Therefore, including a ΔrecF mutation in a Salmonella vaccine strain is unlikely to affect its immunogenicity. Our results with the S. Typhimurium ΔrecA strain are consistent with two previous, independent studies showing that recA mutations

reduce Salmonella virulence [51, 52]. To evaluate the potential effect of ΔrecA mutation on immunogenicity, mice inoculated with the recA mutant were challenged with a lethal dose of virulent wild-type Ro 61-8048 cell line S. Typhimurium. All the challenged mice survived, indicating that a ΔrecA mutant retains immunogenicity and therefore may be suitable for use in a vaccine. However, since it does not affect virulence, inclusion of a ΔrecF mutation into a Salmonella vector that has been attenuated by other means to reduce the frequency of intra- and interplasmid recombination, may be more desirable than a ΔrecA mutation. Studies are currently underway to investigate these possibilities. Our data show that ΔrecA and ΔrecF mutations resulted in reduced frequencies of intraplasmid recombination in all Salmonella strains tested, which included three serovars, when there was an intervening sequence between the direct duplications (Table 3). Our results also show that it is likely that deletions in recA, recF or recJ will not be useful for reducing interplasmid recombination in S. Typhi vaccine strains, since we did not observe

any reduction in interplasmid recombination frequency. This result was disappointing, since the majority of human trials with live Salmonella vaccines have focused on S. Typhi. In the case of S. Typhi, it appears that the best approach to preventing interplasmid selleck products recombination will be in the careful design of each plasmid, avoiding any stretches of homology. However, for vaccines based on S. Typhimurium or S. Paratyphi A, introduction of a ΔrecF mutation into attenuated Rolziracetam Salmonella vaccine strains carrying multiple plasmids is a useful approach to reduce unwanted plasmid/plasmid or plasmid/chromosome recombination without further attenuating the strain or negatively influencing its immunogenicity. The ΔrecA mutation had a similar or more pronounced effect on reducing various classes of recombination

and it clearly had an effect on virulence. We did not examine the effect of a ΔrecA mutation on the immunogenicity of a vectored antigen. Based on its effect on virulence, it may affect the immunogenicity of the vectored antigen in some attenuation backgrounds and therefore may not be applicable for all attenuation strategies. Conclusions In this study we showed that ΔrecA and ΔrecF mutations reduce intraplasmid recombination in S. Typhimurium, S. Typhi and S. Paratyphi while there is an intervening sequence between the duplicated sequences. The ΔrecA and ΔrecF mutations reduce interplasmid recombination in S. Typhimurium and S. Paratyphi but not in S. Typhi. The ΔrecF mutations also sharply reduce intraplasmid recombination between direct duplications in S. Typhi.