Comparison of MALDI-TOF MS and AFLP for strain typing of ESBL-producing Escherichia coli.

Typing of bacterial isolates using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) potentially provides an efficient on-site method to monitor the spread of antibiotic-resistant bacteria and rapidly detect outbreaks. We compared MALDI-MS typing results to those of amplified fragment length polymorphism (AFLP) in a collection of 52 ESBL-producing Escherichia coli, isolated in a Dutch nursing home with an on-going outbreak of ST131 E. coli. Specific MALDI types were defined based on spectral data from four replicate colony samples of isolates grown on Columbia agar using multivariate statistical procedures. Type-specific superspectra were computed for four E .coli MALDI-types and tested for the potential of rapid and automated typing. The effect of different incubation conditions on typing performance was tested by analysing five isolates incubated for 24 h and 48 h on five different media. Types defined based on MALDI spectra were largely in agreement with the AFLP results, although some MALDI types comprised of more than one AFLP type. In particular, isolates belonging to ST131 showed distinct mass patterns. The proportion of isolates correctly assigned was substantially lower for isolates incubated on Sabouraud-dextrose and Drigalski agars for 24 h, and for those incubated for 48 h (all media). Our results show that the identification of type-specific peaks potentially allows direct typing of isolates belonging to specific clonal lineages. Both incubation time and media affected type assignment, suggesting that there is a need for a careful standardization of incubation time and culturing conditions when developing MALDI-typing schemes for E. coli.