Killing Resistant Superbugs with Antibiotic Triumvirate

The exponential rise in antibiotic-resistant microbes is a major public health threat worldwide. Moreover, with approval times for new antibiotics taking more than a decade in most cases, finding a viable treatment among previously approved compounds is imperative to stem the drug-resistant tide.

Now, researchers at the University at Buffalo (UB) have assembled of three antibiotics that when used in concert together, are capable of eradicating deadly Escherichia coli carrying the mcr-1 and ndm-5 genes, making the bacterium immune to last-resort antibiotics.

Findings from the new study were published recently in mBio in an article entitled “Polymyxin Combinations Combat Escherichia coli Harboring mcr-1 and blaNDM-5: Preparation for a Postantibiotic Era.”

“The threat of Gram-negative bacteria, including E. coli carrying mcr-1, is worrisome,” remarked lead study investigator Zackery Bulman, Pharm.D., who is currently an assistant professor at the University of Illinois at Chicago College of Pharmacy. “We believe that the appearance of mcr-1 and ndm-5 in patients may be a harbinger for what is to come. The golden era of antibiotics isn’t over yet, but we wanted to help clinicians prepare therapeutically for the occurrence of these strains.”

In the current study, the researchers found that a novel combination of aztreonam, amikacin, and polymyxin B—a last-resort antibiotic—was able to kill E. coli carrying mcr-1 and ndm-5 genes within 24 hours while also preventing regrowth. Traditional combinations of these antibiotics were unable to kill the E. coli and resulted in rapid resistance.

“We assessed the bacterial killing of 15 different FDA-approved antibiotics alone, and in combination with polymyxin B in time-killing experiments against Escherichia coli MCR1_NJ, the first reported isolate in the United States to coharbor mcr-1 and a New Delhi metallo-β-lactamase gene (bla_NDM-5),” the authors wrote. “The most promising regimens were advanced to the hollow-fiber infection model (HFIM), where human pharmacokinetics for polymyxin B, aztreonam, and amikacin were simulated over 240 h. Exposure to polymyxin B monotherapy was accompanied by MCR1_NJ regrowth but not resistance amplification (polymyxin B MIC from 0 to 240 h [MIC_0h to MIC_240h] of 4 mg/liter), whereas amikacin monotherapy caused regrowth and simultaneous resistance amplification (amikacin MIC_0h of 4 mg/liter versus MIC_240h of >64 mg/liter). No MCR1_NJ colonies were observed for any of the aztreonam-containing regimens after 72 h. However, HFIM [hollow fiber infection model] cartridges for both aztreonam monotherapy and the polymyxin B-plus-aztreonam regimen were remarkably turbid, and the presence of long, filamentous MCR1_NJ cells was evident in scanning electron microscopy, suggestive of a nonreplicating persister (NRP) phenotype. In contrast, the 3-drug combination of polymyxin B, aztreonam, and amikacin provided complete eradication (>8-log₁₀ CFU/ml reduction) with suppression of resistance and prevention of NRP formation.”

Fewer than two dozen cases of E. coli carrying mcr-1 have been reported in the U.S. However, with additional cases reported worldwide, the bacteria’s immunity to available antibiotics has left the medical community vulnerable to a massive outbreak of infections. The rapid increase in antibiotic-resistant bacteria has resurrected the importance of polymyxins, a class of antibiotics that are effective but employed as a last resort because of the damage they can cause to the kidneys.

“The mcr-1 and ndm-5 strains represent an urgent threat, because of the high-degree of resistance combined with the potential for rapid spread in the community setting,” noted senior study author Brian Tsuji, Pharm.D., associate professor in the School of Pharmacy and Pharmaceutical Sciences at UB. “We had to work quickly and think outside of the box, beyond traditional antibiotic combinations.”

To avoid prescribing high dosages of polymyxins and to make up for the antibiotic’s weaknesses, the researchers decided to turn to new dosing strategies and multiple antibiotic combinations. After conducting studies on dozens of combinations of more than 15 antibiotics paired with polymyxin B, the researchers discovered two effective treatments. Combinations of polymyxin B with either aztreonam or amikacin resulted in undetectable bacterial counts after 24 hours.

Interestingly, the microbes were able to regrow to initial levels after 96 hours, and a subpopulation of amikacin-resistant strains arose after ten days when exposed to the combination of polymyxin B and amikacin. Polymyxin B and aztreonam pushed the E. coli into a persistent but nonreplicating state. Only the triple combination eliminated the E. coli strain and prevented regrowth.

“We knew that polymyxins alone couldn’t work. Only the three drugs combined were able to work synergistically to suppress and kill the bacteria,” Dr. Bulman concluded. “We overcame the bacteria by pushing it as far as possible with an agent that it was resistant to while simultaneously administering two other antibiotics.”