Impact of revised breakpoints on the categorization of susceptibility of Enterobacterales to temocillin.

Background: To harmonize with the EUCAST breakpoints, the French Society of Microbiology introduced a change in the inhibition diameter breakpoint (17 mm versus 20 mm previously) of temocillin. We assessed the impact of the new breakpoints on categorizing susceptibility of Enterobacterales to temocillin. Methods: This was a multicentric retrospective study including all Enterobacterales isolates routinely tested for temocillin susceptibility with the disc diffusion method between 1 January 2016 and 31 July 2022 in four centres. Categorization using the breakpoints of 20 mm (French guidelines CA-SFM/EUCAST 2020 v.1.1) and 17 mm (French guidelines CA-SFM/EUCAST 2021 v1.0 and EUCAST guidelines v11.0) was performed. Results: Overall, 36 416 Enterobacterales isolates were included. The overall rate of temocillin resistance decreased from 11.3% to 4.7% (relative difference of 58.5%) when using the 17 mm breakpoint instead of the 20 mm breakpoint, respectively. The relative change ranged from -44.0% in Klebsiella aerogenes to -72.7% in Klebsiella oxytoca. The median inhibition diameter was 23 mm (IQR 21-25). The isolates with a diameter of 20 mm appeared overrepresented, whereas those with a diameter of 18 and 19 mm were underrepresented. We therefore reviewed the diameters between 18 and 21 mm of 273 isolates. Thirty-two (11.7%) of them categorized as susceptible at first measure were controlled resistant at second measure. Conclusions: The new breakpoint induced a decrease in the rate of isolates categorized as resistant to temocillin, increasing therapeutic choice including for Extended-spectrum beta-lactamase-producing Enterobacterales (ESBL-PE). We suggest the bias in measuring the inhibition diameter is probably related to the fact that temocillin is considered remarkably stable against broad-spectrum ß-lactamases.