Macolacin – a new solution for antimicrobial resistance?

Antimicrobial resistance is a global problem and one of the main priorities of public health. There are many causes of resistance, and include irrational prescribing of antibiotics and improper regime of their use. Science is constantly looking for new compounds and drugs that would work on bacteria that have already developed resistance to a wide range of available therapies.

representation of a drastic increase in the resistance of Acinetobacter baumannii to imipenem, meropenem and the combination of ampicillin + sulbactam in Croatia

Colistin effectiveness in jeopardy

Colistin is used as the last line of treatment for serious infections caused by Gram-negative bacteria. Colistin is a bactericidal polypeptide antibiotic from the polymyxin group. Structurally, it is a cationic lipo-cyclodecapeptide and acts by binding to the lipid portion of the lipopolysaccharide and displacing divalent cations. Consequently, the integrity of the bacterial membrane is compromised and cell death occurs. A problem in the effectiveness of colistin occurred with the discovery of the plasmid gene for colistin resistance mcr-1. This gene encodes a phosphoethanolamine transferase that joins phosphoethanolamine phosphate to lipid A. The interaction between colistin and lipopolysaccharide is thereby reduced, making bacteria resistant to colistin.

How was macolacin discovered?

Colistin is part of structurally related metabolites encoded by biosynthetic gene clusters (BGCs). Given the association between natural and clinical resistance, the researchers sought a solution for mcr-1 gene-mediated resistance among natural colistin congeners. Of the nearly 11,000 bacterial genomes screened, 35 BGCs have been identified that should encode antibiotics from the polymyxin family. Most sequenced compounds of the polymyxin family contain decapeptides that are identical to the consensus sequence or differ in one or two amino acids. As an exception, a decapeptide was discovered that differed from the consensus by four amino acids – it was called macolacin. The natural development of resistance to previous antibiotics is often accompanied by the creation of new congeneric structures, so macolacin has been observed as a potentially effective antibiotic to highly resistant bacterial species.

Structure of colistin (left) and macolacin (right)

Did macolacin pass the antibacterial activity test?

The antibacterial activity of macolacin was initially tested against ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) which are the leading causes of antibiotic-resistant hospital-acquired infections. Since phosphoethanolamine transferase is a key problem of colistin resistance, the researchers used strains of K. pneumoniae and A. baumannii resistant to colistin (formed by transforming a plasmid containing mcr-1) to examine the antibacterial activity of macolacin. To monitor antibacterial activity, a minimum inhibitory concentration (MIC) was measured – the lowest concentration of antibiotics that will inhibit visible bacterial growth. In the case of macolacin, the MIC increased 2-4-fold during mcr-1 gene expression, while in colistin it increased up to 32-fold. We can conclude that the activity against colistin-resistant strains is significantly improved. However, the concentration of macolacin used exceeds the threshold for clinical use, which is why the optimization of the lipid substitute macolacin produced its more active analogue – biphenyl-macolacin.

In the case of macolacin, the MIC increased 2-4-fold during mcr-1 gene expression, while in colistin it increased up to 32-fold.

Did the in vivo study meet expectations?

A neutropenic thigh infection model was used to evaluate the efficacy of biphenyl-macolacin in vivo. Two colistin-resistant strains of A. baumannii were selected – carbapenem-resistant A. baumannii (CRAB) transformed with mcr-1 and extremely resistant (XDR) clinical strain of A. baumannii.  Mice used in the study were exposed to the pathogen and received a dose of colistin or biphenyl-macolacin subcutaneously 2 hours later. After 24 hours, a bacterial load was determined that colistin failed to reduce while biphenyl-macolacin showed strong antibacterial activity.

Translated by: Sara Koren