Insights on cross-resistance can improve potency of sequential antibiotic treatment

A new study published in the eLife journal notes that sequential treatment with antibiotics that are similar but often swapped around can effectively kill bacteria and avoid drug resistance.

An agar plate with the human pathogen Pseudomonas aeruginosa (green) and three antibiotics (labelled A, B, and C). Image Credit: Roderich Roemhild.

The findings of the study not only challenge a wider assumption that the use of similar antibiotics boosts cross-resistance to drugs but also demonstrate that existing antibiotics could provide unexplored, highly potent treatment options.

We are currently in an antibiotic crisis, where the overuse of antibiotics is leading to increased antibiotic resistance and certain infections have become difficult and even impossible to treat.”

Aditi Batra, Study First Author and Graduate student, Max Planck Institute for Evolutionary Biology and University of Kiel

“It is the ability of pathogens to evolve and adapt to drugs that underlies this resistance, but evolutionary theory predicts that adaptation is difficult when the environment changes rapidly. We wanted to test if we could use sequential antibiotic treatment to slow down the evolution of human pathogens and limit drug resistance,” added Batra.

Pseudomonas aeruginosa (P. aeruginosa), a bacterium that can cause pneumonia and other infections in humans, was used by the researchers for the tests. Three different sequences of antibiotics were tested under laboratory conditions and their potency to kill different sub-populations of evolved bacterial cells were examined.

Two sets of antibiotics fell in a class of drugs known as β-lactams, with a common structural component—a β-lactam ring. The other set of antibiotics functioned through different mechanisms.

Surprisingly, treatment with both sets of β-lactam antibiotics was found to be better at eliminating bacterial populations compared to other unrelated antibiotics. Furthermore, fast switching between the individual antibiotics led to much better elimination of bacterial populations than a slower switch between antibiotics.

This shows that rapid switching between antibiotics restricted the ability of the bacteria to adapt to the drugs. In view of this unpredicted result, the researchers explored the mechanisms behind this evolutionary constraint.

They analyzed the changes in growth, whole genome sequences, and resistance profiles of the P. aeruginosa populations treated with the most potent sequence of β-lactam antibiotics, combining doripenem, carbenicillin, and cefsulodin.

When the sequences were switched rapidly, the growth of bacteria during a shift to doripenem was much lower than for the other two antibiotics. This shows that resistance to this drug might occur more slowly.

The researchers also analyzed whether physiological changes occurring as a result of drug treatment rendered the bacteria resistant or more vulnerable to the other drugs in the sequence. Spontaneous development of resistance was found to be much lower for doripenem compared to the other two drugs. The cross-resistance toward this drug was also less compared to that toward the other two antibiotics.

Such a lack of cross-resistance might denote the presence of what is called collateral sensitivity. This implies that the mutant cells that have become resistant to one drug maintain at least ancestral levels of vulnerability against the second drug. It is known that collateral sensitivity is crucial for the effectiveness of sequential treatment.

“Although sequential treatments with such similar antibiotics should have sped up resistance evolution, we found this is not the case if resistance to one of the antibiotics cannot emerge easily, and if the antibiotics show collateral sensitivity to each other,” says Hinrich Schulenburg, senior author of the study and Fellow of the Max Planck Institute for Evolutionary Biology and Professor at the University of Kiel.

It is ironic that the differential cross-resistance profile of the β-lactam drugs was a key factor to treatment potency, even though this is usually used to reject treatment that exclusively uses these drugs. Our study shows that spontaneous resistance rates of component antibiotics could be used as a guiding principle for sequential treatments and could improve the potency of sequential protocols.”

Hinrich Schulenburg, Fellow, Max Planck Institute for Evolutionary Biology