Isolation of Highly Persistent Mutants of Salmonella enterica Serovar Typhimurium Reveals a New Toxin-Antitoxin Module

Bacterial persistence is characterized by the ability of a subpopulation within bacterial cultures to survive exposure to antibiotics and other lethal treatments. The surviving persisters are not the result of genetic changes but represent epigenetic variants that are in a physiological state where growth is inhibited. Since characterization of persisters has been performed mainly in Escherichia coli K-12, we sought to identify mechanisms of persistence in the pathogen Salmonella enterica serovar Typhimurium. Isolation of new highly persistent mutants revealed that the shpAB locus (Salmonella high persistence) imparted a 3- to 4-order-of-magnitude increase in survival after ampicillin exposure throughout its growth phase and protected the population against exposure to multiple antibiotics. Genetic characterization revealed that shpAB is a newly discovered toxin-antitoxin (TA) module. The high-persistence phenotype was attributed to a nonsense mutation in the 3' end of the shpB gene encoding an antitoxin protein. Characteristic of other TA modules, shpAB is autoregulated, and high persistence depends on the Lon protease.