Background: Emergence and spread of multidrug resistance bacteria due to indiscriminate use of antibiotics is a worldwide problem. Bacterial resistance can lead to increased mortality, longer hospitalizations and higher therapeutic cost. Antimicrobial peptides (AMPs) are reported as the class of cationic antimicrobials with broad spectrum antimicrobial activity, with limited potential for bacteria to develop resistance against them. The aim of current study was to evaluate whether Pseudomonas aeruginosa and Staphylococcus aureus could develop resistance to the cationic peptides melimine and mel-4 and mode of action of these peptides.
Methods: Development of resistance to the two peptides was evaluated against P. aeruginosa 6294, ATCC 27853 and S. aureus 38, ATCC 25923 through serial passage assay, with development of resistance to ciprofloxacin used as a control. The membrane depolarising effect of the peptides was studied using the potential gradient dye DiSC3-(5) and viable count. Bacterial-lytic effect of the peptides was analysed through bacteriolysis and measurement of the amount 260nm absorbing material (DNA/RNA) in the surrounding fluid.
Results: P. aeruginosa and S. aureus did not develop resistance to melimine and mel-4 after 30 days of serial exposure to sub-inhibitory concentrations, but resistance developed to ciprofloxacin, with resistance increasing to >32 times. Both peptides dissipate the membrane potential of P. aeruginosa and S. aureus to varying degrees, depending on peptide concentrations. Mel-4 depolarization efficiency was higher with 250% increase in fluorescence against P. aeruginosa and 143% against S. aureus compared with melimine 230% and 88% respectively (p<0.05). Both peptides exhibited bacterial lytic effect against P. aeruginosa and S. aureus when tested at inhibitory concentrations or higher. Melimine showed better activity than mel-4 against P. aeruginosa with 8.4 times increase in OD260nm (p<0.05) but no significant difference was observed against S. aureus (p>0.05).
Conclusion: Melimine and mel-4 showed excellent membrane depolarisation activity. This was higher against P. aeruginosa than S. aureus. Both the bacteria could develop resistance against ciprofloxacin, but not against melimine and mel-4.