In vitro evaluation of the potential for resistance development to ceragenin CSA-13.
OBJECTIVE
Though most bacteria remain susceptible to endogenous antimicrobial peptides, specific resistance mechanisms are known. As mimics of antimicrobial peptides, ceragenins were expected to retain antibacterial activity against Gram-positive and -negative bacteria, even after prolonged exposure. Serial passaging of bacteria to a lead ceragenin, CSA-13, was performed with representative pathogenic bacteria. Ciprofloxacin, vancomycin and colistin were used as comparators. The mechanisms of resistance in Gram-negative bacteria were elucidated.


METHODS
Susceptible strains of Staphylococcus aureus, Pseudomonas aeruginosa and Acinetobacter baumannii were serially exposed to CSA-13 and comparators for 30 passages. MIC values were monitored. Alterations in the Gram-negative bacterial membrane composition were characterized via mass spectrometry and the susceptibility of antimicrobial-peptide-resistant mutants to CSA-13 was evaluated.


RESULTS
S. aureus became highly resistant to ciprofloxacin after <20 passages. After 30 passages, the MIC values of vancomycin and CSA-13 for S. aureus increased 9- and 3-fold, respectively. The Gram-negative organisms became highly resistant to ciprofloxacin after <20 passages. MIC values of colistin for P. aeruginosa and A. baumannii increased to ≥100 mg/L after 20 passages. MIC values of CSA-13 increased to ∼20-30 mg/L and plateaued over the course of the experiment. Bacteria resistant to CSA-13 displayed lipid A modifications that are found in organisms resistant to antimicrobial peptides.


CONCLUSIONS
CSA-13 retained potent antibacterial activity against S. aureus over the course of 30 serial passages. Resistance generated in Gram-negative bacteria correlates with modifications to the outer membranes of these organisms and was not stable outside of the presence of the antimicrobial.
Department of Chemistry and Biochemistry,  Brigham Young University, Provo, UT 84602,  USA.
Jake E
JE
Pollard
Jason
J
Snarr
Vinod
V
Chaudhary
Jacob D
JD
Jennings
Hannah
H
Shaw
Bobbie
B
Christiansen
Jonathan
J
Wright
Wenyi
W
Jia
Russell E
RE
Bishop
Paul B
PB
Savage
D040981:Acinetobacter baumannii / Q000187:drug effects*; D000890:Anti-Infective Agents / Q000494:pharmacology*; D002462:Cell Membrane / Q000737:chemistry; D024881:Drug Resistance, Bacterial; D008050:Lipid A / Q000032:analysis; D013058:Mass Spectrometry; D008826:Microbial Sensitivity Tests; D011550:Pseudomonas aeruginosa / Q000187:drug effects*; D013211:Staphylococcus aureus / Q000187:drug effects*; D013256:Steroids / Q000494:pharmacology*
D000890:Anti-Infective Agents; D008050:Lipid A; D013256:Steroids; C521528:ceragenin CSA-13