Why Does Enterobacteriaceae Protect Our Immune Response?

Enterobacteriaceae are gram-negative bacteria that are part of our endogenous microbial population. In normal circumstances our natural flora keeps a balance between the Enterobacteriaceae and the immune response, however, when these natural flora are destroyed infections might occur since there is no balance anymore.
Immunology
Our immune response can target Enterobacteriaceae in several ways. Since Enterobacteriaceae are Gram-negative bacteria, they have LPS in their outer membrane. LPS can activate the innate immune response. Through this activation the complement system (the alternate to be precise) will be activated as well as cytokine secretion. Endotoxins like LPS cause the major systemic manifestations in inflammation so therefore they are important targets for the immune system. Another target might be the toxins secreted by some of the Enterobacteriaceae. Fig.1 Neutralization of antigens by antibodies Our immune system can normally neutralize these antibodies by neutralization.
First of all, they can regulate their antigen-expression, thereby being able to evade being killed via antibodies. Secondly they have a capsule which protects them against phagocytes. The capsule does this by repelling the hydrophobic phagocyte membrane since the capsule antigens are hydrophilic. Third come the type III secretion systems. Through transfer of bacterial effector proteins into the target cell, by means of a needle like projection, they can stimulate host cells to take up the bacterium. In addition, new effector proteins are secreted once the bacterium is in the host, which will induce apoptosis of this host cell. So in this way, the bacteria can evade phagocytosis by killing the phagocyte from the inside. Fig. 2 Type III secretion