DNA base excision repair potentiates the protective effect of Salmonella Pathogenicity Island 2 within macrophages

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Abstract
Reactive oxidants are a primary weapon of the macrophage antibacterial arsenal. The ability of virulentSalmonellato repair oxidative DNA lesions via the base-excision repair system (BER) enables its survival and replication within the macrophage, but is not required for extracellular growth.Salmonellaalso inhibits the targeting of oxidant generators to theSalmonella-containing vacuole (SCV) viaSalmonellaPathogenicity Island 2 (SPI2). Accordingly, the relative contributions of these two discrete systems toSalmonellaresistance to both oxidative mutagenesis and lethality within RAW 264.7 macrophages were investigated. A mutant unable to initiate BER was constructed by deleting all three BER bifunctional glycosylases (Δfpg/nth/nei), and was significantly impaired for early intramacrophage survival. Mutations in various SPI2 effector (sifAandsseEFG) and structural (ssaV) genes were then analysed in the BER mutant background. Loss of SPI2 function alone appeared to increase macrophage-induced mutation. Statistical analyses of the reduced intramacrophage survival of SPI2 mutants and the corresponding SPI2/BER mutants indicated a synergistic interaction between BER and SPI2, suggesting that SPI2 promotes intramacrophage survival by protectingSalmonellaDNA from exposure to macrophage oxidants. Furthermore, this protection may involve the SseF and SseG effectors. In contrast, the SifA effector did not seem to play a major role in oxidant protection. It is speculated thatSalmonellainitially stalls oxidative killing by preserving its genomic integrity through the function of BER, until it can upregulate SPI2 to limit its exposure to macrophage oxidants.

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