Detection of volatile compounds emitted by Pseudomonas aeruginosa using selected ion flow tube mass spectrometry

Identification of volatile organic compounds produced by bacteria using HS-SPME-GC-MS

The analysis of volatile organic compounds (VOCs) as a tool for bacterial identification is reported. Headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography-mass spectrometry (GC-MS) was applied to the analysis of bacterial VOCs with the aim of determining the impact of experimental parameters on the generated VOC profiles. The effect of culture medium, SPME fiber type and GC column were fully evaluated with the Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae and the Gram-positive species Staphylococcus aureus. Multivariate analysis, including cluster analysis and principal component analysis, was applied to VOC data to determine whether the parameters under investigation significantly affected bacterial VOC profiles. Culture medium, and to a lesser extent, SPME fiber type, were found to significantly alter detected bacterial VOC profiles. The detected VOCs varied little with the polarity of the GC column. The results indicate that the generated bacterial VOC profiles need careful evaluation if they are to be used for clinical diagnostics. The whole process is limited by the need to grow the bacteria in broth (18 h) before extraction and analysis (63 min).

Volatile metabolites of pathogens: a systematic review

Ideally, invading bacteria are detected as early as possible in critically ill patients: the strain of morbific pathogens is identified rapidly, and antimicrobial sensitivity is known well before the start of new antimicrobial therapy. Bacteria have a distinct metabolism, part of which results in the production of bacteria-specific volatile organic compounds (VOCs), which might be used for diagnostic purposes. Volatile metabolites can be investigated directly in exhaled air, allowing for noninvasive monitoring. The aim of this review is to provide an overview of VOCs produced by the six most abundant and pathogenic bacteria in sepsis, including Staphylococcus aureus, Streptococcus pneumoniae, Enterococcus faecalis, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli. Such VOCs could be used as biological markers in the diagnostic approach of critically ill patients. A systematic review of existing literature revealed 31 articles. All six bacteria of interest produce isopentanol, formaldehyde, methyl mercaptan, and trimethylamine. Since humans do not produce these VOCs, they could serve as biological markers for presence of these pathogens. The following volatile biomarkers were found for identification of specific strains: isovaleric acid and 2-methyl-butanal for Staphylococcus aureus; 1-undecene, 2,4-dimethyl-1-heptane, 2-butanone, 4-methyl-quinazoline, hydrogen cyanide, and methyl thiocyanide for Pseudomonas aeruginosa; and methanol, pentanol, ethyl acetate, and indole for Escherichia coli. Notably, several factors that may effect VOC production were not controlled for, including used culture media, bacterial growth phase, and genomic variation within bacterial strains. In conclusion, VOCs produced by bacteria may serve as biological markers for their presence. Goal-targeted studies should be performed to identify potential sets of volatile biological markers and evaluate the diagnostic accuracy of these markers in critically ill patients.

Secondary electrospray ionization-mass spectrometry (SESI-MS) breathprinting of multiple bacterial lung pathogens, a mouse model study

Bacterial pneumonia is one of the leading causes of disease-related morbidity and mortality in the world, in part because the diagnostic tools for pneumonia are slow and ineffective. To improve the diagnosis success rates and treatment outcomes for bacterial lung infections, we are exploring the use of secondary electrospray ionization-mass spectrometry (SESI-MS) breath analysis as a rapid, noninvasive method for determining the etiology of lung infections in situ. Using a murine lung infection model, we demonstrate that SESI-MS breathprints can be used to distinguish mice that are infected with one of seven lung pathogens: Haemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Moraxella catarrhalis, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pneumoniae, representing the primary causes of bacterial pneumonia worldwide. After applying principal components analysis, we observed that with the first three principal components (primarily comprised of data from 14 peaks), all infections were separable via SESI-MS breathprinting (P < 0.0001). Therefore, we have shown the potential of this SESI-MS approach for rapidly detecting and identifying acute bacterial lung infections in situ via breath analysis.

Detecting bacterial lung infections: in vivo evaluation of in vitro volatile fingerprints

The identification of bacteria by their volatilomes is of interest to many scientists and clinicians as it holds the promise of diagnosing infections in situ, particularly lung infections via breath analysis. While there are many studies reporting various bacterial volatile biomarkers or fingerprints using in vitro experiments, it has proven difficult to translate these data to in vivo breath analyses. Therefore, we aimed to create secondary electrospray ionization-mass spectrometry (SESI-MS) pathogen fingerprints directly from the breath of mice with lung infections. In this study we demonstrated that SESI-MS is capable of differentiating infected versus uninfected mice, P. aeruginosa-infected versus S. aureus-infected mice, as well as distinguish between infections caused by P. aeruginosa strains PAO1 versus FRD1, with statistical significance (p < 0.05). In addition, we compared in vitro and in vivo volatiles and observed that only 25-34% of peaks are shared between the in vitro and in vivo SESI-MS fingerprints. To the best of our knowledge, these are the first breath volatiles measured for P. aeruginosa PAO1, FRD1, and S. aureus RN450, and the first comparison of in vivo and in vitro volatile profiles from the same strains using the murine infection model.

