Low‐oxygen tensions found in S almonella ‐infected gut tissue boost S almonella replication in macrophages by impairing antimicrobial activity and augmenting S almonella virulence

Summary In Salmonella infection, the S almonella pathogenicity island‐2 ( SPI ‐2)‐encoded type three secretion system ( T3SS 2) is of key importance for systemic disease and survival in host cells. For instance, in the streptomycin‐pretreated mouse model SPI ‐2‐dependent S almonella replication in lamina propria CD 11c − CXCR 1 − monocytic phagocytes/macrophages ( M Φ) is required for the development of colitis. In addition, containment of intracellular S almonella in the gut critically depends on the antimicrobial effects of the phagocyte NADPH oxidase ( PHOX ), and possibly type 2 nitric oxide synthase ( NOS 2). For both antimicrobial enzyme complexes, oxygen is an essential substrate. However, the amount of available oxygen upon enteroinvasive S almonella infection in the gut tissue and its impact on S almonella– M Φ interactions was unknown. Therefore, we measured the gut tissue oxygen levels in a model of S almonella enterocolitis using luminescence two‐dimensional in vivo oxygen imaging. We found that gut tissue oxygen levels dropped from ∼78 T orr (∼11% O 2 ) to values of ∼16 T orr (∼2% O 2 ) during infection. Because in vivo virulence of S almonella depends on the S almonella survival in M Φ, S almonella – M Φ interaction was analysed under such low oxygen values. These experiments revealed an increased intracellular replication and survival of wild‐type and t3ss2 non‐expressing S almonella . These findings were paralleled by blunted nitric oxide and reactive oxygen species ( ROS ) production and reduced S almonella   ROS perception. In addition, hypoxia enhanced SPI ‐2 transcription and translocation of SPI ‐2‐encoded virulence protein. Neither pharmacological blockade of PHOX and NOS 2 nor impairment of T3SS 2 virulence function alone mimicked the effect of hypoxia on S almonella replication under normoxic conditions. However, if t3ss2 non‐expressing S almonella were used, hypoxia did not further enhance S almonella recovery in a PHOX and NOS 2‐deficient situation. Hence, these data suggest that hypoxia‐induced impairment of antimicrobial activity and S almonella virulence cooperate to allow for enhanced S almonella replication in M Φ.