False aminoglycoside resistance in Enterobacterales and non-fermenters by an automated testing system: a descriptive study

Abstract

Antimicrobial sensitivity data are important to guide antimicrobial therapy. In microbiological laboratories, routine sensitivity measurements are typically performed with automated testing systems such as VITEK2 and Phoenix. Using data from the Dutch national surveillance system for antimicrobial resistance over a 6-year period, we found that the measured minimum inhibitory concentrations for aminoglycosides in Enterobacterales and non-fermenters were too high for the Phoenix system. In addition, we observed a yearly increase in resistance for several species measured by Phoenix. These findings might have consequences for clinical treatment of patients with sepsis.

Keywords: aminoglycosides; antimicrobial susceptibility testing; automated testing system; gentamicin.

Fig 1

Minimal inhibitory concentrations for gentamicin as measured by Phoenix (black bars) and VITEK2 (gray bars) in all clinical isolates between 2016 and 2021. The red box indicates borderline susceptible isolates (minimum inhibitory concentration = 2 mg/L), except for Acinetobacter species (4 mg/L) and Pseudomonas aeruginosa (no EUCAST breakpoints for gentamicin).

Fig 2

Minimal inhibitory concentrations for tobramycin as measured by Phoenix (black bars) and VITEK2 (gray bars) in all clinical isolates between 2016 and 2021. The red box indicates borderline susceptible isolates (minimum inhibitory concentration = 2 mg/L), except for Acinetobacter species (4 mg/L).

Fig 3

Yearly gentamicin MIC distributions for Providencia and P. mirabilis isolates. The proportion of “borderline” resistant isolates (minimum inhibitory concentration 4 mg/L) increases yearly from 2016 to 2020 for P. mirabilis and Providencia species but not for other species (data not shown).