Cheap and safe peptide will overcome multidrug-resistant bacteria

The world is running out of effective ways to tackle simple bacterial diseases as multidrug-resistant bacteria become more and more widespread. More than 700,000 people die each year from antimicrobial resistance, and by 2050, antimicrobial resistance is expected to kill more people than cancer does today. Since the bacteria become resistant to new antibiotics at an alarming rate, the interest in investing in and developing new antibiotics declines. This is a massive societal problem, which is why new Danish-German research on solving the problem is particularly good news.

“We have found some very potent peptides in the human body’s own defence system known as defensins, which are among the most conserved peptides in nature and are found in all multicellular organisms. Despite their evolutionary stability, they have not developed resistance. Unfortunately, many of these peptides are toxic in high concentrations, and manufacturing them on an industrial scale is almost impossible due to their complex structure,” explains a researcher involved in the study, Benjamin Anderschou Holbech Jensen from the Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen.

The peptides linearize in a reduced environment, which makes them extremely unstable because the body’s own proteases can degrade them. The researchers initially thought that these peptides were destroyed and inactivated during this process, but it turns out that one of the six resulting fragments is more potent than the full-length peptide and then loses its toxicity to cells from multicellular organisms. 

"However, it can attack a broad spectrum of pathogens, and since it uses several simultaneous mechanisms, resistance is unlikely to occur and, perhaps most importantly, the peptide is both safe and inexpensive, so there is a good chance that someone will invest in developing and marketing it."

Tiny peptide with tiny fragments

The new study is based on a discovery made a year ago, in which the researchers examined two defensins in the human small intestine, called HD-5 and HD-6. At the time, it was difficult to understand why enzymes cleaved these two defensins into tiny fragments as soon as they entered the intestine. However, on closer examination, the researchers discovered that the tiny fragments of defensins were actually more active than the large ones.

“We were somewhat surprised that the fragments behaved like a fragmentation bomb, in which the individual tiny peptide fragments each showed strong antimicrobial activity. Since peptide molecules similar to these can be artificially produced both physically and economically, we decided to try to cleave several of the defensins into fragments to determine whether any of the small peptides are more effective against bacteria than others,” says Benjamin Anderschou Holbech Jensen.

Defensins are tiny positively charged peptide molecules comprising 18–45 amino acids. Humans have 10 different characterizations, of which six are alpha-defensins – the ones on which the researchers are currently focusing most in relation to fragmentation. Four of the six alpha-defensins are expressed in the immune system, HNP1–4, and two in the intestine, HD-5 and HD-6. The researchers cleaved HNP1–4 into fragments with the enzyme trypsin and then set out to investigate in detail the effects of the many small defensin fragments.

“When we cleaved HNP4, one of the tiny fragments was an 11-amino-acid peptide chain that proved especially interesting. Not only was this small peptide far more effective than HNP4; it was also active against both gram-positive and gram-negative bacteria and against both non-resistant and multidrug-resistant bacteria,” explains Benjamin Anderschou Holbech Jensen.

Affects 3–4 sites simultaneously

The researchers examined how the tiny peptide killed the bacteria and discovered something even more important. One challenge of traditional antibiotics is that they often attack bacteria at one vulnerable point.

“The bacteria therefore experience selective pressure, and before long one mutates so that it can survive anyway, and that bacterium will thus have a growth advantage and therefore this clone will quickly crowd out the other bacteria. The tiny new HNP4 peptide was different. It attacked bacteria in 3–4 places simultaneously, so resistance is very unlikely since it is molecularly close to impossible for a bacterium to alter several genes simultaneously and still be viable. Further, despite the enormous conservative structure of the defensins, with them being preserved for tens of thousands of years, resistance has not yet been reported,” says Benjamin Anderschou Holbech Jensen.

The researchers used electron microscopy and could see how the small peptide depolarized and destroyed the cell membrane of the bacteria. Some of the peptide fragments also formed small networks that encapsulated and killed the invading bacteria. However, killing dangerous pathogenic bacteria is one thing, but another is not to harm the naturally occurring health-promoting bacteria in the body, which often assist digestion in the intestines, for example.

“The defensins occur naturally and are therefore harmless and safe in principle. We demonstrated this for both full-length peptides and also several of our fragments. We are working intensely to elucidate whether this specific HNP4 fragment, which is unusually potent against several multidrug-resistant bacteria, has the same beneficial properties. Initial and as yet unpublished experiments, however, suggest that, using small fatty acid modifications, we can design very specific antibiotics that all protect the naturally occurring bacteria; this is a huge advantage over conventional antibiotics, which target across a broad spectrum, thereby partly increasing both the risk of resistance and the risk of inappropriate recolonization following antibiotic treatment,” explains Benjamin Anderschou Holbech Jensen.

All in all, the new peptide is such a promising candidate in combatting multidrug-resistant bacteria, skin irritation and gastrointestinal problems that the research team is engaged in very close and intense negotiations with a company to begin clinical trials of the peptides.

“Traditional antibiotics are expensive to develop and produce, and since resistance develops quickly, investment returns in this field are poor. Defensin peptides have the obvious advantage that no resistance occurs, but in addition, they are also extremely easy and inexpensive to manufacture, so we are very confident that they will end up being an essential weapon in combatting multidrug-resistant bacteria,” concludes Benjamin Anderschou Holbech Jensen.

“Fragmentation of human neutrophil α-defensin 4 to combat multidrug resistant bacteria” has been published in Frontiers in Microbiology. In 2017, the Novo Nordisk Foundation awarded a grant to Benjamin Anderschou Holbech Jensen for the project A Novel Oral Combination Therapy Targeting the Gut Microbiome to Alleviate Insulin Resistance and Type 2 Diabetes–linked Aortic Valve Stenosis.