Six bacterial pathogens with a propensity for developing multi-drug resistance (MDR) are specifically warned for by the Infectious Disease Society of America (IDSA) (ESKAPE:
Enterococcus faecium,
Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and extended spectrum beta lactamase (ESBL)-producing Enterobacteriaceae). These species are causing the majority of human infections and efficiently acquire additional resistance traits [
1], which implies that new antibiotics have to be effective against these actively evolving MDR pathogens. The incidence of vancomycin-resistant Enterococci (VREs) has increased dramatically over recent years [
2].
S. aureus, especially methicillin-resistant
S. aureus (MRSA), currently causes more deaths in the USA annually than HIV and tuberculosis combined [
2]. ESBLs continue to be on the rise and limit treatment options [
3].
P. aeruginosa is becoming increasingly resistant to multiple classes of drugs [
4], whereas
Acinetobacters are naturally resistant to many classes of antibiotics [
5]. Increasing antibiotic resistance leads to extended hospitalization, rising treatment costs, and increased morbidity and mortality.
The rise of antibiotic-resistant pathogens has sparked research into currently disregarded antimicrobial peptides including gramicidin S (GS). GS is naturally produced by
Aneurinibacillus migulanus [
6] and was first discovered in 1941 [
7]. GS shows antimicrobial activity against both Gram-positives and Gram-negatives in a MIC range from 4–64 μg/ml [
8]. The lowest MICs are seen for Gram-positive bacterial species [
9]. Despite its good antimicrobial activity, GS cannot be used systemically due to its haemolytic side-effect [
10] and is therefore only applied topically to treat superficial infections [
11]. GS is a cyclic, C2-symmetrical decapeptide with the sequence cyclo(Pro-
DPhe-Leu-Orn-Val)
2. The two Pro-
DPhe dipeptides form two type II β-turns, and the two Leu-Orn-Val stretches form an antiparallel β-sheet. GS has been reported to kill bacteria by forming pores in the outer membranes [
8]. Native GS is a natural scaffold for amino acid alteration in such a way that antimicrobial activity is retained but toxicity is reduced. In previous attempts to modify GS, several strategies have been followed: non-natural amino-acids were included [
12], the size of the ring has been modified [
8], and the β-turn region [
13] and β-strand region have been changed [
14]. Still, few new derivatives of GS have been identified that show retention of antimicrobial activity with reduced toxicity [
8,
15‐
18]. We here study the β-strand-modified GS analogue VK7 [
14] and the β-turn modified derivative 20 [
13]. Studying naturally occurring antimicrobial peptides such as GS could help with the design and development of novel derivative drugs to combat multidrug resistance.