Bacteria have antibiotic-defying tricks besides genetically determined resistance. They commonly resort to general defense mechanisms such as persistence, the dialing down of metabolism, growth, and proliferation. Although persistence has been known to help bacterial subpopulations outlast treatment, the phenomenon has yet to motivate the development of any specific countermeasures. One possible exception is the development of small molecule drugs that prevent bacteria from generating hydrogen sulfide.

Hydrogen sulfide, it happens, is critical to the innate ability of bacteria to survive normally lethal levels of antibiotics. Essentially, hydrogen sulfide protects bacteria against the toxic effects of oxidative stress. To deprive bacteria of hydrogen sulfide, scientists based at New York University (NYU) targeted the bacterial enzyme called cystathionine γ-lyase (CSE). Inhibiting this enzyme, the scientists reported, “suppresses bacterial tolerance, disrupting biofilm formation and substantially reducing the number of persister bacteria that survive antibiotic treatment.”

These findings appeared June 10 in Science, in an article titled, “Inhibitors of bacterial H2S biogenesis targeting antibiotic resistance and tolerance.”

“We identified CSE as the primary generator of H2S in two major human pathogens, Staphylococcus aureus and Pseudomonas aeruginosa, and discovered small molecules that inhibit bacterial CSE,” the article’s authors wrote. “These inhibitors potentiate bactericidal antibiotics against both pathogens in vitro and in mouse models of infection.”

To find the inhibitors, the researchers obtained an X-ray structure of S. aureus CSE and used it to “virtually screen” millions of drug-like compounds and identify those with the right shape and properties to block the enzyme’s action without side effects. The researchers selected lead compounds that inhibited the bacterial CSE, blocked H2S production by both S. aureus and P. aeruginosa, and strengthened the effect of bactericidal antibiotics from different classes. One of the lead compounds increased the potency of antibiotic effect in mouse models of S. aureus and P. aeruginosa infection.

Unexpectedly, further testing revealed that the compounds markedly diminished persisters and suppressed biofilm formation in both pathogens. How exactly H2S contributes to tolerance remains to be established, but there are some hints.

“Bacteria appear to use controlled, self-poisoning with H2S to slow down their metabolism, preventing the antibiotics from using the bacteria’s energy production system to kill them,” said Evgeny Nudler, PhD, the Julie Wilson Anderson professor of biochemistry at NYU Langone Health and a corresponding author of the current study. “Interfering with the H2S-based defenses represents a largely unexplored alternative to the traditional antibiotic discovery. Our results suggest that a new kind of small molecule potentiator can strengthen the effect of major classes of clinically important antibiotics.”

The authors noted several opportunities for designing conceptually novel antimicrobial therapeutics by combining H2S-blocking potentiators with antibiotics. Such combinations may have better efficacy against bacterial biofilms. Other potential applications include overcoming intermediate-level antibiotic resistance; reducing antibiotic dose and related toxicity while maintaining efficacy; and enhancing the bacteria-killing (bactericidal) effect at the same antibiotic dose.

“The combined trends toward resistant infections and fewer new antimicrobials are projected to kill 10 million people annually by the year 2050,” Nudler pointed out. “New approaches are urgently needed to prevent this, and our study suggests that suppressing bacterial H2S would make different antibiotics more potent.”