Quantification of hydrogen cyanide (HCN) in breath using selected ion flow tube mass spectrometry--HCN is not a biomarker of Pseudomonas in chronic suppurative lung disease

Hydrogen cyanide (HCN) in exhaled breath has been proposed as a biomarker for airway inflammation, and also a marker of the presence in the airways of specific organisms, especially Pseudomonas aeruginosa. However the production of HCN by salivary peroxidase in the oral cavity increases orally exhaled concentrations, and may not reflect the condition of the lower airways. Using SIFT-MS we aimed to determine an appropriate single-exhalation breathing maneuver which avoids the interference of HCN produced in the oral cavity. We have established that the SIFT-MS Voice200™ is suitable for the online measurement of HCN in exhaled breath. In healthy volunteers a significantly higher end exhaled HCN concentration was measured in oral exhalations compared to nasal exhalations (mean ± SD) 4.5 ± 0.6 ppb versus 2.4 ± 0.3 ppb, p < 0.01. For the accurate and reproducible quantification of end exhaled HCN in breath a nasal inhalation to full vital capacity and nasal exhalation at controlled flow is recommended. This technique was subsequently used to measure exhaled HCN in a group of patients with chronic suppurative lung disease (CSLD) and known microbiological colonization status to determine utility of HCN measurement to detect P. aeruginosa. Median nasal end exhaled HCN concentrations were higher in patients with CSLD (3.7 ppb) than normal subjects (2.0 ppb). However no differences between exhaled HCN concentrations of subjects colonized with P. aeruginosa and other organisms were identified, indicating that breath HCN is not a suitable biomarker of P. aeruginosa colonization.

On the features, successes and challenges of selected ion flow tube mass spectrometry

The major features of the selected ion flow tube mass spectrometry (SIFT-MS) analytical method that was conceived and designed for the analysis, in real time, of air obviating sample collections into bags or extraction by pre-concentration of trace compounds onto surfaces are reviewed. The unique analytical capabilities of SIFT-MS for ambient analysis are stressed that allow quantification of volatile organic and inorganic compounds directly from the measurement of physical parameters without the need for regular instrumental calibration using internal or external standards. Then, emphasis is placed on the challenging real-time accurate analysis of single exhalations of humid breath, which is now achieved and readily facilitates wider applications of SIFT-MS in other fields where trace gas analysis has value. The quality of the data obtained by SIFT-MS is illustrated by the quantification of some exhaled breath metabolites that are of immediate relevance to physiology and medicine, including that of hydrogen cyanide in the breath of patients with cystic fibrosis. The current status of SIFT-MS is revealed by a form of a strengths, weakness, opportunities and threats (SWOT) analysis intended to present an objective view of this analytical technique and the likely way forward towards its further development and application.

Detection of Pseudomonas aeruginosa in sputum headspace through volatile organic compound analysis

INTRODUCTION

Chronic pulmonary infection is the hallmark of cystic fibrosis lung disease. Searching for faster and easier screening may lead to faster diagnosis and treatment of Pseudomonas aeruginosa (P. aeruginosa). Our aim was to analyze and build a model to predict the presence of P. aeruginosa in sputa.

METHODS

Sputa from 28 bronchiectatic patients were used for bacterial culturing and analysis of volatile compounds by gas chromatography-mass spectrometry. Data analysis and model building were done by Partial Least Squares Regression Discriminant analysis (PLS-DA). Two analysis were performed: one comparing P. aeruginosa positive with negative cultures at study visit (PA model) and one comparing chronic colonization according to the Leeds criteria with P. aeruginosa negative patients (PACC model).

RESULTS

The PA model prediction of P. aeruginosa presence was rather poor, with a high number of false positives and false negatives. On the other hand, the PACC model was stable and explained chronic P. aeruginosa presence for 95% with 4 PLS-DA factors, with a sensitivity of 100%, a positive predictive value of 86% and a negative predictive value of 100%.

CONCLUSION

Our study shows the potential for building a prediction model for the presence of chronic P. aeruginosa based on volatiles from sputum.

Variability in the concentrations of volatile metabolites emitted by genotypically different strains of Pseudomonas aeruginosa

AIMS

To characterize the volatile metabolites produced by genotypically diverse strains of Pseudomonas aeruginosa in order to evaluate their potential for use as biomarkers of lung infection in noninvasive breath analysis.

