Advanced Techniques in Diagnostic Microbiology

7. MALDI-TOF MS 121
for analysis by MALDI-TOF MS and is referred to as SELDI-TOF MS. The target
plates can be hydrophobic, hydrophilic, electrophilic, and so forth, depending on
the separation required, and has proved useful in exploiting the difference of closely
related organisms (Lancashire et al., 2005).
The simplest and most rapid preparation technique, however, is the direct deposition
of the bacterial cells from the culture plate onto the MALDI target plate,
followed by addition. Immediate co-crystallization of the sample enables the analysis
of mainly surface proteins and produces a more selective spectral pattern. For
all these preparation techniques, the unique spectral fingerprint can be used to identify
a bacterium by comparison of unknown with known bacterial fingerprints. For
this to be successfully achieved, however, the fingerprints must be derived using
the same standardized protocol (van Baar, 2000; Keys et al., 2004).
Application of MALDI-TOF MS for Rapid Identification
of Bacteria
All the above mass spectrometry techniques are capable of bacterial identification.
Acquisition of the mass spectra in each case is very rapid, making all the
candidate techniques feasible for rapid data acquisition and analysis. However, for
the majority of techniques, the sample preparation from pure culture is complex
and time consuming and can often involve the use of costly chemical kits specifically
designed for the technique. Furthermore, interpretation of the mass spectral
data often requires the specialist knowledge normally residing in research institutions
or specialized contract laboratories. Consequently, some techniques are
therefore unsuitable for many routine microbiology laboratories. One technique,
however, offers a rapid, simple sample preparation and analysis, and because the
mass spectrometer is fully automated, provides a strong candidate technique for
rapid bacterial identification in the more routine microbiology laboratory. The
use of whole bacterial cells for MALDI-TOF MS, in which the spectral fingerprint
of an unknown bacterial cell is compared with a database of known library
fingerprints, offers the most attractive solution for rapid bacterial identification
(Table 7.1). Currently, there is only one system in which the mass spectral acquisition
is fully automated and the data acquired searched seamlessly against a fully
curated database from validated bacterial strains. This is the Microbelynx bacterial
identification system (Waters Corporation, Manchester, UK). The following section
therefore focuses upon the MicrobeLynx system with respect to its suitability
for rapid routine bacterial identification.
MicrobeLynx System for Automatic Bacterial Identification
The MirobeLynx rapid bacterial identification system has been developed in collaboration
between Manchester Metropolitan University (MMU; Manchester, UK),
the Molecular Identification Service Unit of the Health Protection Agency (MISU;
London, UK), and the Waters Corporation (Manchester, UK). The system has