Effect of strain and enviromental conditions on the virulence of Streptococcus agalactiae (Group B Streptococcus; GBS) in red tilapia (Oreochromis sp.)

Abstract

Streptococcus agalactiae, also known as Group B Streptococcus (GBS) is a major threat to the global aquaculture industry, especially to tilapia farming. The susceptibility of fish to GBS is affected by environmental conditions, which are expected to deteriorate in many areas with tilapia production due to human population growth and climate change. The ability to cause natural infection in fish is limited to three clonal complexes (CC) of GBS, of which one, CC283, has recently been recognized as an emerging foodborne pathogen in humans with widespread distribution throughout Southeast Asia. Using an experimental challenge model in tilapia, we compared the virulence of GBS sequence type (ST) 283 with the virulence of GBS from CC552, which is fish-adapted, and GBS of human origin, and explored the impact of environmental conditions (temperature, pH) on mortality caused by different strains. Isolates belonging to the newly emerged ST283 were more virulent to fish after intraperitoneal challenge than an isolate from globally established CC552, whilst human-derived ST651, a member of CC103, was non-pathogenic. Survival of tilapia after challenge was highest at water pH 7.5 and with water temperatures of 18 to 20 °C, suggesting that the severity and geographic range of streptococcosis outbreaks in tilapia may increase as water conditions change due to global warming, environmental pollution and acidification of rivers.