Intracellular sensors of extracellular bacteria

Immunol Rev. 2011 Sep;243(1):9-25. doi: 10.1111/j.1600-065X.2011.01039.x.


Initial recognition of bacteria by the innate immune system is thought to occur primarily by germline-encoded pattern recognition receptors (PRRs). These receptors are present in multiple compartments of host cells and are thus capable of surveying both the intracellular and extracellular milieu for bacteria. It has generally been presumed that the cellular location of these receptors dictates what type of bacteria they respond to: extracellular bacteria being recognized by cell surface receptors, such as certain Toll-like receptors, and bacteria that are capable of breaching the plasma membrane and entering the cytoplasm, being sensed by cytoplasmic receptors, including the Nod-like receptors (NLRs). Increasingly, it is becoming apparent that this is a false dichotomy and that extracellular bacteria can be sensed by cytoplasmic PRRs and this is crucial for controlling the levels of these bacteria. In this review, we discuss the role of two NLRs, Nod1 and Nod2, in the recognition of and response to extracellular bacteria.

Publication types

  • Review

MeSH terms

  • Animals
  • Bacterial Infections / immunology*
  • Extracellular Space* / immunology
  • Extracellular Space* / microbiology
  • Host-Pathogen Interactions
  • Humans
  • Immunity, Innate
  • Intracellular Space* / immunology
  • Intracellular Space* / microbiology
  • Nod1 Signaling Adaptor Protein / immunology*
  • Nod1 Signaling Adaptor Protein / metabolism
  • Nod2 Signaling Adaptor Protein / immunology*
  • Nod2 Signaling Adaptor Protein / metabolism
  • Signal Transduction / immunology


  • NOD1 protein, human
  • NOD2 protein, human
  • Nod1 Signaling Adaptor Protein
  • Nod2 Signaling Adaptor Protein