#3 Mass Spectrometry for Bacterial Identification

Overview

Clinical microbiology laboratories perform essential tests that allow physicians to diagnose and treat bacterial infections. Specimens such as blood, cerebrospinal fluid, or urine are collected from patients with a suspected infection and sent to the laboratory for culture. The specimen is inoculated on an agar plate, incubated and examined daily for growth. Each type of colony growing on the plate must be identified and classified as normal flora or a possible pathogen responsible for the infection.

In this age of advanced medical technology, identification of bacteria growing in culture can still require days or weeks.

However, clinical microbiology laboratories throughout the world are now implementing new mass spectrometry (MS) technology to provide rapid organism identification that is more accurate and less expensive than current biochemical methods.

 Mass spectrometry has been used for the past 50 years to ionize and then identify molecules by determining their individual mass-to-charge ratio. However, early MS ionization methods were destructive and could not be used to analyze large molecules such as proteins. 

In 2002, the Nobel Prize was awarded for the development of a soft ionization technique called matrix assisted laser desorption/ionization, or MALDI. The major benefit of MALDI is that it does not fragment large molecules. A special matrix solution protects large molecules from fragmentation by absorbing photonic energy from the laser in a process that is known as “desorption.”

When used to determine the composition of a sample, for example, single charged molecules are created and they travel in a tube towards a detector. The time of flight (TOF) of these molecules is directly proportional to mass and this is what’s used to calculate a mass-to-charge ratio. Charting of the mass-to-charge ratios for individual ions creates a series of peaks called a spectrum, which is then compared to a reference database. Identifications generated using MALDI-TOF mass spectrometry have an accuracy that is similar to molecular sequencing methods.

Using one of the two MALDI-TOF mass spectrometry systems currently available in the United States is very simple. A small amount of bacterial growth from a culture plate is applied to a target plate and covered with a drop of matrix solution. The target is placed in the instrument where a laser shoots short pulses of light and irradiates the sample to create ions inside the instrument’s vacuum chamber. The time of flight before detection of these electrically charged particles is based on their particular masses and is used to create the spectrum or “signature.”

 Computer software compares the spectrum to a database and if there is a match, the identification is generated within minutes of ionization. The cost for labor and reagent needed to generate this highly accurate identification is approximately $.50.

Rapid organism identification now allows clinicians to prescribe the most appropriate treatment sooner and de-escalate therapy from broad-spectrum agents that drive antimicrobial resistance. At a time when bacterial infections account for a large proportion of people admitted to hospitals each year—as well as some acquired by patients already under medical care—quick and accurate detection of these microorganisms to help guide appropriate patient treatment and improve outcomes is more critical than ever.

The use of MALDI-TOF mass spectrometry to provide more accurate identifications of bacteria in minutes—rather than days—is a major advance in treating infections.

Where Are They Now

With recent advances in MALDI mass spectrometry, the process of identifying the causative bacteria in infection cases has become faster and more accurate, resulting in more efficient diagnoses. Mass spectrometry can be used for many different applications and it is this flexibility that has made the process so popular in the analytical world. The global market value is expected to rise by USD 2,360 million between 2015 and 2020.

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