Detection of Staphylococcus succinus strains isolated from mice of natural death from Sonid Left Banner

  Objective  To explore the cause of the mass death of Meriones unguiculatus from Sunite Left Banner in 2019 through the laboratory testing.

  Methods  After all mice of natural death were dissected, their organs were inoculated on Hirsch medium and cysteine heart agar media enriched with blood（CHAB）. Being considered not to be Yersinia pestis, the isolate was inoculated on CHAB supplemented with antibiotics, and tested by Francisella tularensis-specific antigen latex agglutination. Secondly, for all isolates DNA was extracted, and then amplified using universal primers of 16S rRNA gene. The PCR products amplified with target fragment were sequenced, and then their nucleotide sequences obtained were aligned and performed phylogenetic analysis with published 16S rRNA gene sequences of Staphylococcus succinus online. In addition, the experimental mice were infected with the S. succinus suspension, and their mental and physical conditions were observed within 14 days.

  Results  All isolated strains did not grow on the selective medium for F. tularensis and were negative for F. tularensis-specific antigens in latex agglutination, so they were not F. tularensis. After sequencing and alignment of 16S rRNA gene, all isolates were identified as 4 strains of S. succinus, 1 strain of Lactobacillus reuteri, L. vaginalis and Enterococcus faecalis. The phylogenetic analysis indicated that 3 strains of S. succinus obtained in this study were clustered with 9 from the rest of the world on the clade of the major lineage. The rest strain of S. succinus was assigned to a clade with that from Xinjiang. Through the experimental mouse infected model it was found that the minimum lethal dose of S. succinus is 5×10⁸ CFU/mL, and the sublethal dose is 10⁸ CFU/mL.

  Conclusion  For the first time, the strains of S. succinus were isolated from mice of natural death from Sonid Left Banner in Inner Mongolia Autonomous Region. It is speculated that infection of S. succinis may be the cause of mass death of local Meriones Unguiculatus.