Summation of peaks and L34 ribosomal protein in the presence and absence of antibiotics enables susceptibility testing using MALDI-TOF mass spectrometry in 2h from Escherichia coli-positive blood cultures.

INTRODUCTION: We have developed a MALDI-TOF-mediated phenotypic method, which determines antibiotic susceptibility (AS) from positive blood cultures (BCs) in 2h. We developed a software for process automation. We report results on Escherichia coli-positive BCs with cefotaxime (CTX) and ciprofloxacin (CIP). METHODS: We studied CIP and CTX activity in 18 and 17 real E. coli-positive BCs, and in 56 and 45 spiked BCs, respectively. Positive BCs were incubated for 2h without any antibiotics, and with 2mg/l and 4mg/l of CIP and CTX. The extraction was performed using ethanol/formic acid. Spectra were processed with specifically developed software which compares the peaks' intensity and the size of specific peaks. RESULTS: The set cut-off point was a 3-fold decrease in the summation of all peaks and/or the 5382m/z peak value (ribosomal protein L34). In simulated BCs, the correlation of CIP 2mg/l and 4mg/l with Etest® was 94.6% and 98.2%, respectively; for CTX 2mg/l and 4mg/l, this correlation was 95.6%. In real BCs, the correlations were 100% for CIP (2mg/l and 4mg/l) and 88.2% and 94.1% for CTX 2mg/l and 4mg/l, respectively. Resistant isolates were always correctly classified. CONCLUSION: This method provides accurate, fast and inexpensive AS information. The method can be automated, making it easier to implement in a microbiology laboratory routine.