In continuation of our search for new antimicrobial secondary metabolites from Bacillus cereus associated with rhabditid entomopathogenic nematode, a new microbial diketopiperazine, cyclo(L-Pro-D-Arg), was isolated from the ethyl acetate extract of fermented modified nutrient broth. The chemical structures of the isolated compounds were identified based on their 1D, 2D NMR and high-resolution electrospray ionisation-mass spectroscopy data. Antibacterial activity of the compound was determined by minimum inhibitory concentration and disc diffusion method against medically important bacteria, and the compound was recorded to have significant antibacterial activity against test bacteria. The highest activity was recorded against Klebsiella pneumoniae (1 μg/mL). Cyclo(L-Pro-D-Arg) was recorded to have significant antitumor activity against HeLa cells (IC50 value 50 μg/mL), and this compound was recorded to have no cytotoxicity against normal monkey kidney cells (VERO) up to 100 μg/mL). To the best of our knowledge, this is the first time that cyclo(L-Pro-D-Arg) has been isolated from a microbial natural source.

• MeSH
• Anti-Bacterial Agents chemistry   isolation & purification   pharmacology   MeSH
• Bacillus cereus chemistry   metabolism   MeSH
• Chlorocebus aethiops MeSH
• Peptides, Cyclic chemistry   isolation & purification   pharmacology   MeSH
• Diketopiperazines chemistry   isolation & purification   pharmacology   MeSH
• HeLa Cells MeSH
• Humans MeSH
• Microbial Sensitivity Tests MeSH
• Models, Molecular MeSH
• Molecular Structure MeSH
• Nuclear Magnetic Resonance, Biomolecular MeSH
• Antineoplastic Agents chemistry   isolation & purification   pharmacology   MeSH
• Rhabditida microbiology   MeSH
• Vero Cells MeSH
• Cell Survival drug effects   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

The synergistic anticandidal activity of three diketopiperazines [cyclo-(L-Pro-L-Leu) (1), cyclo-(D-Pro-L-Leu) (2), and cyclo-(D-Pro-L-Tyr) (3)] purified from a Bacillus sp. N strain associated with entomopathogenic nematode Rhabditis (Oscheius) in combination with amphotericin B and clotrimazole was investigated using the macrodilution method. The minimum inhibitory concentration and minimum fungicidal concentration of the diketopiperazines was compared with that of the standard antibiotics. The synergistic anticandidal activities of diketopiperazines with amphotericin B or clotrimazole were assessed using the checkerboard and time-kill methods. The results of the present study showed that the combined effects of diketopiperazines with amphotericin B or clotrimazole predominantly recorded synergistic (<0.5). Time-kill study showed that the growth of the Candida was completely attenuated after 12-24 h of treatment with 50:50 ratios of diketopiperazines and antibiotics. These results suggest that diketopiperazines combined with antibiotics may be microbiologically beneficial and not antagonistic. These findings have potential implications in delaying the development of resistance as the anticandidal effect is achieved with lower concentrations of both drugs (diketopiperazines and antibiotics). The cytotoxicity of diketopiperazines was also tested against two normal human cell lines (L231 lung epithelial and FS normal fibroblast) and no cytotoxicity was recorded for diketopiperazines up to 200 μg/mL. The in vitro synergistic activity of diketopiperazines with antibiotics against Candida albicans is reported here for the first time.

• MeSH
• Amphotericin B pharmacology   MeSH
• Bacillus chemistry   MeSH
• Cell Line MeSH
• Candida albicans drug effects   MeSH
• Diketopiperazines isolation & purification   pharmacology   toxicity   MeSH
• Clotrimazole pharmacology   MeSH
• Humans MeSH
• Microbial Sensitivity Tests MeSH
• Microbial Viability drug effects   MeSH
• Drug Synergism * MeSH
• Cell Survival drug effects   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH