Relationship between virulence of Streptococcus iniae strains, induction of oxidative activity and cytotoxicity in rainbow trout (Oncorhynchus mykiss) spleen macrophages

In the present study, the effect of the high and low virulence Streptococcus iniae strains on oxidative activity and cytotoxic effects of trout spleen macrophages were studied. Spleen macrophages were collected from trout. Luminol was used as chemiluminigenic probe. Phorbolmyristate acetate (PMA) also was used as positive control activating agent of respiratory burst and significantly stimulated production of reactive oxygen species (ROS) by normal macrophages. Sensitivity of S. iniae strains to H2O2 and cytotoxicity of bacterial strains for trout macrophages was also determined using the antibacterial effect of H2O2 and the release of lactate dehydrogenase from infected macrophages, respectively. Spleen macrophages produced ROS upon stimulation with both bacterial strains. Approximately twofold as much ROS production was induced by the high virulence strain compared to the low virulence strain. The low virulence strain was killed approximately 6 times more by H2 O2 compared to the high virulence strain. In spleen macrophages, the highly virulent strain caused approximately twice as much cytotoxic effects compared to the low virulent strain. Results indicate that, high virulence in S. iniae appears to be correlated with higher macrophage cytotoxicity and resistance to ROS. Moreover, significant differences between the strains in their capacity to stimulate the macrophages were observed (P < 0.05).