MALDI-TOF MASS SPECTROMETRY

Matrix-assisted laser desorption/ionization (MALDI) is a very soft ionization technique in that large molecules, such as proteins, can be analyzed without fragmentation of the molecules. For this the sample is mixed and crystallized with a matrix solution that protects the target molecules when submitted to shots from a laser beam, absorbing the applied energy. Following this, the matrix ions transfer the energy to the sample molecules, resulting in ions of low charge (+1/ + 2). Subsequently, the ions produced are separated in an electric field with relation to their mass/charge (m/z) ratio by a time of flight (TOF) analyzer.

TABLE 2.1 Bacterial Food Contaminants Studied by Fingerprinting Techniques With the Corresponding References

Species

Method

Reference

Acinetobacter baumanii

MALDI-TOF MS

Alvarez-Buylla etal. (2012), Sousa etal. (2014)

Bacillus spp.

MALDI-TOF MS

Branquinho et al. (2014), Fernandez-No et al. (2013)

SERS

Patel et al. (2008)

Campylobacter spp.

MALDI-TOF MS

Bessede et al. (2011), Kolfnska et al. (2008), Mandrell et al. (2005), Zautner et al. (2013)

Clostridium spp.

MALDI-TOF MS

Grosse-Herrenthey et al. (2008), Reil et al. (2011)

FT-IR

Kirkwood et al. (2006)

Escherichia coli

MALDI-TOF MS

Christner et al. (2014), Clark etal. (2013), Khot and Fisher (2013), Matsumura etal. (2014), Novais etal. (2014), Siegrist et al. (2007)

FT-IR

Al-Qadiri et al. (2006)

Raman/SERS

Cho et al. (2015), Yang and Irudayaraj (2003)

LIBS

Barnett et al. (2011), Diedrich et al. (2007), Marcos- Martinez et al. (2011), Mohaidat et al. (2011), Multari etal. (2013)

Legionella spp.

MALDI-TOF MS

Gaia et al. (2011), Moliner et al. (2010), Pennanec et al. (2010)

Listeria spp.

MALDI-TOF MS

Barbuddhe etal. (2008), Jadhav etal. (2014)

FT-IR

Al-Holy et al. (2006), Janbu et al. (2008), Rebuffo et al. (2006)

SERS

Green et al. (2009)

Salmonella enterica

MALDI-TOF MS

Dieckmann etal. (2008), Dieckmann and Malorny (2011), Kuhns et al. (2012), Sparbier et al. (2012)

FT-IR

Al-Qadiri et al. (2008), Baldauf et al. (2006), Mannig et al.

(2008)

SERS

Duan et al. (2015)

LIBS

Barnett etal. (2011), Marcos-Martinez etal. (2011), Multari et al. (2013)

(Continued)

TABLE 2.1 (Continued)

Species

Method

Reference

Shigella spp.

MALDI-TOF MS

Khot and Fisher (2013)

Staphylococcus aureus

MALDI-TOF MS

BOhme et al. (2012), Carbonnelle et al. (2007), Du et al. (2002), Dubois et al. (2010), Jackson et al. (2005), Josten et al. (2013), Szabados et al. (2010), Wolters et al. (2010)

FT-IR

Amiali et al. (2011)

Raman

Harz et al. (2005)

LIBS

Barnett et al. (2011)

Vibrio spp.

MALDI-TOF MS

Dieckmann et al. (2010), Erler et al. (2015), Hazen et al. (2009)

Yersinia enterocolitica

MALDI-TOF MS

Ayyadurai et al. (2010), Lasch et al. (2010), Stephan et al. (2011)

FT-IR

Kuhm etal. (2009)

MALDI-TOF MS emerged as a new tool for bacterial differentiation due to its speed, simplicity, and cost effectiveness. Whole bacterial cells can be analyzed directly without any sample pretreatment and the resulting spectral profiles are highly specific, representing a fingerprint for the corresponding organism (Clark et al., 2013). Furthermore, culturing conditions and growth media have been shown to influence the fingerprints to a lower extent, allowing the comparison and identification of bacterial strains even if different protocols have been applied. With MALDI-TOF MS soluble and low-weight proteins (1,500—20,000 Da) are detected and the majority have been associated with ribosomal proteins and to a lower degree with structural proteins, such as cold-shock proteins and DNA-binding proteins (Ryzhov and Fenselau, 2001). The number of studies aimed at the application of MALDI- TOF MS fingerprinting to microbial identification has increased significantly in the last decades and several detailed reviews give an overview of the methodology, sample preparation and application to bacteria, yeasts, and fungi (Clark et al., 2013; Welker and Moore, 2011). The applicability of MALDI- TOF MS fingerprinting to clinical routine analysis has been demonstrated in a number of studies, achieving 92—98% of correct species identification. This is a significantly better result than that obtained with commonly applied microbial identification tools. MALDI-TOF MS has been compared to conventional phenotypic and molecular methods, highlighting the higher discrimination potential, in addition to the speed and cost effectiveness of the proteomic approach. More recently, MALDI-TOF MS fingerprinting has also been applied to the detection of foodborne pathogens isolated from food products (Bohme et al., 2013).

Bacterial identification by MALDI-TOF MS is carried out by comparing the spectral profile to a previously created library of reference spectra. The whole spectral profile is representative for the corresponding strain and the determination of a number of characteristic peaks allows the classification of the strain at the genus, species, and even subspecies or strain level.

 
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