Unlike viruses, bacteria are not as susceptible to silencing by siRNA. This is largely due to how bacteria replicate. Bacteria replicate outside of the host cell and do not contain the necessary machinery for RNAi to function. However, bacterial infections can still be suppressed by siRNA by targeting the host genes that are involved in the immune response caused by the infection or by targeting the host genes involved in mediating the entry of bacteria into cells. For instance, siRNA was used to reduce the amount of pro-inflammatory cytokines expressed in the cells of mice treated with lipopolysaccharide (LPS). The reduced expression of the inflammatory cytokine, tumor necrosis factor α (TNFα), in turn, caused a reduction in the septic shock felt by the LPS-treated mice. In addition, siRNA was used to prevent the bacteria, Psueomonas aeruginosa, from invading murine lung epithelial cells by knocking down the caveolin-2 (CAV2) gene. Thus, though bacteria cannot be directly targeted by siRNA mechanisms, they can still be affected by siRNA when the components involved in the bacterial infection are targeted.