Evaluation of antimicrobial potential of synthetic Solenopsis invicta (red imported fire ant) venom alkaloids

The imported fire ant, Solenopsis invicta, produces venom alkaloids that have antibacterial activity. Nine synthetic fire ant venom alkaloids were tested: (+/-)-solenopsin A (Sol A), (2R , 6R)-solenopsin A (Sol A), (2S, 6S)-solenopsin B (Sol B), (+/-)-isosolenopsin A (Iso A), (2S, 6R)-isosolenopsin A (Iso A), (2R, 6S)-isosolenopsin A (Iso A), (+)-isosolenopsin B (Iso B), (2S, 6R)-isosolenopsin B (Iso B), and (2R, 6S)-isosolenopsin B (Iso B). Minimum inhibitory concentration (MIC) was determined by microdilution assay against ATCC strains of three gram-positive bacteria (Streptococcus pneumoniae, SP; Staphylococcus aureus, SA; and Enterococcus faecalis, EF), and three gram-negative bacteria ( Escherichia coli, EC; Stenotrophomonas maltophilia, SM; and, Pseudomonas aeruginosa, PA). Microorganisms were added to microtiter plates containing 10 fold dilutions of each alkaloid, with a range from 0.0005 to 50 mug/mL. None of the venom alkaloids inhibited EC or PA, four of the alkaloids, (2S, 6S)-Sol B, (+/-)-Iso B, (2S, 6R)-Iso B and (2R, 6S)-Iso B inhibited SP, EF, SA, and SM at various concentrations. The MICs were repeated using two-fold dilutions. SP was most sensitive, with MIC values of 1 mug/mL to all alkaloids tested.{09}EF, SA, and SM had MICs ranging from 2-4 mug/mL. Clinical isolates of SP, SA, and EF had similar MICs compared to ATCC strains. Thus, significant dose-related antibacterial activity against human pathogens is present in S. invicta venom alkaloids. Minimum bactericidal concentration (MBC) was determined. MBC values were above MIC values in all experiments at a ratio favorable for antimicrobial agents. Microorganisms were evaluated for bactericidal or bacteriostatic growth by time kill kinetics. All four active alkaloids inhibited SP and SA in a bactericidal manner, resulting in >3 log10 colony forming units (CFU)/mL decrease in 6 hours. For EF, (+/-)-Iso B decreased EF by < 3 log10 CFU/mL while the remaining alkaloids decreased EF by > 3 log10 CFU/mL. Electron microscopic analysis of SP, SA, and EF revealed substantial ultrastructural changes in the cell wall and cell membrane in response to selected alkaloids. These studies have shown that these four alkaloids possess the potential for future use as disinfectants or possibly as therapeutic agents.