Temperature strongly affects the health of aquatic poikilotherms. In Nile tilapia
(Oreochromis niloticus), elevated water temperatures increase the severity of strepto-
coccosis. Here we investigated the effects of temperature on the vulnerability and
inflammatory response of Nile tilapia to Streptococcus agalactiae (Group B streptococci;
GBS). At 35 and 28 8C, GBS took 4 and 7 h, respectively to reach the log-phase and, when
incubated with tilapia whole blood, experienced survival rates of 97% and 2%, respectively.
The hemolysis activity of GBS grown at 35 8C was five times higher than that of GBS grown
at 28 8C. GBS expressed cylE (b-hemolysin/cytolysin), cfb (CAMP factor) and PI-2b (pili-
backbone) much more strongly at 35 8C than at 28 8C. Challenging Nile tilapia reared at 35
and 28 8C with GBS resulted in accumulated mortalities of about 85% and 45%,
respectively. At 35 8C, infected tilapia exhibited tremendous inflammatory responses
due to a dramatic up-regulation (30–40-fold) of inflammatory-related genes (cyclooxy-
genase-2, IL-1b and TNF-a) between 6 and 96 h-post infection. These results suggest that
the increase of GBS pathogenicity to Nile tilapia induced by elevated temperature is
associated with massive inflammatory responses, which may lead to acute mortality