Innate immune evasion
Cells of the innate immune system play a crucial role in preventing the unchecked growth of bacteria within the body. However, many pathogens have evolved sophisticated mechanisms to evade or subvert these defences, allowing them to persist and cause disease.
Common evasion strategies include intracellular replication, as seen in Mycobacterium tuberculosis, which hides within host cells to avoid detection. Some bacteria, such as Salmonella, produce a protective capsule that shields them from complement-mediated lysis and phagocytosis.
Certain normally mutualistic bacteria, like species of Bacteroides a major component of the mammalian gut microbiota can act as opportunistic pathogens under specific conditions. For example, Bacteroides fragilis can cause peritoneal infections and evades immune detection by inhibiting phagocytosis, either by interfering with phagocytic receptor signalling or by mimicking host cell surfaces, thereby avoiding recognition as foreign.
Staphylococcus aureus employs yet another tactic by inhibiting the phagocyte’s response to chemokine signals, reducing its ability to reach and eliminate pathogens. Other organisms, including M. tuberculosis, Streptococcus pyogenes, and Bacillus anthracis, have evolved mechanisms to directly destroy phagocytes, further disabling the innate immune response.