METHODS AND RESULTS

Volatile organic compounds (VOCs) emitted from 36 clinical strains of Ps. aeruginosa (belonging to different multilocus sequence types) cultured in liquid and on solid media were analysed by gas chromatography mass spectrometry (GC-MS) and selected ion flow tube mass spectrometry (SIFT-MS). Several previously identified VOCs were detected, including ethanol, acetone, 2-butanone, 2-pentanone, isoprene, aminoacetophenone, dimethyl sulphide, dimethyl disulphide, dimethyl trisulphide and methyl thiocyanate. Additionally, significant production of 3-methyl-butanone, acetophenone, methylthioacetate and methyl thiobutanoate was observed for the first time in this study. SIFT-MS quantifications of VOCs showed high variability between genotypically distinct strains.

CONCLUSIONS

The data obtained indicate that the production rates of the volatile biomarkers of Ps. aeruginosa vary by two orders of magnitude between different strains cultured under the same conditions. Similar variability was observed for both liquid and solid media.

SIGNIFICANCE AND IMPACT OF THE STUDY

Inter-strain genotypic variability strongly influences the concentrations of the volatile biomarkers from Ps. aeruginosa. A group of several biomarkers quantified in real time in exhaled breath may thus provide a more valuable indicator of the course of pulmonary infections compared to a single biomarker.

Bacterial volatile discovery using solid phase microextraction and comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry

Bacteria produce unique volatile mixtures that could be used to identify infectious agents to the species, and possibly the strain level. However, due to the immense variety of human pathogens, and the close relatedness of some of these bacteria, the robust identification of the bacterium based on its volatile metabolome is likely to require a large number of volatile compounds for each species. We applied comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC×GC-TOFMS) to the identification of the headspace volatiles of Pseudomonas aeruginosa PA14 grown for 24 h in lysogeny broth. This is the first reported use of GC×GC-TOFMS for the characterization of bacterial headspace volatiles. The analytical purity that is afforded by this chromatographic method facilitated the identification of 28 new P. aeruginosa-derived volatiles, nearly doubling the list of volatiles for this species.

Quantification of methyl thiocyanate in the headspace of Pseudomonas aeruginosa cultures and in the breath of cystic fibrosis patients by selected ion flow tube mass spectrometry

Infection by Pseudomonas aeruginosa (PA) is a major cause of morbidity and mortality in patients with cystic fibrosis (CF). Breath analysis could potentially be a useful diagnostic of such infection, and analyses of volatile organic compounds (VOCs) emitted from PA cultures are an important part of the search for volatile breath markers of PA lung infection. Our pilot experiments using solid-phase microextraction, SPME and gas chromatography/mass spectrometric (GC/MS) analyses of volatile compounds produced by PA strains indicated a clear presence of methyl thiocyanate. This provided a motivation to develop a method for real-time online quantification of this compound by selected ion flow tube mass spectrometry, SIFT-MS. The kinetics of reactions of H(3)O(+), NO(+) and O(2)(+•) with methyl thiocyanate at 300 K were characterized and the characteristic product ions determined (proton transfer for H(3)O(+), rate constant 4.6 × 10(-9) cm(3) s(-1); association for NO(+), 1.7 × 10(-9) cm(3) s(-1) and nondissociative charge transfer for O(2)(+•) 4.3 × 10(-9) cm(3) s(-1)). The kinetics library was extended by a new entry for methyl thiocyanate accounting for overlaps with isotopologues of hydrated hydronium ions. Solubility of methyl thiocyanate in water (Henry's law constant) was determined using standard reference solutions and the linearity and limits of detection of both SIFT-MS and SPME-GC/MS methods were characterized. Thirty-six strains of PA with distinct genotype were cultivated under identical conditions and 28 of them (all also producing HCN) were found to release methyl thiocyanate in headspace concentrations greater than 6 parts per billion by volume (ppbv). SIFT-MS was also used to analyze the breath of 28 children with CF and the concentrations of methyl thiocyanate were found to be in the range 2-21 ppbv (median 7 ppbv).

Volatile compounds in health and disease

PURPOSE OF REVIEW

To summarize the recent progress made in noninvasive monitoring of volatile compounds in exhaled breath and above biological liquids, as they are becoming increasingly important in assessing the nutritional and clinical status and beginning to provide support to conventional clinical diagnostics and therapy. To indicate the potential of these developments in medicine and the specific areas which are currently under investigation.

RECENT FINDINGS

The significance of the following breath gases and their concentrations are reported: acetone and the influence of diet; ammonia confirmed as an indicator of dialysis efficacy; hydrogen and the (13)CO(2)/(12)CO(2) ratio (following the ingestion of (13)C-labeled compounds) as related to gastric emptying and bowel transit times; hydrogen cyanide released by Pseudomonas and its detection in breath of children with cystic fibrosis; and multiple trace compounds in breath of patients with specific pathophysiological conditions and 'metabolic profiling'.

