Antibiogram types of Streptococcus agalactiae strains isolated from Oreochromis spp. in Guangxi during 2011-2016

ObjectiveThe study investigated the laws of change of antibiogram types of tilapia Streptococcus agalac-tiae strains in Guangxi so as to offer reference for effective control and scientific medication of S. agalactiae disease in ti-lapia.MethodThe antibiotic resistances of 48 strains of S. agalactiae isolated from diseased tilapia cultured in Nanning, Yulin,Liuzhou,Hechi,Beihai and Baise of Guangxi were tested by K-B disc diffusion method. Clustering of S. agalac-tiae was analysed by SPSS 18.0,using the inhibition zone diameter of antibiotics as a variable. Homogeneous relationship of antibiotics resistance among the strains was indicated by squared Euclidean distance.ResultResults showed that 48 strains of tilapia S. agalactiae formed 19 antibiogram types with an antibiogram abundance of 39.6%. All the antibiogram types were multi-resistant patterns and contained at least three kinds of antibiotics to which the strains were resistant. Ac-cording to the sampling places,isolates from Nanning,Yulin,Liuzhou,Hechi,Beihai and Baise possessed five,nine, eight,two,one and two antibiogram types respectively. The antibiogram types in 2011-2016 were as follows:there were ten kinds of antibiogram types(A,B,F,J,K,N,O,P,Q and S)in 2011,four kinds(B,H,N and S)in 2012,two kinds(L and M)in 2013,three kinds(B,D and S)in 2014,nine kinds(B,C,D,E,G,H,J,R and S)in 2015,and three kinds(B,I and J)in 2016. Cluster analysis showed that 48 strains of tilapia S. agalactiae were clustered into two groups(Group I and Group II)and six subgroups(i-vi). Groups I and II each contained 11 kinds of antibiogram types with possessed antibiogram abundances of 37.9%and 57.9%respectively. The difference of antibiotics sensitivity of S. agalac-tiae between Group I and Group II mainly depended on the rates of sensitivity or resistance of the strains against enrofloxa-cin,florfenicol and ciprofloxacin.ConclusionThe S. agalactiae isolates from tilapia in Guangxi during 2011-2016 dis-play polymorphic antibiogram types with moderately high abundance. The antibiogram types of strains are diverse because of different source places and interannual variation. Besides,the clustering analysis can effectively cluster the S. agalac-tiae strains according to the antibiotics resistances of the isolates. It is recommended that this clustering analysis can be ex-tended and applied as a typing method for epidemiology of S. agalactiae.