Analysis Of Mass Spectra Research Articles

Identification of Shiga-Toxigenic Escherichia coli outbreak isolates by a novel data analysis tool after matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

The fast and reliable characterization of bacterial and fungal pathogens plays an important role in infectious disease control and tracking of outbreak agents. DNA based methods are the gold standard for epidemiological investigations, but they are still comparatively expensive and time-consuming. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a fast, reliable and cost-effective technique now routinely used to identify clinically relevant human pathogens. It has been used for subspecies differentiation and typing, but its use for epidemiological tasks, e. g. for outbreak investigations, is often hampered by the complexity of data analysis. We have analysed publicly available MALDI-TOF mass spectra from a large outbreak of Shiga-Toxigenic Escherichia coli in northern Germany using a general purpose software tool for the analysis of complex biological data. The software was challenged with depauperate spectra and reduced learning group sizes to mimic poor spectrum quality and scarcity of reference spectra at the onset of an outbreak. With high quality formic acid extraction spectra, the software’s built in classifier accurately identified outbreak related strains using as few as 10 reference spectra (99.8% sensitivity, 98.0% specificity). Selective variation of processing parameters showed impaired marker peak detection and reduced classification accuracy in samples with high background noise or artificially reduced peak counts. However, the software consistently identified mass signals suitable for a highly reliable marker peak based classification approach (100% sensitivity, 99.5% specificity) even from low quality direct deposition spectra. The study demonstrates that general purpose data analysis tools can effectively be used for the analysis of bacterial mass spectra.

Comparative Studies on Physicochemical Properties of Bovine Serum Albumin–Glucose and Galactose Conjugates Formed by Glycation Combined with Ultrasonic Pretreatment

Abstract The effects of ultrasonication on the physicochemical properties of bovine serum albumin (BSA)–glucose and the galactose conjugates formed by glycation was investigated. Fourier transform ion cyclotron resonance mass spectra analysis showed that the conjugates had a higher molecular weight than the native and ultrasonicated BSA. Ultrasonicated BSA had significantly higher emulsifying and foaming properties than native BSA. The browning intensity, surface hydrophobicity, emulsifying property and foaming capacity of the conjugates were substantially improved while the free amino groups, intrinsic fluorescence emission and foaming stability were decreased compared to native and ultrasonicated BSA. The results of this study indicate that ultrasonication is an efficient technique to improve the physicochemical properties of proteins. The glycation between ultrasonicated BSA and monosaccharide can effectively improve the physicochemical properties of BSA, and the glycation rate order is galactose \gt glucose. It also shows the critical role of monosaccharide conformational changes in improving the glycation and physicochemical properties of proteins.

Constructing new pseudoscalar meson nonets with the observed X(2100) , X(2500) , and η(2225)

Stimulated by the BESIII observation of $X(2100)$, $X(2500)$, and $\eta(2225)$, we try to pin down new pseudoscalar meson nonets including these states. The analysis of mass spectra and the study of strong decays indicate that $X(2120)$ and $\eta(2225)$ associated with $\pi(2070)$ and the predicted kaon $K(2150)$ may form a new pseudoscalar meson nonet. In addition, more experimental data for $X(2100)$ are necessary to determine its structure of nonets. Then, $X(2500)$, $X(2370)$, $\pi(2360)$, and the predicted kaon $K(2414)$ can be grouped into another new nonet. These assignments to the discussed pseudoscalar states can be further tested in experiment.

Hyper-EMG: A new probability distribution function composed of Exponentially Modified Gaussian distributions to analyze asymmetric peak shapes in high-resolution time-of-flight mass spectrometry

A new probability distribution function (PDF) called hyper-Exponentially Modified Gaussian (hyper-EMG) is introduced for the analysis of high-resolution spectra from multiple-reflection time-of-flight mass spectrometers. The hyper-EMG consists of a central Gaussian distribution modified by multiple exponential tails with different strengths at one or both sides. The basic statistical properties of the new PDF are given and the analysis of mass spectra containing separated and overlapping peaks is presented. The main requirement is to accurately determine the positions and areas of the individual mass peaks. From the distances of positions the mass values can be determined, from the areas the population of different ground and isomeric states can be obtained. The hyper-EMG has been applied to high-resolution time and mass spectra characterized by mass resolving powers of 140,000 and 520,000 obtained with Cs+133 and K+39 ions, respectively. From the measured mass distribution of K+39 ions, an overlapping distribution of two peaks with an area ratio of 1:10 and a mass difference of 2.6ppm (parts-per-million) is generated and analyzed. The results reveal significant advantages of the new PDF for the evaluation of overlapping distributions for accurate mass and area determinations compared with commonly used PDFs which are more than one order of magnitude less accurate. It is obvious that the hyper-EMG can be favorably applied also to other fields.

