Cholic acid: a novel steroidal uncompetitive inhibitor against β-lactamase produced by multidrug-resistant isolates

β-lactam antibiotics are the most frequently prescribed class of drugs worldwide, due to its efficacy and good safety profile. However, the emergence of β-lactamase producing bacterial strains eliminated the use of β-lactam antibiotics as a chemotherapeutic choice. To restore their usability, a non-antibiotic adjuvant in conjunction with β-lactam antibiotics is now being utilised. Cholic acid potentially acts as an adjuvant since it can blunt the pro-inflammatory activity in human. Our main objective is to scrutinise the inhibition of β-lactamase-producing bacteria by adjuvant cholic acid, synergism of the test drugs and the primary mechanism of enzymatic reaction. Antibacterial effect of the cholic acid-ampicillin (CA-AMP) on 7 β-lactamase positive isolates were evaluated accordingly to disc diffusion assay, antibiotic susceptibility test, as well as checkerboard analysis. Then, all activities were compared with ampicillin alone, penicillin alone, cholic acid alone and cholic acid-penicillin combination. The CA-AMP displayed notable antibiotic activity on all test bacteria and depicted synergistic influence by representing low fractional inhibitory concentration index (FIC ≤ 0.5). According to kinetic analyses, CA-AMP behaved as an uncompetitive inhibitor against beta lactamase, with reducing values of Michaelis constant (K_m) and maximal velocity (V_max) recorded. The inhibitor constant (Ki) of CA-AMP was equal to 4.98 ± 0.3 µM, which slightly lower than ampicillin (5.00 ± 0.1 µM).