Livestock & Aquaculture - Unravelling bile resistance in zoonotic and invasive Salmonella serovars

Salmonella enterica is responsible for an estimated 98 million cases of human non-typhoidal salmonellosis annually, which are commonly acquired via the food chain from farmed animals. Salmonella also causes disease in these animals, thus affecting their welfare and productivity. There are over 2600 serovars of S. enterica, which, depending on their host and tissue of preference, can broadly be grouped into two types: promiscuous serovars that cause gastroenteritis in several hosts (e.g. Typhimurium in humans and livestock) and invasive serovars that cause systemic disease (e.g. Typhi in humans, Dublin in cattle and Choleraesuis in pigs). Currently effective vaccines exist against human typhoid but not against non-typhoidal serovars in pigs and cattle. Understanding how promiscuous and invasive serovars respond to host- and tissue-specific cues in animals can identify targets to develop vaccines to reduce livestock infections and prevent zoonoses. Bile is one such cue encountered by all Salmonella serovars in the intestine but invasive serovars can persist in the gallbladder where bile is concentrated and stored, which may be associated with a chronic asymptomatic carrier state. Bile salts are bactericidal owing to their detergent-like properties but Salmonella has various mechanisms for bile resistance. Interestingly, exposure to bile affects promiscuous and invasive serovars differently; for example, by increasing the ability of S. Typhi to invade host cells and form biofilms but hampering these processes in S. Typhimurium1. The source of bile is also important; for example, S. Typhi specifically makes more curli fimbriae in human bile than in cattle bile2. Salmonella senses environmental cues via gene pairs called two-component systems (TCS), which co-ordinate other genes to adapt to changes in the environment. TCSs like PhoP/PhoQ and SirA/BarA help S. Typhimurium to adapt to bile. S. Dublin has thirty-one TCSs but little is known about their role in bile resistance3.