2.2.1. Determination of Minimum Inhibitory Concentrations (MICs)

Antimicrobial studies were determined by the broth microdilution technique using the Clinical Laboratory Standards Institute (CLSI) recommendations [23,24]. Mueller–Hinton broth (BD Difco, Detroit, MI, USA catalog number: DF0757-17-6) for bacteria ((Staphylococcus aureus (ATCC 29213), Staphylococcus epidermidis (ATCC 12228), Enterococcus faecalis (ATCC 29212), Pseudomonas aeruginosa (ATCC 27853), Escherichia coli (ATCC 25922), Klebsiella pneumoniae ATCC 4352), and Proteus mirabilis (ATCC 14153)), and RPMI-1640 medium buffered to pH 7.0 with MOPS (Sigma, St. Louis, MO, USA, R6504, M1254) for the yeast strains ((Candida albicans (ATCC 10231), Candida tropicalis (ATCC 750), and Candida parapsilosis (ATCC 22019)) were used as the test media. Then 10 mg/mL stock solution of each molecule was prepared using DMSO (Merck, Catalog number: 67685). Then, serial twofold dilutions ranging from 1250 µg/mL to 0.6 µg/mL were prepared in the medium. The inoculum was diluted in broth media to give a final concentration of 5 × 10⁵ CFU/mL for bacteria and 0.5 × 10³ to 2.5 × 10³ CFU/mL for yeast in the test tray. The MIC was defined as the lowest concentration of compound producing complete inhibition of visible growth [23,24]. The antimicrobial effects of the solvents were also investigated against the test microorganisms as controls, and the results were evaluated according to the values of the controls. According to the results, in vitro activities of QQ2 and QQ6 were investigated against each of the 31 resistant Staphylococcus aureus strains obtained clinically by the broth microdilution technique recommended in CLSI [23,24]. Thirty-one nonduplicates, nosocomially acquired methicillin-resistant Staphylococcus spp. isolated from blood specimens between June and December 2018, were obtained from the Department of Infectious Diseases and Clinical Microbiology, Faculty of Medicine, Istanbul Medipol University. API STAPH (bioM’erieux) was used to identify the studied clinical strains. Additionally, all the tested Staphylococcus spp. were selected by using oxacillin susceptibility to examine the methicillin-resistant isolates, approved by CLSI (MIC ≥ 4 µg/mL).