A novel virulence factor was identified in Salmonella enterica serovar Typhimurium.This factor was determined to be a protein named SseL.SseL is translocated from the bacteria into the infected cell by a type III secretion system.SseL has de-ubiquitylating activity in vitro and a infection of cells with a SseL mutant leads to an accumulation of ubiquitylated proteins.This mutant strain also fails to cause macrophage cell death which has been implicated in the spread and systemic infection of Salmonella.Other serovars, including Typhi, Paratyphi A and Choleraesuis contain sequences encoding a protein nearly identical to SseL suggesting that de-ubiquitylating enzymes play an important role in the pathogenesis of other Salmonella serovars.