Images of Salmonella Dublin (green) growing in human epithelial cells. Image credit: Sia et al. (CC BY 4.0)
Salmonella bacteria often live harmlessly in animals but can make people sick. For example, milk or meat from cows infected with a type called Salmonella Dublin can cause serious blood infections. Over time, many Salmonella strains have become harder to treat because they carry genes that let them survive antibiotic treatment. These survival genes often sit on small DNA circles called plasmids, which can move between bacteria.
By studying the DNA from Salmonella Dublin samples from around the world, scientists can learn how different groups of this bacterium have evolved to resist drugs and cause infections in humans. Understanding these changes may help explain why some types of Salmonella Dublin spread more easily to humans and may point to better ways to detect and control infections.
Sia et al. identified two main lineages of Salmonella Dublin: the widespread ST10 group and a less common ST74 group. The experiments compared DNA from more than 1300 strains of Salmonella Dublin from 13 countries on five continents. The ST10 strains from North America often had genes that made them resistant to antibiotics and heavy metals. Resistance to heavy metals may make it easier for bacteria to develop resistance to drugs. Most Australian strains of ST10 remained drug sensitive. ST74 strains lack a key secretion system gene set but have more genes overall than ST10. In laboratory tests using human immune cells, ST74 bacteria multiplied up to eight times more over 24 hours and triggered weaker immune responses in the cells than ST10. Both groups of bacteria lacked the Vi capsule gene, which some vaccines against bacterial infections have targeted.
The experiments may also help explain why some strains of the bacteria cause more disease and point to new ways to prevent or treat infections. Knowing that ST74 does not trigger a strong immune response may help scientists develop new therapies to strengthen the immune response and prevent the bacteria from multiplying and spreading. Vaccines targeting the Vi capsule may not be effective against Salmonella Dublin, because most strains do not have it. Instead, scientists may want to use the data to look for alternate targets. More research is needed to support the study's findings.
