COMPARATIVE SUSCEPTIBILITY PROFILES OF CLINICAL AND REFERENCE BACTERIAL STRAINS ACROSS MULTIPLE ANTIBIOTIC CLASSES.

Antimicrobial resistance (AMR) is a major global public health threat, undermining the efficacy of commonly used antibiotics. Resistance patterns differ across bacterial taxa, including Enterobacteriaceae, non-fermenting Gram-negative bacilli, and Gram-positive cocci. This study aimed to provide a comparative analysis of antimicrobial susceptibility among reference strains with defined susceptible and resistant phenotypes, alongside selected clinical isolates, to evaluate the preservation of phenotypic traits and the impact of antibiotic use. Reference susceptibility strains exhibited high susceptibility across most antibiotics, whereas resistant reference strains demonstrated multidrug resistance. Among Enterobacteriaceae, reference strains harboring ESBL and AmpC mechanisms displayed resistance to penicillins, cephalosporins, and carbapenems. Non-fermenters, including Pseudomonas aeruginosa and Acinetobacter baumannii, showed both intrinsic and acquired resistance to multiple classes, particularly carbapenems and fluoroquinolones. Gram-positive cocci largely retained susceptibility to glycopeptides and linezolid, while MRSA, high-level aminoglycoside-resistant enterococci, and penicillin-resistant Streptococcus pneumoniae posed significant therapeutic challenges. Comparative analysis revealed that antimicrobial susceptibility is influenced not only by bacterial taxonomy but also by patterns of uncontrolled or inappropriate antibiotic use. Clinical strains of Klebsiella pneumoniae and Streptococcus pneumoniae displayed reduced and more variable susceptibility compared to the predictable profiles of reference strains. These findings highlight the importance of continuous surveillance, strict adherence to antimicrobial stewardship, and the use of standardized reference strains to ensure reliable susceptibility testing. Early detection of emerging resistance patterns is essential to guide effective therapy and mitigate the public health impact of multidrug-resistant pathogens.