Giving Old Drugs New Life: Metabolite Disruption of Bacterial Persistence
Antibacterial resistance makes waves in news headlines, but another insidious phenomenon – bacterial persistence – can also affect the outcome of an infection. Fortunately, researchers are hard at work to understand and attack persistent bacteria. A new Antimicrobial Agents and Chemotherapy report extends previous findings that a metabolite, administered with antibacterial drugs, can increase drug efficacy.
Persistence is a quasidormant state, where bacterial cells are largely metabolically inactive. This inactivity allows the cells to avoid the activity of most antibacterial drugs, which require cells to be metabolically active (and often, actively growing) for the drugs to affect the bacteria. Persister cells are especially problematic in chronic infections, or those that require a long course of treatment, such as Mycobacterium tuberculosis. In this study, first author Martina Koeva and senior scientist Diane Joseph-McCarthy worked with a team to study Pseudomonas aeruginosa, which can cause recalcitrant infections in cystic fibrosis (CF) patients. P. aeruginosa grows in the thick mucus that accumulates in the lungs of CF patients, often in a biofilm state, which increases treatment difficulty.
Fumarate increases tobramycin efficacy in two biofilm models. (A) A 96-well plate biofilm assay and (B) a colony biofilm assay. Source.
The research team confirmed that tobramycin can more effectively eradicate persister cells when administered with fumarate. This prediction was based on previous experiments that demonstrated gentamycin and tobramycin efficacy increased when administered in the presence of various bacterial metabolites; for P. aeruginosa, tobramycin efficacy increased in the presence of fumarate. Fumarate is part of the glycolytic pathway; inside the cells, it activates glycolysis, generating ATP and creating the proton motor force necessary to allow tobramycin uptake by active transport.
The researchers expanded the previous findings by testing different Pseudomonas clinical isolates in a variety of conditions. All but two of 21 clinical isolates showed better killing when fumarate was administered along with tobramycin. They observed the tobramycin-plus-fumarate effect on isolates tested in the presence of patient sputum, and when the bacteria grew as a biofilm (see above right) – two conditions that mimic the environment and growth pattern of CF patient P. aeruginosa infections. Importantly, they saw no pathological effects on in vitro cell culture, so the combination therapy would be unlikely to have negative effects on patients.
Bacterial persisters lead to antibiotic tolerance and failure to eradicate the infection. Source.
Increasing tobramycin efficacy would be a big benefit to treating CF patients, who are often prescribed tobramycin to control P. aeruginosa infection. Because of the presence of persister cells, tobramycin never eliminates all bacteria, and its use selects for persister cell populations, leading to decreased drug efficacy over time (see right). There are still numerous experiments necessary before this research changes CF patient treatment regimens, but it is a promising step toward making an already-approved and used drug more effective.
In their discussion, the authors write “the precise and often stochastic mechanism by which bacteria naturally enter and emerge from a persistent stat is not fully known.” Ongoing studies are investigating these mechanisms, with two mBio articles published using transposon mutagenesis to look for the genetic basis of persistent cells, and an mBio commentary on manipulation of the bacterial metabolism to increase drug efficacy. Learn more: