Filamentous fungal characterizations by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

Matrix‐assisted laser desorption/ionization time‐of‐flight intact cell mass spectrometry (MALDI‐TOF ICMS) is coming of age for the identification and characterization of fungi. The procedure has been used extensively with bacteria. UV‐absorbing matrices function as energy mediators that transfer the absorbed photoenergy from an irradiation source to the surrounding sample molecules, resulting in minimum fragmentation. A surprisingly high number of fungal groups have been studied: (i) the terverticillate penicillia, (ii) aflatoxigenic, black and other aspergilli, (iii) Fusarium, (iv) Trichoderma, (iv) wood rotting fungi (e.g. Serpula lacrymans) and (v) dermatophytes. The technique has been suggested for optimizing quality control of fungal Chinese medicines (e.g. Cordyceps). MALDI‐TOF ICMS offers advantages over PCR. The method is now used in taxonomic assessments (e.g. Trichoderma) as distinct from only strain characterization. Low and high molecular mass natural products (e.g. peptaibols) can be analysed. The procedure is rapid and requires minimal pretreatment. However, issues of reproducibility need to be addressed further in terms of strains of species tested and between run variability. More studies into the capabilities of MALDI‐TOF ICMS to identify fungi are required.     

Evaluation of VITEK matrix‐assisted laser desorption ionization–time of flight mass spectrometry for identification of anaerobes

Rapid and adequate identification of anaerobic bacterial species still presents a challenge for most diagnostic laboratories, hindering the selection of appropriate therapy. In this study, the identification capacity of 16S rRNA sequence analysis, VITEK 2 (BioMérieux, Lyon, France) compact analysis and VITEK MS‐mediated identification for anaerobic bacterial species was compared. Eighty‐five anaerobic bacterial isolates from 11 provinces in China belonging to 14 genera were identified by these three methods. Differences in identification between these three methods were compared. Consistent identification results were obtained for 54 (54/85, 63.5%) isolates by all three methods, the most discordant results being concentrated in Clostridium XI (n = 8) and Bacteroides fragilis (n = 9) clusters. Using the VITEK MS system, 74 (74/90, 82.2%) isolates were identified as single species consistent with 16S rRNA sequence analysis, which was significantly better than the results obtained with VITEK 2 Compact (P < 0.01). Misidentifications by the Vitek 2 Compact and Vitek MS systems were mainly observed in the Clostridium XI (n = 8)and B. fragilis clusters (n = 9). VITEK MS identified anaerobic bacteria even after they had been exposed to oxygen for a week. Identification by the Vitek MS system was more consistent with 16S rRNA sequence analysis than identification by Vitek 2 Compact. Continuous expansion of the VITEK MS database with rare described anaerobic species is warranted to improve both the efficiency and accuracy of VITEK MS identification in routine diagnostic microbiology.     

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry as a useful tool for the rapid identification of wild flea vectors preserved in alcohol

Flea identification is a significant issue because some species are considered as important vectors of several human pathogens that have emerged or re‐emerged recently, such as Bartonella henselae (Rhizobiales: Bartonellaceae) and Rickettsia felis (Rickettsiales: Rickettsiaceae). Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) has been evaluated in recent years for the identification of multicellular organisms, including arthropods. A preliminary study corroborated the usefulness of this technique for the rapid identification of fleas, creating a preliminary database containing the spectra of five species of flea. However, longterm flea preservation in ethanol did not appear to be an adequate method of storage in the context of specimen identification by MALDI‐TOF MS profiling. The goal of the present work was to assess the performance of MALDI‐TOF MS in the identification of seven flea species [Ctenocephalides felis (Siphonaptera: Pulicidae), Ctenocephalides canis, Pulex irritans (Siphonaptera: Pulicidae), Archaeopsylla erinacei (Siphonaptera: Pulicidae), Leptopsylla taschenbergi (Siphonaptera: Ceratophyllidae), Stenoponia tripectinata (Siphonaptera: Stenoponiidae) and Nosopsyllus fasciatus (Siphonaptera: Ceratophyllidae)] collected in the field and stored in ethanol for different periods of time. The results confirmed that MALDI‐TOF MS can be used for the identification of wild fleas stored in ethanol. Furthermore, this technique was able to discriminate not only different flea genera, but also the two congeneric species C. felis and C. canis.     

Natural products in Glycyrrhiza glabra (licorice) rhizome imaged at the cellular level by atmospheric pressure matrix‐assisted laser desorption/ionization tandem mass spectrometry imaging

The rhizome of Glycyrrhiza glabra (licorice) was analyzed by high‐resolution mass spectrometry imaging and tandem mass spectrometry imaging. An atmospheric pressure matrix‐assisted laser desorption/ionization imaging ion source was combined with an orbital trapping mass spectrometer in order to obtain high‐resolution imaging in mass and space. Sections of the rhizome were imaged with a spatial resolution of 10 μm in the positive ion mode, and a large number of secondary metabolites were localized and identified based on their accurate mass and MS/MS fragmentation patterns. Major tissue‐specific metabolites, including free flavonoids, flavonoid glycosides and saponins, were successfully detected and visualized in images, showing their distributions at the cellular level. The analytical power of the technique was tested in the imaging of two isobaric licorice saponins with a mass difference of only 0.02 Da. With a mass resolving power of 140 000 and a bin width of 5 ppm in the image processing, the two compounds were well resolved in full‐scan mode, and appeared with different distributions in the tissue sections. The identities of the compounds and their distributions were validated in a subsequent MS/MS imaging experiment, thereby confirming their identities and excluding possible analyte interference. The use of high spatial resolution, high mass resolution and tandem mass spectrometry in imaging experiments provides significant information about the biosynthetic pathway of flavonoids and saponins in legume species, combing the spatially resolved chemical information with morphological details at the microscopic level. Furthermore, the technique offers a scheme capable of high‐throughput profiling of metabolites in plant tissues.     

Quantification of plant surface metabolites by matrix‐assisted laser desorption–ionization mass spectrometry imaging: glucosinolates on Arabidopsis thaliana leaves

The localization of metabolites on plant surfaces has been problematic because of the limitations of current methodologies. Attempts to localize glucosinolates, the sulfur‐rich defense compounds of the order Brassicales, on leaf surfaces have given many contradictory results depending on the method employed. Here we developed a matrix‐assisted laser desorption–ionization (MALDI) mass spectrometry protocol to detect surface glucosinolates on Arabidopsis thaliana leaves by applying the MALDI matrix through sublimation. Quantification was accomplished by spotting glucosinolate standards directly on the leaf surface. The A. thaliana leaf surface was found to contain approximately 15 nmol of total glucosinolate per leaf with about 50 pmol mm^?2 on abaxial (bottom) surfaces and 15–30 times less on adaxial (top) surfaces. Of the major compounds detected, 4‐methylsulfinylbutylglucosinolate, indol‐3‐ylmethylglucosinolate, and 8‐methylsulfinyloctylglucosinolate were also major components of the leaf interior, but the second most abundant glucosinolate on the surface, 4‐methylthiobutylglucosinolate, was only a trace component of the interior. Distribution on the surface was relatively uniform in contrast to the interior, where glucosinolates were distributed more abundantly in the midrib and periphery than the rest of the leaf. These results were confirmed by two other mass spectrometry‐based techniques, laser ablation electrospray ionization and liquid extraction surface analysis. The concentrations of glucosinolates on A. thaliana leaf surfaces were found to be sufficient to attract the specialist feeding lepidopterans Plutella xylostella and Pieris rapae for oviposition. The methods employed here should be easily applied to other plant species and metabolites.     

Matrix‐assisted laser desorption/ionization mass spectrometry imaging: a powerful tool for probing the molecular topology of plant cutin polymer

The cutin polymers of different fruit cuticles (tomato, apple, nectarine) were examined using matrix‐assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) after in situ release of the lipid monomers by alkaline hydrolysis. The mass spectra were acquired from each coordinate with a lateral spatial resolution of approximately 100 μm. Specific monomers were released at their original location in the tissue, suggesting that post‐hydrolysis diffusion can be neglected. Relative quantification of the species was achieved by introducing an internal standard, and the collection of data was subjected to non‐supervised and supervised statistical treatments. The molecular images obtained showed a specific distribution of ions that could unambiguously be ascribed to cutinized and suberized regions observed at the surface of fruit cuticles, thus demonstrating that the method is able to probe some structural changes that affect hydrophobic cuticle polymers. Subsequent chemical assignment of the differentiating ions was performed, and all of these ions could be matched to cutin and suberin molecular markers. Therefore, this MALDI‐MSI procedure provides a powerful tool for probing the surface heterogeneity of plant lipid polymers. This method should facilitate rapid investigation of the relationships between cuticle phenotypes and the structure of cutin within a large population of mutants.     

One‐pot sample preparation approach for profiling spatial distribution of gibberellins in a single shoot of germinating cereal seeds

Sample preparation remains a bottleneck in the rapid and reliable quantification of gibberellins (GAs) for obtaining an insight into the physiological processes mediated by GAs. The challenges arise from not only the extremely low content of GAs in complex plant matrices, but the poor detectability of GAs by mass spectrometry (MS) in negative ion mode. In an effort to solve these urgent difficulties, we present a spatial‐resolved analysis method to investigate the distribution of GAs in tiny plant tissues based on a simplified one‐pot sample preparation approach coupled with ultrahigh‐performance liquid chromatography‐tandem MS. By integrating extraction and derivatization into one step, target GAs were effectively extracted from plant materials and simultaneously reacted with N‐(3‐dimethylaminopropyl)‐N′‐ethylcarbodiimide, the sample preparation time was largely shortened, the probability of sample loss was minimized and the detection sensitivity of MS was also greatly improved compared with underivatized GAs. Under optimal conditions, the method was validated from the quantification linearity, limits of detection and limits of quantification in the presence of plant matrices, recoveries, and precision. With the proposed method, 15 endogenous GAs were detected and, among these, 11 GAs could be quantified in 0.50 mg fresh weight (FW) wheat shoot samples, and five GAs were quantified in only 0.15 mg FW developing seed samples of Arabidopsis thaliana. The distribution patterns of GAs along both the non‐13‐hydroxylation pathway and the early 13‐hydroxylation pathway in a single shoot of germinating wheat, rice and maize seeds were finally profiled with a spatial resolution down to approximately 1 mm².