Turbitest: an innovative rapid test for determining in vitro susceptibility to ceftazidime-avibactam plus aztreonam in Enterobacterales

Treating carbapenem-resistant Enterobacterales, mainly those producing metallo-β-lactamases (MBL) is a subject of major concern. The combination aztreonam-avibactam (AZA) was developed in an attempt to overcome this challenge, as avibactam inhibits KPC and other serine-β-lactamases, enabling aztreonam (ATM) to act against MBL. However, in scenarios where AZA is not licensed so far, the combination of ATM with ceftazidime-avibactam (CZA) is an alternative that has been proved in vivo efficacy and in vitro synergic activity. We propose the TurbiTest, an alternative method based on turbidity measurement of bacterial growth after a short incubation period (4 h) in the presence and absence of antibiotics to easily determine susceptibility to this combination. A total of 134 Enterobacterales harboring a variety of resistance genes (52 bla_KPC, 53 bla_NDM, 26 bla_KPC + bla_NDM, 2 bla_NDM + bla_OXA-48-like, 1 bla_OXA-48-like) as well as 11 Enterobacterales negative for carbapenemase genes by HRM-qPCR were evaluated for susceptibility to CZA, ATM, ATM-CZA and AZA. According to broth microdilution (BMD), 55.9% were resistant to CZA and 69.7% to ATM. All isolates were susceptible to ATM-CZA and AZA. Compared to BMD, TurbiTest from colonies (TTc) presented categorical agreement (CA) of 95.9% for CZA, 97.9% for ATM and ATM-CZA, and 99.3% for AZA. Fifty-eight spiked blood cultures were evaluated, with CA of 98.3% for CZA and ATM, and 100% for ATM-CZA and AZA. Besides, 41 clinical, consecutive Enterobacterales-positive blood cultures were tested (TTbc), with 100% of CA for CZA, 95.1% for ATM and 92.7% for ATM-CZA and AZA. TurbiTest is an inexpensive test that demonstrated a promising performance with considerably reduced turnaround time to results (less than 5 h). Interlaboratory reproducibility, as well as the performance with other bacterial population remain to be explored.