Synthesis and Antimicrobial Activity of Newly Synthesized Nicotinamides

Antioxidants are promising compounds with antimicrobial activity against drug-resistant pathogens, especially when combined with conventional antimicrobials. Our study aimed to characterize the structure of nicotinamides synthesized from nicotinic acid and thiocarbohydrazones and to evaluate their antibacterial and antifungal activity. Seven nicotinic acid hydrazides (NC 1-7) were synthesized using mono-thiocarbohydrazones with hydroxyl group substituents, along with quinolone, phenolic, and pyridine rings known for their antimicrobial activity. The in vitro antimicrobial activity of NC 1-7, at concentrations ranging from 0.001 to 1 mM, was tested against Staphylococcus aureus (ATCC 6538), Enterococcus faecalis (ATCC 29212), Pseudomonas aeruginosa (ATCC 27853), Klebsiella pneumoniae (NCIMB 9111), and Candida albicans (ATCC 24433) using the broth microdilution method per EUCAST 2024 guidelines. Microorganism survival percentages were calculated based on optical density, and target fishing using the PharmMapper database identified potential molecular targets. The results showed that P. aeruginosa was most susceptible to the compounds, while C. albicans was the least susceptible. NC 3 significantly inhibited P. aeruginosa and K. pneumoniae growth at 0.016 mM, while higher concentrations were required for S. aureus, E. faecalis, and C. albicans. NC 5 was most effective against gram-positive bacteria at 0.03 mM. Only NC 4 completely inhibited C. albicans below 1 mM. NC 3, with the lowest concentration for 50% growth inhibition (0.016-0.064 mM), showed promising antibacterial potential against specific AMR-related proteins (bleomycin resistance protein, HTH-type transcriptional regulator QacR, and streptogramin A acetyltransferase), suggesting that this class of compounds could enhance or restore the activity of established antibiotics.