Enzymatic O-GlcNAcylation of α-synuclein reduces aggregation and increases SDS-resistant soluble oligomers.

Neurodegenerative diseases including dementia with Lewy bodies, Lewy body variant of Alzheimer's disease, and Parkinson's disease are associated with the aberrant aggregation of α-synuclein, which is influenced by several post-translational modifications (PTMs). O-GlcNAcylation is one PTM that has an important role in many fundamental processes. The O-GlcNAcylation of endogenous α-synuclein at residues 53, 64, 72 and 87 has been reported in an unbiased mass spectrum analysis. The consequences of O-GlcNAcylation at residues 72 or 87 have been studied by using a synthetic α-synuclein bearing O-GlcNAcylation at threonine residue 72 or serine 87, respectively. O-GlcNAcylation at Thr72 or Ser87 suppresses the aggregation of α-synuclein. However, the effect of enzymatic O-GlcNAcylation of α-synuclein at multiple residues is not clear. Here, we successfully generated O-GlcNAcylated α-synuclein by co-expressing a shorter form of OGT (sOGT) with α-synuclein. The O-GlcNAcylation inhibited α-synuclein aggregation and promoted the formation of soluble SDS-resistant and Thioflavine T negative oligomers. Our data warrant further studies on the role of O-GlcNAcylation in the progression/treatment of Parkinson's disease in animal models.

Abstract 912: ADAM9-plectin interaction involved in prostate cancer malignancy progression

Abstract Introduction: To identify a specific biomarker for malignant prostate cancer progression and therapeutic target, we had demonstrated the correlation of ADAM9 expression and cancer progression. The aim is to clarify the association of ADAM9 and interactive proteins in malignant phenotypic progression. Methods: Knockdown of ADAM9 expression was conducted to explore its role in cell motility and metastasis activities. Pull-down assays were performed to identify the proteins that interact with ADAM9 during cancer cell migration. Proximity ligation assay (PLA) was carried out to validate the protein-protein interaction. Transmission electron microscope (TEM) analysis to confirm ADAM9 and plectin interaction. Protein degradation test was conducted to determine the regulation pathway. Results: Our study demonstrated that ADAM9 play an important role in the regulation of prostate cancer migration, invasion and metastasis in the in vitro assay and xenograft animal models. The combination of ADAM9 pull-down assays and mass spectrum analysis demonstrated hemidesmosome components, plectin, ladinin, and cytoskeleton were pull-down with ADAM9. However, the key element of hemidesmosome, integrin α6β4, did not pull-down together. Both TEM and PLA confirmed the interaction of ADAM9 and plectin in the protrusion end of migrating cells. This interaction can be strengthened after knockdown integrin β4; and vice verse, knockdown of ADAM9 increased interaction of integrin β4 and plecin. These results indicated ADAM9 and integrin β4 compete for plectin binding. Furthermore, shADAM9 attenuated the degradation of integrin β4. Clinical evaluation of 96 patient samples in TMU demonstrated increased of ADAM9 and plectin interaction in tumor region when compared to the benign part of the same patient. Conclusions: Our studies demonstrated that ADAM9 promotes cancer metastasis through alteration of hemidesmosome dynamic by enhancing of integrin β4 degradation. Therefore, these results indicated possible to use ADAM9-plectin interaction to predict prostate cancer malignancy progression. Citation Format: Che-Ming Liu, Chia-Ling Hsieh, Shian-Ying Sung. ADAM9-plectin interaction involved in prostate cancer malignancy progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 912. doi:10.1158/1538-7445.AM2017-912

The Mass Spectra Analysis for α-Ionone and β-Ionone

The presented work of this paper will be the first of a series of MS spectral analysis for some selected well-known compounds. In here the target molecules to have their MS spectra analyzed are α-Ionone and β-Ionone. The analysis will be in detail and show the mechanism of how the major fragments are formed in the mass spectrophotometer; hence, this paper is for educational purposes that could serve both students and instructors.

