In 1945, Sir Alexander Fleming predicted that antibiotic resistance would become a global problem if antibiotics were abused. Today, the World Health Organization and other leading public health institutions have affirmed this prediction – bacteria are rapidly becoming resistant to medical interventions and clinicians are running out of treatment options for bacterial illnesses.
Antimicrobial peptides, or AMPs, are short proteins comprised of 12-50 amino acids and are an integral part of the immune system in nearly all organisms. Bacteria in the human gut and intestine produce certain AMPs to control the proliferation of competing bacteria in their environments. AMPs are also produced by insects, amphibians and mammals to fight off bacterial invasions. These small peptides can kill bacterial cells by aggregating at the cell membrane, inhibiting protein folding or by entering the cell and interfering with molecular processes.
Despite animals producing AMPs of many different shapes and sizes, sometimes our own bodies cannot produce the correct AMP to fight off a bacterial infection. Though not necessarily more effective than conventional antibiotics, AMPs are nonetheless useful because of the multitude of ways in which they can kill bacteria, offering physicians a greater repertoire of tools to combat the rise in antibiotic-resistant bacteria.
The reason for AMPs slow adoption in the clinic, until now, has been their cost. Small peptides are incredibly expensive to produce by chemical synthesis. A single milligram of an AMP can cost anywhere from £60 – £700 to chemically synthesize. In comparison, a milligram of ampicillin, an antibiotic, would set you back less than £0.01.
To address the drastic price differences between antibiotics and AMPs and to demonstrate the potential utility of AMPs in the clinic, the laboratory of Timothy Lu at the Massachusetts Institute of Technology (MIT) has developed a cell-based production platform for AMPs. Published in the journal ACS Synthetic Biology, Lu’s group reports the successful engineering of Pichia pastoris, a type of yeast, to efficiently produce a specific AMP called apidaecin. Apidaecin is a small peptide that is naturally produced by honeybees and has been shown to inhibit the growth of many different bacterial species, including E. coli and Pseudomonas syringae.
First, the scientists created a circular piece of DNA that they inserted into the yeast species. This sequence of DNA instructed the cell to begin producing a protein that contains human serum albumin (HSA) fused to the apidaecin amino acid sequence. It was necessary to fuse the apidaecin protein to HSA because apidaecin will kill cells in its active form, and the HSA protein fusion ensures that the AMP will not be ‘active’ within the cell. Once the cells produce this fusion protein in large quantities, the team extracted the protein and cut off the HSA portion, thus releasing apidaecin. The authors went on to show that the purified apidaecin had antimicrobial activity against E. coli.
This study marks a significant advancement in the production of AMPs and may expedite their implementation in the clinic. The solution is not a perfect one, however, as bacteria can still develop resistance to AMPs in much the same way as they do antibiotics. As biological engineers continue developing platforms for AMP production, physicians will gain access to a growing repertoire of tools to treat complex bacterial diseases.
Niko McCarty is studying for an MSc in Systems and Synthetic Biology at Imperial College London
Banner Images: Antimicrobial assay, Wikimedia Commons