Fig 2.

Differentiation of ampicillin-sensitive and -resistant E. coli strains by MAAST. Sixty E. coli strains were obtained from clinical isolates and previously identified and characterized by a Vitek 2 system, cryo-conserved, and subcultured on Luria-Bertani agar plates for 24 h at 37°C prior to MAAST. (A) The bacterial mass corresponding to a single colony was used for analysis. Using this low inoculum, ESBL and ampicillin-resistant non-ESBL strains showed a significantly higher drug/metabolite ratio than ampicillin-susceptible strains without overlap. Additional, ESBL strains showed a significantly lower drug/metabolite ratio than ampicillin-resistant non-ESBL strains. (B) Using the bacterial mass corresponding to 3 colonies, ESBL and ampicillin-resistant non-ESBL strains showed a significantly higher drug/metabolite ratio than ampicillin-susceptible strains without overlap. (C) Bacteria in positive BC flasks of 24 clinical specimens were biotyped as E. coli using MALDI-TOF MS and used for analysis. MAAST results were retrospectively compared to results of classical AST with a Vitek 2 system. As seen in for subcultures, direct MAAST from BC showed an accuracy of 100% for the detection of β-lactamase activity. #, P < 0.005 versus ampicillin sensitive; ‡, P < 0.05 versus β-lactamase positive (Student's t test).