Immune System Targeted to Fight Drug-Resistant Bacteria

The escalating crisis of antibiotic resistance is complicating treatment for hospital-acquired infections worldwide. Among the most concerning are the ESKAPE pathogens – Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter species – known for evading many conventional therapies. Researchers are now exploring innovative strategies to combat these resilient microbes, including harnessing the power of the immune system.

Targeting a Unique Bacterial Sugar

A team of researchers in Australia is investigating a novel approach to target Acinetobacter baumannii, a pathogen notorious for causing pneumonia and bloodstream infections resistant to last-line antibiotics. Their strategy centers on pseudaminic acid, a sugar that is not produced by humans but is present in the outer coating of many bacteria. This sugar helps bacteria move, adhere to tissues, and evade immune defenses.

Published in Nature Chemical Biology, the research demonstrates that antibodies engineered to recognize pseudaminic acid can effectively clear lethal A. Baumannii infections in mice, essentially using the sugar as a molecular beacon for the immune system.

The Promise of a Universal Target

Pseudaminic acid belongs to a family of unusual sugars found exclusively in bacteria and archaea, appearing on both proteins and larger surface structures like capsules. This exclusivity is crucial because targeting a molecule absent in human cells minimizes the risk of damaging healthy tissue. It offers a potential pathway to circumvent the ongoing antibiotic resistance crisis, where bacteria evolve resistance faster than modern drugs can be developed.

Historically, studying pseudaminic acid on bacterial surfaces has been challenging. To overcome this, the researchers synthesized the sugar in the laboratory. “By precisely building these bacterial sugars in the lab with synthetic chemistry, we were able to understand their shape at the molecular level and develop antibodies that bind them with high specificity,” explained Professor Richard Payne of the University of Sydney.

By creating the sugar from scratch and combining it with protein fragments, the scientists generated clear targets for the immune system to learn from. The resulting antibodies demonstrated the ability to recognize the sugar across various chemical variations.

Antibodies in Action: Clearing Infections in Mice

The antibodies were found to recognize the sugar on several disease-causing bacteria, including Helicobacter pylori, Campylobacter jejuni, and A. Baumannii. The most compelling evidence came from an experiment designed to mimic a real-world medical scenario: treating an established infection.

Mice infected with A. Baumannii were treated with an antibody targeting pseudaminic acid one hour post-infection. Remarkably, all treated mice survived for seven days, even as untreated mice or those receiving a control treatment succumbed to the infection within approximately 12 hours. Blood tests confirmed these findings, showing extremely high bacterial levels in untreated animals and no detectable bacteria in the blood of treated mice.

The antibody doesn’t directly kill the bacteria; instead, it acts as a marker, enhancing the ability of immune cells to identify and engulf the microbes. Laboratory tests confirmed this increased efficiency of immune cell cleanup in the presence of the antibody.

Passive Immunotherapy: A Rapid Response

This approach utilizes passive immunotherapy – administering pre-made antibodies rather than relying on the body’s natural antibody production. This speed is particularly critical for high-risk patients, such as those in intensive care units, where drug-resistant infections can rapidly become life-threatening.

The next phase of research will focus on translating these promising results from mice to human trials. If successful, this work could pave the way for a new class of therapies that bypass antibiotic resistance by training the immune system to recognize bacterial molecular fingerprints.