Resistance mechanisms for Gram-negative bacteria-specific lipopeptides, turnercyclamycins, differ from that of colistin

Abstract

ABSTRACT Turnercyclamycin A and B lipopeptides exhibit Gram-negative bacteria-specific toxicity. This includes CDC urgent threat organisms such as multidrug-resistant Acinetobacter baumannii . Like the last-line agent colistin, turnercyclamycins interact with the lipopolysaccharide (LPS) pathway, but they remain active against clinical isolates that are colistin resistant. Here, we aimed to determine why turnercyclamycins A and B show little cross-resistance with colistin despite some potential mechanistic and structural similarities. The outer membrane was important in the actions of all three agents, since the deletion of lpxC that synthesizes LPS and the addition of exogenous LPS led to resistance to turnercyclamycins and colistin. Even so, it was much more difficult to generate resistance to turnercyclamycin A than turnercyclamycin B. In Escherichia coli , disruption of the mlaA gene, which is involved in outer membrane homeostasis, resulted in resistance to turnercyclamycin B. However, mlaA disruption blocked neither turnercyclamycin A nor colistin activity. This activity was recapitulated in A. baumannii , where transposon mutants were more resistant to turnercyclamycin B. Moreover, the common A. baumannii colistin resistance gene mcr-1 blocked colistin activity in E. coli but did not affect turnercyclamycins. These results demonstrate a unique resistance profile for turnercyclamycins A and B, in addition to suggesting differences in the mechanism of action. Further, turnercyclamycin A was effective in a mouse model of A. baumannii infection, indicating that this compound class may have potential promise in treating drug-resistant infections. IMPORTANCE Bacterial resistance to antibiotics is a crisis. Acinetobacter baumannii is among the CDC urgent threat pathogens in part for this reason. Lipopeptides known as turnercyclamycins are produced by symbiotic bacteria that normally live in marine mollusks, where they may be involved in shaping their symbiotic niche. Turnercyclamycins killed Gram-negative pathogens including drug-resistant Acinetobacter , but how do the mechanisms of resistance compare to other lipopeptide drugs? Here, we define resistance from a truncation of MlaA, a protein involved in regulating bacterial membrane phospholipids. Intriguingly, this resistance mechanism only affected one turnercyclamycin variant, which differed only in two atoms in the lipid tail of the compounds. We could not obtain significant resistance to the second turnercyclamycin variant, which was also effective in an infection model. This study reveals an unexpected subtlety in resistance to lipopeptide antibiotics, which may be useful in the design and development of antibiotics to combat drug resistance.