Fis Is Essential for the Stability of Linear Plasmid pBSSB1 and Affects the Motility of Salmonella enterica Serovar Typhi

pBSSB1 is a 27 kb non-bacteriophage-related linear plasmid first found in Salmonella enterica serovar Typhi (S. Typhi), but the mechanism underlying the replication of pBSSB1 is currently unknown. Previous reports showed that the factor for inversion stimulation (Fis) encoded by fis can affect the replication, transcription and other processes through binding DNA. Here, a fis deletion mutant of S. Typhi (Δfis) was prepared through the homologous recombination mediated by suicide plasmid and the loss of pBSSB1 in Δfis was observed surprisingly by pulsed field gel electrophoresis (PFGE). Subsequently, the loss of pBSSB1 was verified by PCR and Southern blot. In addition, the motility of Δfis was deficient and the flagellin of Δfis could not be detected by 2-dimensional polyacrylamide gel electrophoresis. All these results show that Fis is essential for the stability of pBSSB1 and affects the motility of S. Typhi.