Bile-Induced DNA Damage in Salmonella enterica

Abstract

In the absence of DNA adenine methylase, growth of Salmonella enterica serovar Typhimurium is inhibited by bile. Mutations in any of the mut H, mutL, and mutS genes suppress bile sensitivity in a Dam Ϫ background, indicating that an active MutHLS system renders Dam Ϫ mutants bile sensitive. However, inactivation of the MutHLS system does not cause bile sensitivity. An analogy with Escherichia coli, in which the MutHLS system sensitizes Dam Ϫ mutants to DNA-injuring agents, suggested that bile might cause DNA damage. In support of this hypothesis, we show that bile induces the SOS response in S. enterica and increases the frequency of point mutations and chromosomal rearrangements. Mutations in mut H, mut L, or mutS cause partial relief of virulence attenuation in a Dam Ϫ background (50-to 100-fold by the oral route and 10-fold intraperitoneally), suggesting that an active MutHLS system reduces the ability of Salmonella Dam Ϫ mutants to cope with DNA-damaging agents (bile and others) encountered during the infection process. The DNA-damaging ability of bile under laboratory conditions raises the possibility that the phenomenon may be relevant in vivo, since high bile concentrations are found in the gallbladder, the niche for chronic Salmonella infections.

Repair of DNA Damage Induced by Bile Salts in Salmonella enterica

“…DNA-damaging activity of bile salts: A previous study showed that the DNA-damaging ability of bile could be reproduced with an individual bile component, sodium deoxycholate (Prieto et al 2004). To ascertain whether other bile salts were able to cause DNA damage, we tested their DNA-damaging capacity.…”

“…To ascertain whether other bile salts were able to cause DNA damage, we tested their DNA-damaging capacity. The occurrence of DNA damage was examined by monitoring the activity of an SOS-inducible fusion (ceaTlacZ) carried on plasmid pGE108, as previously described (Prieto et al 2004). Exposure to any of five individual bile salts (sodium cholate, sodium chenodeoxycholate, sodium gycocholate, sodium taurocholate, and sodium glycochenodeoxycholate) turned on the expression of the ceaTlacZ fusion in a RecA 1 background ( Figure 1).…”

“…Plate tests for monitoring b-galactosidase activity were carried out as described elsewhere (Prieto et al 2004), using 5-bromo-4-chloro-3-indolyl-b-dgalactopyranoside (''X-gal,'' Sigma Chemical) as indicator. To monitor the b-galactosidase activities of dpsTlac, katGTlac, nfoTlac, and fumCTlac fusions in response to sodium deoxycholate, exponential cultures in LB were washed with PBS buffer and resuspended in PBS containing 5% sodium deoxycholate.…”

“…An investigation of the causes of bile sensitivity in DNA adenine methylase (Dam À ) mutants of S. enterica serovar Typhimurium indicated that bile causes damage to Salmonella DNA and increases the rates of gene rearrangements and point mutations (Prieto et al 2004). Bile may have similar effects on E. coli DNA, as indicated by the observation that exposure to bile salts induces genes of the SOS network (H. .…”

The SOS response of Listeria monocytogenes is involved in stress resistance and mutagenesis

“…Furthermore, two of the SOSresponse genes are part of a bile-resistance system (lmo1421-lmo1422). Since bile exposure may result in DNA damage (Prieto et al, 2004), activation of this system as part of the SOS response may provide additional protection of cellular DNA. Lastly, the first and the last gene of the comK integrated bacteriophage A118 (lmo2271 and lmo2332) are LexA controlled.…”