Page 93 - FoodFocusThailand No.199 October 2022

STRONG QC & QA

Yagel, 2021). Moreover, further database updates and
optimization of the workflows and procedures can increase
the accuracy of the method. It has been reported that
identifying some species within the family of
Enterobacteriaceae by using MALDI-TOF MS is more
accurate than 16S rRNA gene sequence analysis which is
a highly reputable species identification method.
Functions and Workflow MALDI-TOF MS is an
analyzer based on mass spectroscopy. The machine is
comprised of three parts, including (1) an ionizer or ion
source to generate the ion by using a technique called MALDI
(Matrix-Assisted Laser Desorption/Ionization), (2) a mass
analyzer, and (3) a detector. For routine identification of
MALDI-TOF MS workflows, a single colony is placed on the
target plate, air-dried, and coated with a specific matrix which
co-crystallizes with the bacterial sample. The target plate is
then inserted into the machine, where a vacuum is
established. Subsequently, a laser beam is fired onto the
sample-matrix crystals, evaporating and ionizing the
analytes in the sample. The ions are accelerated in
electrostatic potential, depending on the mass and the
charge of the ionized peptides. After acceleration, the ions
enter a vacuum flight tube and are further separated
according to their m/z ratio and then detected at the end of
the tube. The m/z ratio of an ion is determined by measuring
the time. According to this time of flight (TOF) information,
a spectrum of m/z values is generated and is a unique
spectrum. These characteristic spectra are called Peptide
Mass Fingerprints (PMF). The PMF of the unknown sample
is then compared to a database containing a set of the PMF
from well-characterized microbial samples.
The matrix is an energy-absorbent organic compound
that co-crystalizes with the microbial sample. Examples of
matrices are alpha-4-cyano-4-hydroxycinnamic acid
(HCCA), and 5-chloro-2-mercaptobenzothiazole. They are
suitable for identifying Gram-negative and Gram-positive
bacteria, respectively.
Species identification using MALDI-TOF MS is a highly
rapid technique. The average time from picking up the single
colony from the cultured media to getting the PMF is
approximately 10-20 seconds. After that, it takes about
15-30 seconds to compare the unknown PMF to a set of
PMF in a database and get the result of species identification
in less than 5 minutes. It is cost-effectiveness 7-10 times
lower than the conventional method. Furthermore, it
remarkably reduces the amount of microbiological media;
thus, it is environmentally friendly.
Application of MALDI-TOF MS in food industry and
food microbiological research Currently, MALDI-TOF
MS is widely applied for rapid routine diagnostics in
laboratory hospitals and species identification for laboratories
in the food industry. It can potentially be used in the genus
and species identification of microorganisms, including

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