Comparison of automated system D2 Mini with conventional method and VITEK-2 for identification and antibiotic susceptibility pattern of gram-negative fermentative bacteria

Abstract

This study evaluates and compares the performance of the automated D2 Mini system with conventional microbiological methods and the VITEK-2 system for the identification and antibiotic susceptibility testing (AST) of gram-negative fermentative bacteria. A total of 34 clinical isolates were analyzed, including Escherichia coli (n=9), Klebsiella pneumoniae (n=7), Klebsiella oxytoca (n=1), Salmonella Typhi (n=7), Salmonella Paratyphi (n=1), Serratia marcescens (n=3), Aeromonas hydrophila (n=2), Morganella morganii (n=1), Proteus hauseri (n=1), Proteus mirabilis (n=1), and Edwardsiella hoshinae (n=1). The isolates were recovered from various clinical specimens, including blood, urine, pus, wound swabs, catheter tips, and bronchial wash. Identification was performed using conventional methods, including Gram staining, biochemical tests, and antibiotic susceptibility testing (AST) by the disk diffusion method, followed by automated systems (VITEK-2 and D2 Mini). The results showed that both the VITEK-2 and conventional methods achieved 100% concordance for genus and species identification. However, the D2 Mini system demonstrated high genus-level concordance (100%) for most isolates, except for Klebsiella pneumoniae (85.7%) and Serratia marcescens (66.7%) at the species level. The D2 Mini system failed to identify Salmonella species at the species level. Antibiotic susceptibility testing revealed that both automated systems (VITEK-2 and D2 Mini) exhibited high concordance with the disk diffusion method for several antibiotics, including amoxicillin-clavulanate, ceftazidime, ciprofloxacin, gentamicin, and amikacin. However, discrepancies were observed for antibiotics such as netilmicin and colistin, with low concordance values. The D2 Mini demonstrated a restricted antibiotic panel, lacking profiles for antibiotics such as ceftriaxone colistin, and TZP, while VITEK-2 showed higher concordance but also displayed limitations for certain antibiotics. This study highlights the strengths and limitations of automated systems in microbial diagnostics, emphasizing the need for further improvements in their antibiotic testing capabilities, particularly for last-resort antibiotics and non-fermenting bacteria.