Minocycline and Omadacycline Resistance Among Carbapenem-Resistant Gram-Negative Bacteria: Antimicrobial Susceptibility Testing and Molecular Characterization

Increasing prevalence of multidrug-resistant infections has rendered the healthcare systems ineffective in managing infectious diseases. Drugs of "last resort" like carbapenems and polymyxins are becoming less effective in the management of antibiotic-resistant Gram-negative bacterial infections, leaving the clinicians with limited choices. Evaluation of the efficacy of other available broad-spectrum antibiotics (belonging to a different class) is warranted as a treatment alternative. The current study was undertaken to evaluate the <i>in vitro</i> antibacterial activity of minocycline and a new drug, omadacycline among carbapenem-resistant Gram-negative bacteria (GNB), isolated from clinical samples (pus and sputum) and to genotypically analyze them. A prospective cross-sectional study was conducted in a 3,200-bedded tertiary care medical center, located in Lucknow in the northern part of India. All the clinical isolates recovered from pus and sputum samples of patients admitted in intensive care units were processed according to the standard protocols. Identification and antibiotic susceptibility testing were performed, and carbapenem-resistant Gram-negative bacteria (CRGNB) showing resistance to minocycline were included in the study. Molecular screening of β-lactamase and tetracycline resistance genes was done by the conventional polymerase chain reaction method. Minimum inhibitory concentration analysis was performed using the broth microdilution technique. Among 700 CRGNB, 15.29% (<i>n</i> = 107/700) were minocycline resistant by disk diffusion method. Genetic analysis demonstrated the presence of tetracycline-resistant genes in about one-third isolates, among which the <i>tet</i>(B) gene was present in 41.12% (<i>n</i> = 44/107). Upon broth microdilution analysis, the overall minimum inhibitory concentration for minocycline was raised, wherein 4.76% (<i>n</i> = 5/107) of our clinical Gram-negative isolates were inhibited at ≤8 mg/L and 15.23% (<i>n</i> = 28/107) were inhibited at ≤16 mg/L. Omadacycline was able to inhibit 13.08% (<i>n</i> = 14/107) of the minocycline-resistant isolates at ≤4 mg/L (susceptible breakpoint for <i>Enterobacterales</i>). Based on the cut-off value proposed, 15.09% (<i>n</i> = 16/107) isolates resistant to minocycline were inhibited by omadacycline. High prevalence of multidrug-resistant bugs entails judicious use of minocycline and omadacycline. The presence of <i>tet</i> genes coexisting with <i>bla</i>_NDM and <i>bla</i>_OXA in our bacterial isolates shows that the resistance pattern in Gram-negative bacilli is regularly evolving, and a fully functional surveillance program across the health care system is needed to prevent the emergence and spread of antimicrobial resistance.