Rapid Differentiation of Haemophilus influenzae and Haemophilus haemolyticus by Use of Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry with ClinProTools Mass Spectrum Analysis.

Haemophilus influenzae is associated with severe invasive disease, while Haemophilus haemolyticus is considered part of the commensal flora in the human respiratory tract. Although the addition of a custom mass spectrum library into the matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) system could improve identification of these two species, the establishment of such a custom database is technically complicated and requires a large amount of resources, which most clinical laboratories cannot afford. In this study, we developed a mass spectrum analysis model with 7 mass peak biomarkers for the identification of H. influenzae and H. haemolyticus using the ClinProTools software. We evaluated the diagnostic performance of this model using 408 H. influenzae and H. haemolyticus isolates from clinical respiratory specimens from 363 hospitalized patients and compared the identification results with those obtained with the Bruker IVD MALDI Biotyper. The IVD MALDI Biotyper identified only 86.9% of H. influenzae (311/358) and 98.0% of H. haemolyticus (49/50) clinical isolates to the species level. In comparison, the ClinProTools mass spectrum model could identify 100% of H. influenzae (358/358) and H. haemolyticus (50/50) clinical strains to the species level and significantly improved the species identification rate (McNemar's test, P < 0.0001). In conclusion, the use of ClinProTools demonstrated an alternative way for users lacking special expertise in mass spectrometry to handle closely related bacterial species when the proprietary spectrum library failed. This approach should be useful for the differentiation of other closely related bacterial species.

Tautomerism of uracil and related compounds: A mass spectrometry study.

It has been demonstrated that uracil has a preponderant tautomeric form, but it is also known that different tautomers co-exist in this equilibrium. In this work, mass spectrometry is used as a helpful tool to analyse the equilibria, using derivative compounds to forbid the presence of some tautomers and ion trap mass spectrometry to follow relevant fragmentation pathways. Theoretical calculations were performed to confirm tautomers abundance by energy minimization in gas phase. Analysis of mass spectra of uracil, three methyl-substituted uracils, 2-thiouracil and three benzouracils suggest that uracil exists mainly as three tautomers in gas phase: one major structure that corresponds to the classical structure of uracil (pyrimidine-2,4(1H,3H)-dione) bearing two carbonyls and two NH moieties, and two minor enolic forms (4-hydroxypyrimidin-2(1H)-one and 2-hydroxypyrimidin-4(1H)-one). Such tautomeric distribution is supported by theoretical calculations, which show that they are the three most stable tautomers.

Investigation of corrosion mechanism in Type 304 stainless steel under different corrosive environments: A SIMS study

Surface imaging, depth distribution and mass spectra analysis features of Secondary Ion Mass Spectrometry has been utilized for investigating the corrosion mechanism in Type 304 austenitic stainless steel under different corrosive environments of H2SO4, HCl and HNO3 solutions of 10M each. Surface distribution analysis revealed non-homogenous distribution of alloying elements S, Cr, Mn, Ni and Fe in case of H2SO4 and HCl treated samples, which was due to the formation of corroded layer and its morphology. Depth distribution and mass spectra analysis revealed that oxides and hydroxides containing species are most abundant in H2SO4 treated sample followed by HCl treated sample. Surface distribution and mass spectra analysis indicated localized attack or pitting corrosion of chloride species in case of HCl treated specimen whereas in case of H2SO4 treated sample, attack of sulfur species was associated with an intensification of oxide and hydroxide species. Depth distribution analysis in case of HNO3 treated specimen concluded the formation of a passive layer enriched with Cr, Ni, Mn and Fe which causes the inhibition of corrosion process under the experimental corrosive parameters. The study also provides a methodology to investigate the type of chemical attack and to find out the root cause of corrosion utilizing various analysis features of SIMS technique.

