Mass Spectrometry Identifies Bacterial and Yeast Isolates

Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) is an automated molecular platform that can be used to identify pathogens.

The MALDI-TOF technique effectively circumvents many of the drawbacks of other methods and offers a rapid, straightforward, and inexpensive method to distinguish bacteria and yeasts isolates from human samples.

Scientists at the Mayo Clinic’s Department of Laboratory Medicine and Pathology (Rochester, MN, USA) have recently validated the use of MALDI-TOF in the clinical laboratory. Currently, they only perform MALDI-TOF MS analysis from isolated bacterial and yeast colonies, therefore clinical specimens continue to be plated to solid media and are observed for growth. They use the process referred to as the formic acid-based, direct on-plate preparation method and depending on room humidity, has a total preparation time of 10 to 20 minutes for 24 samples.

The Bruker Biotyper MALDI-TOF MS (Fällanden, Switzerland) system is used for routine identification of bacterial and yeast isolates from culture. The Bruker Biotyper system includes the Microflex LT/SH MS instrument and two software programs: FlexControl for acquisition of protein spectra and Biotyper real-time classification (RTC) for automated spectral analysis.

The team has evaluated 900 bacterial and yeast isolates, with an overall correct identification rate of 94.2% to the genus and 79.2% to the species levels. While genus level identification was above 90% for most of the evaluated organism groups, the lower species level identification percentage was primarily driven by the gram-positive cocci (69.5%). Some closely related bacterial species cannot be differentiated by MALDI-TOF MS, regardless of the system manufacturer. This technology cannot reliably distinguish Escherichia coli from Shigella species or Streptococcus pneumoniae from S. mitis group species.

Identification of unknown isolates by this technology is based on the acquisition of unique protein profiles from isolated colonies and comparison of this data to a library of reference spectra derived from well-characterized isolates. The Mayo scientists have shown that has shown that MALDI-TOF MS is a rapid and inexpensive method able to accurately identify isolates within minutes, compared to the more time-consuming methods associated with other automated microbial identification systems and the potentially subjective nature of classic biochemical analyses.

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