By Eriona Hysolli
Antibiotic resistance has emerged as a major threat to human health today because antibiotics are widely overused and pathogenic bacteria have developed effective evasion strategies. Known mechanisms whereby bacteria counteract drug lethality such as upregulation of efflux pumps to actively export antibiotics, modification of drug targets, and inactivation of drugs by enzymes, result from acquisition of resistance markers through changes in the bacterial DNA sequence. Strikingly, tolerance is also observed in bacterial populations without DNA mutations and thus still susceptible to the antibiotic, due to a subpopulation of trigger-ready dormant cells. Clinicians increasingly recognize that these tolerant cells represent a major hurdle in the successful treatment of patients prone to chronic infections like those suffering from cystic fibrosis (CF).
Now, a team of scientists at the Wyss Institute led by core faculty member James J. Collins, Ph.D. explored the role of metabolism in the antibiotic susceptibility of Pseudomonas aeruginosa, an opportunistic pathogen that frequently lodges in the airways of CF patients and correlates with accelerated pulmonary decline and mortality. “The capacity to evade antibiotics in the absence of identifiable resistance markers is very significant for the clinical world”, said Sylvain Meylan M.D., Ph.D., who is a clinical fellow working with Collins and the first author on a recent study focusing on the problem. “The lack of an active metabolism is one of the ways the bug can evade the drug.”
Based on their in vitro observation that tobramycin, an aminoglycoside that is part of the first line of defense against infections by P. aeruginosa, efficiently killed growing but not quiescent bacteria, the researchers hypothesized that jump-starting metabolism in these quiescent cells could restore susceptibility to the drug.
To identify candidate molecules that would affect tobramycin sensitivity in P. aeruginosa, they treated the bacteria with a range of metabolites involved in key chemical pathways that produce energy (cellular respiration): the tricarboxylic acid cycle (TCA) and glycolysis. Interestingly, two of the most effective metabolites turned out to exhibit opposite effects; the TCA cycle intermediate fumarate stimulated, whereas a second compound, glyoxylate, suppressed tobramycin activity.
The effects of fumarate and glyoxylate occur through opposing changes in cellular respiration. Fumarate stimulates the full TCA cycle and the cell’s oxygen consumption, and therefore increases the force that drives, among others, tobramycin import. In contrast, glyoxylate inhibits metabolites feeding the later stages of TCA cycle, decreases essential metabolic enzymes and factors, and therefore shunts cell energy production and drug susceptibility. This “glyoxylate shunt” emerges as a powerful determinant of antibiotic tolerance, and as such it’s a novel target for therapeutics. Fumarate, on the other hand, could be used as a candidate adjuvant to potentiate the effects of tobramycin as it’s already an approved inhaled substance for asthma patients. How to fine-tune the modulation of fumarate and glyoxylate in order to make the antibiotic more effective is “trying to play chess with the bug” said Meylan.
“The novelty of our work is that we developed a clinically feasible, inexpensive means to treat persistent P. aeruginosa infections, which often pose significant challenges for patients with cystic fibrosis”, explained Collins, professor in the Department of Biological Engineering and Institute for Medical Engineering & Science at MIT. The team plans to exploit the power of metabolic manipulation and “extend this work to other pathogens, including tuberculosis-causing bacteria”, said Collins. In absence of adequate animal models of chronic infections in CF, the researchers aim to replicate their findings in biomimetic human lung-on-a-chip systems pioneered at the Wyss.
The findings were published in the journal Cell Chemical Biology.