Identification performance of MALDI-ToF-MS upon mono- and bi-microbial cultures is cell number and culture proportion dependent

Abstract

Biotyping using matrix-assisted laser desorption ionization-time of flight (MALDI-ToF) mass spectroscopy (MS) has revolutionized microbiology by allowing clinicians and scientists to rapidly identify microbes at genus and species levels. The present study extensively assesses the suitability and reliability of MALDI-ToF biotyping of 14 different aerobic and anaerobic bacterial species as pure and mixed cultures. Reliable identification at species level was possible from biomaterial of older colonies and even frozen biomaterial, although this was species dependent. Using standard instrument settings and direct application of biomaterial onto the MALDI-ToF target plates, it was determined that the cell densities necessary for completely reliable identification of pure cultures varied between 2.40 × 108 and 1.10 × 1010 viable cell counts (VCCs) per mL, depending on the species. Evaluation of the mixed culture algorithm of the Bruker Biotyper® software showed that the performance of the algorithm depends greatly on the targeted species, on their phylogenetic distance, and on their ratio of VCC per mL in the mixed culture. Hence, the use of MALDI-ToF-MS with incorporation of the mixed culture algorithm of the software is a useful pre-screening tool for early identification of contaminants, but due to the great variability in performance between different species and the usually unknown percentage of the possible contaminant in the mixture, it is advisable to combine this method with other microbiology methods.Electronic supplementary materialThe online version of this article (10.1007/s00216-019-02080-x) contains supplementary material, which is available to authorized users.

Rapid direct detection of pathogens for diagnosis of joint infections by MALDI-TOF MS after liquid enrichment in the BacT/Alert blood culture system

“…Five out of 37 samples (14%) contained mixed-species cultures of either Staphylococcus aureus combined with different enterobacteria, or Pseudomonas aeruginosa combined with Entercoccus faecieum (4 samples of the same patient). Mixed cultures have previously been shown to be problematic, where the ration of the organisms is unequal [ 17 , 18 ]. Among our samples, only in one the result indicated a mixed culture with both species, in the other four only a single species was found with high score values, however, plating of the culture indicated no more than 4-fold differences in cfu between each of the two species present.…”

Rapid identification of respiratory bacterial pathogens from bronchoalveolar lavage fluid in cattle by MALDI-TOF MS

“…Since the concentration of pathogens in bovine nBAL samples is generally considered low (average of 1 × 10 4 CFU/mL 20 ), and the MALDI-TOF MS technique requires high bacterial counts to provide reliable results 21 (a minimum of 1 × 10 7 –1 × 10 8 CFU/mL for Pasteurellaceae , data not shown), an incubation step in growth medium is mandatory. Therefore, 2 different growth media were examined.…”

“…Considering the average concentration of target pathogens in bovine BALf samples varies around 1 × 10 4 CFU/mL 20 , whereas MALDI-TOF MS requires a high concentration for reliable identification of pathogens 21 (minimum 1 × 10 7 –1 × 10 8 CFU/mL, data not shown), and considering a doubling time of approximately 30 minutes, a selective enrichment step of maximum 6 hours was deemed necessary to obtain reliable MALDI-TOF identification and still practically feasible for a single-day protocol.…”

The Use of MALDI-TOF Mass Spectrometry to Analyze Commensal Oral Yeasts in Nursing Home Residents

“…The accumulation of MS profiles for reference fungal isolates means that MALDI-TOF MS is an effective identification method for clinical examination. The Bruker database comprises 604 references for yeast and fungus and includes almost all known pathogenic fungi [ 46 ], but mix-cultures are usually misidentified in term of the microorganisms, and only monomicrobial cultures can be specifically identified [ 47 ]. In clinical, the limitation of MALDI-TOF MS is to identify different species in a sample with mix species.…”

“…In clinical, the limitation of MALDI-TOF MS is to identify different species in a sample with mix species. Recently, mix-microbial identification by MALDI-TOF MS has used specific algorithms to determine the mass spectra biomarkers for microorganisms [ 47 ]. Meanwhile, liquid chromatography–tandem MS is also used to identify mix-microbial samples [ 48 , 49 ].…”