A joint research project based at Kumamoto University, Japan, has developed a new, highly sensitive analytical method that can detect degraded β-lactam antibacterial agents used in the treatment of bacterial infections. With this method, the researchers found that reactive sulfur species produced by bacteria degrade and inactivate β-lactam antibiotics.
Bacteria are different from animal cells in that their outer layer is covered with a rigid structure called the cell wall. Antimicrobial agents -lactams interfere with the processes that form the cell wall. As a result, the bacteria can no longer withstand their own internal pressure, they rupture and die. Β-lactam antimicrobial agents are very potent because they selectively inhibit bacterial cell wall synthesis and have few side effects on hosts such as humans. These antimicrobial agents have a common structure called the β-lactam ring which is essential for inhibiting the development of the cell wall. If this ring is degraded, the antimicrobial effect disappears.
Previous studies have reported that hydrogen sulfide (H2S), which bacteria produce during sulfur metabolism, reduces their sensitivity to antimicrobial agents leading to resistance. However, the detailed mechanism behind this is not yet understood. Researchers at Kumamoto University have already shown that the cysteine persulfide molecule, a combination of H2S, and the amino acid cysteine, has an extremely powerful antioxidant effect that is not found in H2S or cysteine alone.
In this study, researchers examined how this reactive sulfur species is involved in the acquisition of resistance to β-lactam antibiotics. They found that β-lactam antibiotics such as penicillin G, ampicillin, and meropenem (carbapenem antibiotics) rapidly lose their bactericidal activity when exposed to cysteine persulfide but not to hydrogen sulfide. A detailed study of the reaction between the antimicrobial agents -lactam and cysteine persulfide revealed that the β-lactam ring, essential for bactericidal action, breaks down and that a sulfur atom is inserted into part of the cycle creating carbothioic acid. The production of carbothioic acid from an antimicrobial agent -lactam appears to be a novel degradation metabolite.
The researchers thus developed a very sensitive analytical method to detect and quantify carbothioic acid by mass spectrometry, then analyzed the production of carbothioic acid from bacteria exposed to anti-lactam antimicrobials. They found that bacteria can take up antimicrobial agents and use cysteine persulfide to break down the agents into carbothioic acid which is then released. This is believed to be a previously undescribed mechanism of inactivation and degradation of β-lactam antimicrobial agents to carbothioic acid by cysteine persulfide.
“Our new analytical method allows quantifying the amount of carbothioic acid released by bacteria with high sensitivity,” said Professor Tomohiro Sawa, who led the study. “We believe that it will be possible to screen for compounds which inhibit bacterial synthesis of cysteine persulfide using carbothioic acid as a biomarker. Such an inhibitor of cysteine persulfide synthesis in combination with β-lactam antibiotics should inhibit degradation of antibiotics and lead to effective treatments with a lower concentration of β-lactam antibiotics This should also help reduce the emergence of new resistant bacteria.
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