrofloxacin nanoemulsion with 10～20 mg/kg.bw can treat most common aquatic diseases.3.In order to reseach the control effect of enrofloxacin nanoemulsion on S.agalactiae in tilapia.The tilapia infection model was induced by intraperitoneal injection of 0.2 m L 1.5×108 cfu/ml S.agalactiaeat water temperature of（30±1）℃.Pathological changes were observed on tilapias such as abdominal hydrops,exophthalmos,corneal opacities,liver hyperemia and swelling,inflammatory bowel that had yellow mucus.Histopathological observation showed that the liver,kidney,brain and intestinal tissue of tilapia were abnormal.Lots of vacuoles in hepatocytes cytoplasm were observed in liver tissue.Pathological calcifications were found in kidney tissue.Intestinal submucosal edema was serious and hyperplasia glial cells were observed in brain tissue.Tilapias were given enrofloxacin nanoemulsion orally at the dose of5mg/kg,10mg/kg and 20 mg/kg.bw daily for 8 d to observe the survival of tilapia.The survival rate of tilapia in each dose group was 26.7%,53.3%,60%respectively within8d,the infection group（20 mg/kg.bw enrofloxacin powder）was 0,and the control group was 53.3%.The results proved that enrofloxacin nanoemulsion had good quality,stable properties and could be prepared simply.It could increase the solubility of enrofloxacin and improve the antibacterial effect of enrofloxacin.Enrofloxacin nanoemulsion had great potential to be developed into a new formulation of antibiotics for aquaculture which can prevent tilapia from S.agalactiae.In summary,the modified enrofloxacin nanoemulsion has high solubility,good stability and strong antibacterial effect.It can quickly reach the peak drug concentration in tilapia and maintain a high drug concentration level for a long time.In addition,enrofloxacin nanoemulsion can also penetrate the blood-brain barrier.It has a good control effect on S.agalactiae in tilapia.This study suggests that enrofloxacin nanoemulsion can be used as a new formulation for the treatment of S.agalactiae infection on tilapia. |
