In the conflict between the immune system and disease-causing microbes, bacterial death can happen in many ways; the mechanism by which it occurs is important because only a subset of the immune responses responsible promotes survival of the infected individual. What mechanisms has innate immunity evolved to use?

Multicellular organisms have developed sets of proteins to both recognize and kill invading or competing microbes. One ancient family of innate immune proteins that is conserved from flies to humans recognizes peptidoglycan, a ubiquitous polymer found in nearly all bacterial cell walls; it is required for the structural integrity of bacteria as well as their growth and survival. In humans, the family of four PGRPs causes bacterial death and/or the cessation of bacterial growth via an unknown mechanism. In flies, PGRPs are themselves signaling receptors, or they can function via Toll receptors to induce an antimicrobial immune response, but a similar role in humans has never been described1. Structurally, human PGRPs bind to peptidoglycan in cell walls and to the outer membrane molecules of Gram-negative bacteria (which have a much thinner cell wall than Gram-positive bacteria) through their conserved peptidoglycan-binding domains; however, the functions of the PGRP sequences outside of these domains is unclear.

Classically, it is thought that bactericidal antibiotics kill by interfering with major bacterial metabolic pathways such as cell wall assembly or protein synthesis. Recently, it has been proposed that the downstream effects of these diverse classes of antibiotics converge on a conserved signaling pathway resulting in cell toxicity owing to the release of hydroxyl radicals2,3. In other words, despite a large variety in antibiotic targets, at the end of the day, they all trigger the same final weapon.

In this issue of Nature Medicine, Kayshap et al.4 show that PGRPs kill bacteria by triggering the same final common bacterial death pathway as many antibiotics. In Gram-positive bacteria, PGRPs bind to the cell wall near the site of daughter cell separation, and in Gram-negative bacteria they bind to the outer cell membrane. They then activate a conserved two-component signaling system that is part of the bacterial stress response, which triggers death in a series of discrete steps (Fig. 1): first, all major biosynthetic processes such as cell wall, protein, and DNA and RNA synthesis cease; second, the bacterial membrane depolarizes, leading to a loss of the proton motive force (which is important in energy generation and transport); and third, hydroxyl radicals build up intracellularly, eventually poisoning the cell. This mechanism of bacterial killing by PGRPs could potentially be exploited to develop new antimicrobials with fewer inflammatory side effects than the antibiotics currently used in the clinic.

Figure 1: A common mechanism of bacterial death.
figure 1

MaryLou Quillen

Kashyap et al.4 show that multiple bactericidal effectors kill bacteria by triggering a stress-responsive two-component sensory transduction system (CssR-CssS in Gram-positive and CpxA-CpxR in Gram-negative bacteria). This signaling leads to a sequence consisting of arrest of macromolecular synthesis, membrane depolarization and the accumulation of toxic hydroxyl radicals. Solid lines indicate effects that are thought to be direct, whereas dashed lines show those that are likely to be downstream or indirect.

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Notably, PGRP-induced death was greatly reduced in both Gram-positive Bacillus subtilis and Gram-negative Escherichia coli when the homologous stress-control two-protein systems consisting of CssR-CssS or CpxA-CpxR, respectively, were deleted4. This result is similar to that found for antibiotic treatment of E. coli, in which antibiotic-induced death was preceded by membrane depolarization and hydroxyl radical accumulation and required the CpxA-CpxR system2,3. Taken together, these studies indicate that both antibiotics and PGRPs kill bacteria by inducing a response through stress sensors; the end result of this stress signaling is that bacteria succumb to a lack of metabolic control in which oxidative damage irreparably harms the cell.

The findings of Kashyap et al.4 lead to several tempting speculations regarding the development of antimicrobials. Antibiotics that target the bacterial cell wall such as penicillin (or other β-lactams) are bacteriolytic and have long been both effective and problematic in controlling life-threatening infections. β-lactam treatment is associated with substantial inflammation in infected tissues as a result of the liberation of highly inflammatory cell wall fragments. In some cases, this is as damaging to tissues as infection with a pathogen5. A nonlytic β-lactam would therefore be highly desirable. Although the target is the same as for β-lactam antibiotics, binding of PGRPs to the cell wall does not lead to the breakdown and release of cell wall fragments before bacterial death6. The identification of the stress-response pathways triggered by PGRPs may make it possible to manipulate these pathways to induce bacterial death while greatly reducing the release of damaging bacterial cell wall fragments.

Several key areas in the PGRP-induced bacterial death pathway must be better understood before this pathway can truly be considered a prime target for drug development. Many of the key aspects of cell death by antibiotics and PGRPs are not fully explained by the current model of CpxA-CpxR and CssR-CssS signaling. Primarily, activation of CpxA-CpxR or CssR-CssS in response to their physiological signals (secretion stress) does not lead to a massive downregulation of the cell wall synthetic machinery, nor does a constitutively active stress response owing to mutated CpxA alleles lead to cell death in E. coli7. This suggests that if antibiotics or PGRP directly activate the CpxA-CpxR and CssR-CssS systems, this activation may occur in a different manner than that elicited by secretion stress. It is also possible that CpxA-CpxR or CssR-CssS are necessary but not sufficient for induction of the death signal. This suggests the involvement of additional unidentified pathways that, when altered in combination with secretion stress–response activation, lead to triggering of the cell death pathway. Finally, it has been shown that certain PGRPs can form complexes with the eukaryotic chaperone HSP70, thereby leading to a strong cytotoxic effect on host cells8 through largely unknown mechanisms; accordingly, this might require the development of therapeutic PGRP constructs that include only the nontoxic portions of the protein.