Scientists develop reporter strain to detect inhibition of bacterial cell-wall biosynthesis

A new bacterial “alarm system” could make it dramatically easier to discover antibiotics that attack one of microbes’ most vulnerable structures: the cell wall. Researchers in Germany have engineered reporter strains of Bacillus subtilis that glow when drugs interfere with cell-wall construction, creating a rapid way to identify promising compounds even when their precise molecular targets are unknown.

The approach exploits WalRK, a two-component regulatory system that acts as a surveillance network for the cell envelope. Two-component systems typically consist of a sensor protein embedded in or associated with the cell membrane and a response regulator that changes gene expression. In the WalRK system, the sensor kinase WalK detects disturbances linked to cell-wall biosynthesis and activates the regulator WalR. Because WalRK is essential in many Gram-positive bacteria, including Bacillus subtilis and the pathogen Staphylococcus aureus, disrupting its function can ultimately cause lethal damage to the cell envelope.

The researchers designed reporter bacteria by placing luciferase genes under the control of two WalRK-dependent promoters, known as PiseA and PssaA. A promoter is a stretch of DNA that determines when a gene is transcribed. When WalRK activity changes, these promoters alter the production of luciferase, the enzyme responsible for the light generated by fireflies and several marine organisms. Measuring luminescence therefore provides an indirect, continuous readout of the cell’s response to stress. Instead of waiting for bacterial colonies to stop growing or die, scientists can monitor the molecular alarm as it is activated.

The PiseA reporter proved particularly broad in its response. It reacted to nearly all of the established cell-wall biosynthesis inhibitors tested by the team, while showing no discernible response to antibiotics that act through unrelated mechanisms. The panel included β-lactams, which block penicillin-binding proteins involved in building the cell wall; glycopeptides such as vancomycin, which bind cell-wall precursors; tunicamycin, which interferes with the production of lipid-linked building blocks; daptomycin, which disrupts the bacterial membrane and envelope; and fosfomycin, which inhibits an early step in peptidoglycan synthesis.

Peptidoglycan is the tough mesh-like polymer that surrounds most bacteria and prevents them from bursting under internal osmotic pressure. Its construction requires a carefully coordinated sequence of enzymatic reactions. Precursors are assembled inside the cell, attached to lipid carriers, transported across the membrane and joined into a cross-linked external matrix. Antibiotics can block different stages of this pathway, but the resulting cellular stress may vary depending on the precise lesion. A reporter that records the downstream regulatory response could therefore identify cell-wall damage without requiring researchers to know which enzymatic step a compound targets.

To broaden the detection system, the team combined the new WalRK reporters with an established reporter controlled by the LiaRS two-component system. LiaRS also responds to envelope stress, but it senses and regulates a partially different set of disturbances. The comparison revealed that WalRK and LiaRS do not function as interchangeable alarms. Rather, they respond differently to specific forms of cell-envelope injury, including stress caused by antibiotics that target lipid II, a membrane-associated precursor essential for peptidoglycan assembly. Using both reporters produced a complementary pattern of signals that expanded the range of potential cell-wall inhibitors detectable in a single screening workflow.

The scientists also investigated PrkC, an unusual eukaryotic-like serine/threonine kinase found in Bacillus subtilis. Unlike the more familiar histidine kinases that control many bacterial two-component systems, PrkC phosphorylates proteins on serine or threonine residues and participates in processes associated with growth, cell-wall sensing and developmental transitions. Removing the prkC gene increased the PiseA luminescence produced in response to certain antibiotics and after longer incubation periods. At the same time, the deletion reduced P liaI expression in the presence of drugs that target lipid II, emphasizing that bacterial stress networks can amplify one signal while dampening another.

The practical test came with natural products whose modes of action were not known. Such compounds are a valuable source of antibiotic candidates, but they can be difficult to classify: a decrease in bacterial growth alone does not reveal whether a molecule damages DNA, blocks protein synthesis, collapses the membrane or interferes with the cell wall. Several of the natural products generated reporter patterns resembling those produced by vancomycin. That similarity does not by itself prove that the compounds bind the same target, but it marks them as candidates for further investigation as possible cell-wall inhibitors. Follow-up experiments would be needed to determine their precise targets, toxicity and ability to overcome resistant bacteria.

The new system could help address a persistent bottleneck in antibiotic discovery. High-throughput screens can test thousands of natural products or synthetic molecules, but identifying how an active compound works often requires laborious biochemical and genetic experiments. Reporter strains provide an early triage step: luminescence patterns can indicate which cellular process is under attack and distinguish likely cell-wall activity from effects associated with other antibiotic classes. Because the signal can be followed in living bacteria over time, the method may also reveal differences in the speed and intensity of envelope stress responses that endpoint assays miss.

The researchers describe the PiseA and PssaA strains as tools for continuous in vivo observation rather than direct measurements of WalRK itself. The light output reflects the combined effects of promoter regulation, gene expression, luciferase production and cellular physiology, so it must be interpreted alongside controls and complementary assays. Even so, pairing WalRK-dependent reporters with LiaRS-based sensing offers a richer map of bacterial envelope stress than either system alone. The authors say this combined strategy provides a rapid and powerful route for discovering antibiotics aimed at cell-wall biosynthesis, a target class that remains clinically important as resistance erodes the effectiveness of existing drugs.