Distribution and localization of Streptococcus agalactiae in different tissues of artificially infected tilapia (Oreochromis niloticus)

China's tilapia industry has the most complete industrial chain in the world, but, in recent years, tilapia streptococcosis have frequently occurred, which has become a bottleneck restricting the development of tilapia industry. Although plenty of studies have been conducted on Streptococcus agalactiae (Group B Streptococcus, GBS) in tilapia, the mechanism of GBS invasion is still unclear. In order to further understand the invasion route, we used the qPCR method to study bacteria load in liver, intestine, gill, spleen, head kidney, kidney, and brain tissues of GIFT tilapia infected by a GBS virulent strain WC1535 at different infection time and the histological method to systematically analyze bacterial distribution and localization in above infected tissue. Through the dynamic analysis of bacteria load, the bacterial copy number of WC1535 reached the peak in each tissue at 24 h post-infection (hpi). Therefore, tissues at 24 hpi were taken as the research object to investigate the distribution and localization of WC1535 by paraffin section and electron microscopy. The results of tissue section showed that WC1535 existed in three forms in blood: free scattering without attachment to any structure, phagocytized by phagocytes (mainly macrophages) and a few red blood cells, and attached to the inner wall of blood vessels. The results of qPCR and tissue section both showed that the density of bacteria in head kidney, spleen and gill was high, at the same time, these tissues had abundant blood and capillaries, so this suggested that the S. agalactiae existed mainly in the blood of tilapia. There were almost no pathogens in the parenchymal cells of each tissue, but the surface of epithelial cells such as vascular endothelial cells and fibroblasts adhered a lot, which indicated that S. agalactiae had epithelial cell affinity. The results of electron microscopy showed that S. agalactiae can survive and proliferate intracellularly after being phagocytosed by macrophages, suggesting that macrophages might play as pathogen carriers or "Trojan horses" to achieve the immune escape, break the blood-brain barrier (BBB), and lead the meningitis in tilapia. Since GBS of tilapia can be internalized by macrophages, antibiotics with strong tissue distribution, cell penetration and good meningeal penetration are recommended for the treatment in the early stage of streptococcosis.