Organics in comet 67P – a first comparative analysis of mass spectra from ROSINA–DFMS, COSAC and Ptolemy

The ESA Rosetta spacecraft followed comet 67P at a close distance for more than 2 yr. In addition, it deployed the lander Philae on to the surface of the comet. The (surface) composition of the comet is of great interest to understand the origin and evolution of comets. By combining measurements made on the comet itself and in the coma, we probe the nature of this surface material and compare it to remote sensing observations. We compare data from the double focusing mass spectrometer (DFMS) of the ROSINA experiment on ESA's Rosetta mission and previously published data from the two mass spectrometers COSAC (COmetary Sampling And Composition) and Ptolemy on the lander. The mass spectra of all three instruments show very similar patterns of mainly CHO-bearing molecules that sublimate at temperatures of 275 K. The DFMS data also show a great variety of CH-, CHN-, CHS-, CHO2- and CHNO-bearing saturated and unsaturated species. Methyl isocyanate, propanal and glycol aldehyde suggested by the earlier analysis of the measured COSAC spectrum could not be confirmed. The presence of polyoxymethylene in the Ptolemy spectrum was found to be unlikely. However, the signature of the aromatic compound toluene was identified in DFMS and Ptolemy data. Comparison with remote sensing instruments confirms the complex nature of the organics on the surface of 67P, which is much more diverse than anticipated.

Analytical Characterization of Methyl-β-Cyclodextrin for Pharmacological Activity to Reduce Lysosomal Cholesterol Accumulation in Niemann-Pick Disease Type C1 Cells.

Methyl-β-cyclodextrin (MβCD) reduces lysosomal cholesterol accumulation in Niemann-Pick disease type C1 (NPC1) patient fibroblasts. However, the pharmacological activity of MβCD reported by different laboratories varies. To determine the potential causes of this variation, we analyzed the mass spectrum characteristics, pharmacological activity of three preparations of MβCDs, and the protein expression profiles of NPC1 patient fibroblasts after treatment with different sources of MβCDs. Our data revealed varied mass spectrum profiles and pharmacological activities on the reduction of lysosomal cholesterol accumulation in NPC1 fibroblasts for these three preparations of MβCDs obtained from different batches and different sources. Furthermore, a proteomic analysis showed the differences of these three MβCD preparations on amelioration of dysregulated protein expression levels in NPC1 cells. The results demonstrate the importance of prescreening of different cyclodextrin preparations before use as a therapeutic agent. A combination of mass spectrum analysis, measurement of pharmacological activity, and proteomic profiling provides an effective analytical procedure for characterization of cyclodextrins for therapeutic applications.

Separated Immobilization of Incompatible Enzymes on Polymer Substrate via Visible Light Induced Living Photografting Polymerization.

The use of the mixed catalytic system with several enzymes can provide multiple benefits in terms of the cost, simplification of a multistep reaction, and effectiveness of complex chemical reactions. Although study of different enzyme coimmobilization systems has attracted increasing attention in recent years, separately immobilizing enzymes which can not coexist on one support is still one of the great challenges. In this paper, a simple and effective strategy was introduced to separately encapsulate incompatible trypsin and transglutaminase (TGase) into different poly(ethylene glycol) (PEG) network layer grafted on low-density polyethylene (LDPE) film via visible light induced living photografting polymerization. As a proof of concept, this dual-enzyme separately loaded film was used to catalyze the synthesis of a new target antitumor drug LTV-azacytidine. The final results demonstrated that this strategy could maintain higher activities of both enzymes than the mixed coimmobilization method. And the mass spectra analysis results demonstrated that LTV-azacytidine was successfully synthesized. We believe that this facile and mild separately immobilizing incompatible enzyme strategy has great application potential in the field of biocatalysis.

Isolation and purification of osthole and imperatorin from Fructus Cnidii by semi-preparative supercritical fluid chromatography

ABSTRACTFructus Cnidii, the dried ripe fruit of Cnidium monnieri (L.) Cusson., has been widely used in traditional Chinese medicine. Osthole and imperatorin are the main active ingredients of Fructus Cnidii and had been found of antispasmodic, anti-HIV, anti-fungal, anti-viral, anti-tumor, anti-mutagenic, anti-arrhythmic, hypotensive, and broad-spectrum antimicrobial effects. A supercritical fluid chromatography (SFC) method for isolation and purification of osthole and imperatorin from Fructus Cnidii was established in this work. The separation conditions, including the stationary phase, the organic modifier, the composition and the flow rate of the mobile phase, column backpressure and column temperature, were optimized on analytical scale at first. And then a semi-preparative SFC (SP-SFC) method was developed based on the conditions of analytical scale SFC. SP-SFC was accomplished on YMC-Pack NH2 column. Ethanol was used as the modifier and its percentage in the mobile phase was 3%. The flow rate of the mobile phase was 20 mL/min, column backpressure was 13 MPa, column temperature was 318 K, detection wavelength was 310 nm, and injection volume was 0.2 mL. Under the optimum conditions, osthole and imperatorin were obtained with high purities as determined by high performance liquid chromatography. The chemical structures of the obtained compounds were identified by nuclear magnetic resonance and mass spectrum analysis.

