Determination of antimicrobial susceptibility ofnon-fermentative bacteria isolated from kancultures and comparison of vitek-2 automatedsystem with standard liquid microdilution methodfor colistin susceptibility testing

Gram-negative bacteria are one of the main causes of nosocomial infections, particularly in patients in intensive care units. This group of bacteria is also among the leading opportunistic pathogens in immunocompromised individuals. The aim of this study was to determine the antimicrobial susceptibility profiles of non-fermentative bacteria isolated from blood cultures at Tokat Gaziosmanpaşa University Training and Research Hospital, and to compare the sensitivity to colistin between the Vitek-2 Automated system and the standard liquid microdilution method. Non-fermentative Gram-negative bacteria are responsible for severe opportunistic nosocomial infections in immunosuppressed patients. Infections caused by multidrug-resistant Gram-negative bacteria have become an increasingly significant issue, particularly in healthcare settings. For these types of infections, where treatment options are limited, colistin is one of the last-resort antibiotics. However, the rising resistance to colistin is becoming a global concern. A total of 50 Acinetobacter baumannii and 28 Pseudomonas aeruginosa isolates, obtained from various departments, were included in the study. Antibiotic susceptibility profiles were determined using the Vitek 2 Compact automated system to detect multidrug resistance. Colistin susceptibility was determined using the ISO-standard liquid microdilution method (20776-1) as the reference method, according to the guidelines of the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST). The colistin minimum inhibitory concentration (MIC) values obtained with the Vitek 2 Compact system were compared to those determined by the liquid microdilution method. In our study, the most resistant antimicrobial agents for Acinetobacter baumannii isolates were imipenem, meropenem, and ciprofloxacin (98%), while for Pseudomonas aeruginosa isolates, the most resistant agent was piperacillin-tazobactam (32.14%). The most sensitive antimicrobial agent for both Acinetobacter baumannii and Pseudomonas aeruginosa isolates was colistin. Furthermore, four isolates that were classified as resistant by the Vitek 2 system, according to EUCAST-2021 standards, were found to be sensitive to colistin by the liquid microdilution method. In conclusion, the Vitek 2 automated system tends to report higher colistin MIC values than the gold-standard liquid microdilution method and may generate resistant results for isolates that are actually susceptible.