Identification of Rare Bacterial Pathogens by 16S rRNA Gene Sequencing and MALDI-TOF MS

The overall goal of this MALDI-TOF Mass Spectrometry protocol is to improve typing of rare pathogens through a rapid and cost-effective procedure. This method can help to answer key questions in the field of diagnostic microbiology, it allows rapid edification of rare bacterial pathogens. Today, we want to talk about odoratimimus and bacteria of the genus Myroides.

To demonstrate the method and to talk about the interpretation of the results. Generally, individuals new to this method might find it difficult since transfer of bacteria, overlay with metrics, and interpretation of results require technical skills and microbiological expertise. We first had the idea to use MALD-TOF MS to screen for rare bacteria when we had a patient in our hospital suffering from a diabetic food infection caused by myroides odoratimus.

Begin this protocol with the extraction of bacterial DNA, 16S ribosomal DNA PCR and Sanger sequencing as described in the text protocol. Use a wooden toothpick to transfer a single bacterial colony to a well of a 96 well steel target. Spot one microliter of 70%formic acid on top of the air-dried organism on the steel target, and leave to dry for approximately two to three minutes.

Overlay the spot with one microliter of matrix solution containing 50 mg/ml of CHCA in an organic solvent composed of 50%acetonitrile, 2.5 trifluoroacetic acid, and 47.5%water. Let the smear and overlaid matrix air dry for about five minutes in a hood. To introduce the samples, aerate the sample loading port of the mass spectrometer by pressing the in/out button of the instrument.

Open the loading port and insert the MALDI-TOF MS target plate with the dried bacterial smear plus matrix. Close the port and evacuate again. Observe the vacuum and when it reaches 4.5 million millibar start the measurement.

Open the MMIS analysis software and click the file menu. Select New Classification”and a new window called MALDI Biotyper Real Time Classification Wizard”should open. Type the project name in the field Project Name”and click New, under the new dialogue box named New Project, check the project name and proceed by clicking the Okay”button.

In the MALDI Biotyper Real Time Classification Wizard, observe the Analyte placement window. Select target positions, right click any selected position, and select Add samples. In the automatically opened table view, type in the sample name, sample ID, and optionally, a comment.

Click the Next”button to proceed. Next, observe the selection of MALDI Biotyper methods window. Select Bruker Taxonomy”from the MSPs from Taxonomy Trees and click Next”to continue.

Then, observe the project summary window. Check the inserted entries given above for the actual classification project. Start the classification run by clicking the Finish”button.

Measurements start automatically when the appropriate vacuum is reached. Follow the classification run until the measurement is successfully completed. Once the measurement is complete, open the menu View”and click Results”to view the results table in the MALDI Biotyper Real Time Classification Project One.

View the Bruker-Daltonik MALDI Biotyper classification results as an html file in the default web browser. Print and save the results. Shown here are spectra obtained by MALDI-TOF MS that are measured and classified in real time.

The spectra are assigned a species name and a score value reflecting the quality of the measurement. Scores above 2.300 represent a highly-probable species identification. A score between 2.000 and 2.300 indicates a secure species identification.

A score below 1.700 and 2.000 designates a probable species identification. And a score below 1.700 is non-reliable. The identification of bacterial pathogens grown on a pure culture on a solid medium takes two to three minutes.

It is important to remember that rare bacteria pathogens can only be found with MALDI-TOF MS when the instruments reliably contains the appropriate spectra. Otherwise, it has to be expunged by integrating MaSDA spectra from typed strains of their characterized isolates. In the case where score results do not lead to an unequivocal strain identification, further methods, such as biochemical profiling or 16S rDNA sequencing can be applied.

Though this method can provide insight into bacterial detection, it can also be applied to other microorganisms such as fungi or anaerobes. Furthermore, bacteria-resistance can be measured. Don’t forget working with a matrix which is a hazardous substance, wear gloves, protective goggles, and work under a hood.

Furthermore, it is absolutely necessary to dry the matrix and smear thoroughly to avoid splash overs in the analyzation chamber.