Accurate differentiation of Mycobacterium chimaera from Mycobacterium intracellulare by MALDI-TOF MS analysis.

The increasing number of infections caused by nontuberculous mycobacteria (NTM) has prompted the need for rapid and precise identification methods of these pathogens. Several studies report the applicability of MALDI-TOF mass spectrometry (MS) for identification of NTM. However, some closely related species have very similar spectral mass fingerprints, and until recently, Mycobacterium chimaera and M. intracellulare could not be separated from each other by MALDI-TOF MS. The conventional identification methods used in routine diagnostics have similar limitations. Recently, the differentiation of these two species within the Mycobacterium avium complex has become increasingly important due to reports of M. chimaera infections related to open heart surgery in Europe and in the USA. In this report, a method for the distinct differentiation of M. chimaera and M. intracellulare using a more detailed analysis of MALDI-TOF mass spectra is presented. Species-specific peaks could be identified and it was possible to assign all isolates (100 %) from reference strain collections as well as clinical isolates to the correct species. We have developed a model for the accurate identification of M. chimaera and M. intracellulare by MALDI-TOF MS. This approach has the potential for routine use in microbiology laboratories, as the model itself can be easily implemented into the software of the currently available systems by MALDI-TOF MS manufacturers.

IN VITRO ANTI-HELICOBACTER PYLORI ACTIVITY AND GC-MS ANALYSIS OF ENICOSTEMMA AXILLARE (LAM). RAYNAL

Traditional healers possess a rich knowledge on the use of medicinal plants for the treatment of various diseases. Enicostemma axillare (Lam). Raynal is one plant, used by healer to treat gastric cancer or ulceration. We assessed the phytochemical components of leaf extract for its antibacterial activity, to add value and provide an evidence-base for their traditional use. The antibacterial potential of the leaf extract was tested against Helicobacter pylori using agar well diffusion method. The minimum inhibitory concentration (MIC) of ethanol extract exhibited the (MIC) against H. pylori ranged from 200µg/ml - 250µg/ml followed by chloroform extract. Gas chromatography and mass spectrum (GC-MS) analysis confirms the occurrence of different components in the ethanol and chloroform leaf extract of the studied species.

Detection of small molecules using SBA-15 modified CHCA as a novel matrix of MALDI-TOF MS

It is challenging to apply matrix-assisted laser desorption/ionization−time of flight mass spectrometry (MALDI-TOF MS) with analysis of small molecules efficiently. In this study, we describe the use of SBA-15@APTES@CHCA as a novel silica-containing matrix to aid the analysis of small molecules with MALDI-TOF MS. To synthesize this novel matrix, 3-aminopropyltriethoxysilane (APTES) was used to bond α-cyano-4-hydnoxycinnamic acid (CHCA) and SBA-15 via SiO and three CH2 alkyl chain. Furthermore, APTES also acts as flexible chains to allow CHCA molecules spreading over the surface of SBA-15, which increase the capacity of matrix to absorb energy and crystallize with sample. This novel matrix is more stable than an ionic macro-complex matrix modified with SBA-15 and can be vacuum dried or air dried easily and stored for a long period as powder. The structure of matrix was characterized by FTIR spectra and SEM. Twelve quinolone antibiotics (danofloxacin, enrofloxacin, ciprofloxacin, difloxacin, enoxacin, gatifloxacin, fleroxacin, lomefloxacin, sparfloxacin, norfloxacin, levofloxacin, pefloxacin, coumaphos, fenobucarb, carbendazim) and three pesticides (carbendazim, coumaphos, bassa) were applied to verify the feasibility of SBA-15@APTES@CHCA as a matrix for small molecules analysis. Several factors which affected the application of this matrix with mass spectrum analysis are discussed. The factors include suspension agent, matrix concentration, matrix/analyte ratio (v: v), sample preparation methods Compared with CHCA, as a matrix, SBA-15@APTES@CHCA displays distinct advantages on analysis of small molecules such as minimal chemical noise and improved analyte signal intensity. It is therefore a promising matrix to be used in MALDI-TOF MS analysis of low molecular weight analytes.

