Top 10 Innovations for 2013
#3 Mass Spectrometry for Bacterial Identification
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 technology to provide rapid organism
identification that is more accurate and less expensive than current biochemical
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 mass
spectrometry 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 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
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 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.