Etiological characteristics and analysis of 19 isolates of Streptococcus suis serotype 4 in China, 2018

Abstract:

[Background] In recent years, the clinical isolation rate of Streptococcus suis serotype 4 (SS4) increased gradually, but systematic studies on SS4 were scarce.[Objective] In this research, we aimed to study on etiological characteristics of 19 isolates of SS4 in China, 2018. [Methods] 2 strains of Streptococcus suis serotype 2 (SS2) used as reference, 19 strains of SS4 were researched by cultural characteristics and morphological observation, biochemical test, animal pathogenicity test, virulence gene detection, determination of biofilm formation ability and multilocus sequence typing. [Results] Compared with 2 strains of SS2, 19 strains of SS4 had a larger colony diameter, the same hemolytic characteristics and microscopy morphology and incompletely same reaction to lactose, raffinose, synanthrin and mushroom sugar. All the strains of SS4 could caused meningitis and death in mice, and the LD₅₀ of the 7 most virulent strains of SS4 and 2 strains of SS2 were in the order of 10⁷-10⁸CFU. There were 2 to 6 virulence genes and 5 virulence genotypes in SS4, and epf+mrp+sly+gapdh+fbps+orf2+ was the dominant (36.8%, 7/19). All the strains of SS4 had the ability of biofilm formation, weak (1+) mainly (89.5%, 17/19), and the reticular biofilm structure was observed under scanning electron microscope. There were 3 ST types in SS4, of which ST94 were prevalent (89.5%, 17/19), while ST1158 and ST1224 were new found.[Conclusion] The culture and morphological characteristics of 19 strains of SS4 were uniform, the biochemical characteristics diverse, the virulence genotypes diverse, the biofilm formation ability generally owned, with strong pathogenicity to mice and genetic diversity.