Termination reaction of living poly(3-hexylthiophene) using thiophene Grignard reagents: Substituent effect on the functionalization

Two 2-chloromagnesiothiophenes having the methoxy group at the 3- and 4-position were applied for the termination reaction of living poly(3-hexylthiophene) (P3HT). The number-averaged molecular weights of obtained P3HT approximately agreed with the theoretical value, and the high regioregularity of polymers were confirmed. The comparison of the 1H NMR spectra of P3HT with those of model compounds in addition to the analysis of the MALDI-TOF mass spectra suggested the incorporation of the methoxythiophene unit. The termination reaction using 2-chloromagnesio-3-methoxythiophene (GT1) and 2-chloromagnesio-4-methoxythiophene (GT2) resulted in P3HT with the non-uniform terminal structure and that having the terminating unit at both chain ends, respectively. These facts can be ascribed to the efficiency of the elementary process (transmetallation, reductive elimination, reverse chain walking, and oxidative addition) in the catalytic cycle. On the other hand, 2-chloromagnesio-3-oxazolylthiophene (GT3) exhibited the poor transmetallation activity due to the pronounced steric hindrance. It was found that the addition of LiCl for accelerating the transmetallation step and the coexistence of styrene for trapping the zero-valent nickel complex give P3HT bearing the oxazolylthiophene unit at the propagating end.

Structure and Biosynthesis of Branched Wax Compounds on Wild Type and Wax Biosynthesis Mutants of Arabidopsis thaliana

The cuticle is a waxy composite that protects the aerial organs of land plans from non-stomatal water loss. The chemical make-up of the cuticular wax mixture plays a central role in defining the water barrier, but structure-function relationships have not been established so far, in part due to gaps in our understanding of wax structures and biosynthesis. While wax compounds with saturated, linear hydrocarbon tails have been investigated in detail, very little is known about compounds with modified aliphatic tails, which comprise substantial portions of some plant wax mixtures. This study aimed to investigate the structures, abundances and biosynthesis of branched compounds on the species for which wax biosynthesis is best understood: Arabidopsis thaliana. Microscale derivatization, mass spectral interpretation and organic synthesis identified homologous series of iso-alkanes and iso-alcohols on flowers and leaves, respectively. These comprised approximately 10-15% of wild type wax mixtures. The abundances of both branched wax constituents and accompanying unbranched compounds were reduced on the cer6, cer3 and cer1 mutants but not cer4, indicating that branched compounds are in part synthesized by the same machinery as unbranched compounds. In contrast, the abundances of unbranched, but not branched, wax constituents were reduced on the cer2 and cer26 mutants, suggesting that the pathways to both types of compounds deviate in later steps of chain elongation. Finally, the abundances of branched, but not unbranched, wax compounds were reduced on the cer16 mutant, and the (uncharacterized) CER16 protein may therefore be controlling the relative abundances of iso-alkanes and iso-alcohols on Arabidopsis surfaces.

Contribution of bacteria-like particles to PM2.5 aerosol in urban and rural environments

We report highly time-resolved estimates of airborne bacteria-like particle concentrations in ambient aerosol using an Aerodyne aerosol mass spectrometer (AMS). AMS measurements with a newly developed PM2.5 and the standard (PM1) aerodynamic lens were performed at an urban background site (Zurich) and at a rural site (Payerne) in Switzerland. Positive matrix factorization using the multilinear engine (ME-2) implementation was used to estimate the contribution of bacteria-like particles to non-refractory organic aerosol. The success of the method was evaluated by a size-resolved analysis of the organic mass and the analysis of single particle mass spectra, which were detected with a light scattering system integrated into the AMS. Use of the PM2.5 aerodynamic lens increased measured bacteria-like concentrations, supporting the analysis method. However, at all sites, the low concentrations of this component suggest that airborne bacteria constitute a minor fraction of non-refractory PM2.5 organic aerosol mass. Estimated average mass concentrations were below 0.1 μg/m3 and relative contributions were lower than 2% at both sites. During rainfall periods, concentrations of the bacteria-like component increased considerably reaching a short-time maximum of approximately 2 μg/m3 at the Payerne site in summer.