SUMMARY

Advanced analytical methods, especially exploiting mass spectrometry, are moving breath analysis towards the clinical setting; some trace gas metabolites are already being exploited in diagnosis and therapy. Much effort is being given to the search for biomarkers of tumours in the body. HCN as an indicator of the presence of Pseudomonas in the airways has real potential in therapeutically alleviating the symptoms of cystic fibrosis.

Time-resolved selected ion flow tube mass spectrometric quantification of the volatile compounds generated by E. coli JM109 cultured in two different media

Preliminary measurements have been made of the volatile compounds emitted by the bacterium E. coli JM109 cultured in the commonly used media Dulbecco's modified Eagle's medium (DMEM) and lysogeny broth (LB) using selected ion flow tube mass spectrometry, SIFT-MS, as a step towards the real time, non-invasive monitoring of accidental infections of mammalian cell cultures. In one procedure, the culture medium alone and the E. coli cells/medium combination were held at 37 °C in bottles sealed with septa for a given time period, usually overnight, to allow the bacterium to proliferate, after which the captured headspace was analysed directly by SIFT-MS. Several compounds were seen to be produced by the E. coli cells that depended on the liquid medium used: when cultured in DMEM, copious amounts of ethanol, acetaldehyde and hydrogen sulphide were produced; in LB ammonia is the major volatile product. In a second procedure, to ensure aerobic conditions prevailed in the cell culture, selected volatile compounds were monitored by SIFT-MS in real time for several hours above the open-to-air E. coli/DMEM culture held at close to 37 °C. The temporal variations in the concentrations of some compounds, which reflect their production rates in the culture, indicate maxima. Thus, the maxima in the ethanol and acetaldehyde production are a reflection of the reduction of glucose from the DMEM by the vigorous E. coli cells and the maximum in the hydrogen sulphide level is an indication of the loss of the sulphur-bearing amino acids from the DMEM. Serendipitously, emissions from DMEM inadvertently infected with the bacterium C. testosteroni were observed when large quantities of ammonia were seen to be produced. The results of this preliminary study suggest that monitoring volatile compounds might assist in the early detection of bacterial infection in large-scale bioreactors.

Background levels and diurnal variations of hydrogen cyanide in breath and emitted from skin

The hydrogen cyanide (HCN) concentration in exhaled human breath and skin gas samples collected with different sampling techniques was measured using near-infrared cavity ring-down spectroscopy. The median baseline HCN concentrations in samples provided by 19 healthy volunteers 2-4 h after the last meal depended on the employed sampling technique: 6.5 parts per billion by volume (ppbv) in mixed (dead space and end-tidal) mouth-exhaled breath collected to a gas sampling bag, 3.9 ppbv in end-tidal mouth-exhaled breath, 1.3 ppbv in end-tidal nose-exhaled breath, 1.0 ppbv in unwashed skin and 0.6 ppbv in washed skin samples. Diurnal measurements showed that elevated HCN levels are to be expected in mouth-exhaled breath samples after food and drink intake, which suggests HCN generation in the oral cavity. The HCN concentrations in end-tidal nose-exhaled breath and skin gas samples were correlated, and it is concluded that these concentrations best reflect systemic HCN levels.

Ambient analysis of trace compounds in gaseous media by SIFT-MS

The topic of ambient gas analysis has been rapidly developed in the last few years with the evolution of the exciting new techniques such as DESI, DART and EESI. The essential feature of all is that analysis of trace gases can be accomplished either in the gas phase or those released from surfaces, crucially avoiding sample collection or modification. In this regard, selected ion flow tube mass spectrometry, SIFT-MS, also performs ambient analyses both accurately and rapidly. In this focused review we describe the underlying ion chemistry underpinning SIFT-MS through a discourse on the reactions of different classes of organic and inorganic molecules with H(3)O(+), NO(+) and O(2)(+)˙ studied using the SIFT technique. Rate coefficients and ion products of these reactions facilitate absolute SIFT-MS analyses and can also be useful for the interpretation of data obtained by the other ambient analysis methods mentioned above. The essential physics and flow dynamics of SIFT-MS are described that, together with the reaction kinetics, allow SIFT-MS to perform absolute ambient analyses of trace compounds in humid atmospheric air, exhaled breath and the headspace of aqueous liquids. Several areas of research that, through pilot experiments, are seen to benefit from ambient gas analysis using SIFT-MS are briefly reviewed. Special attention is given to exhaled breath and urine headspace analysis directed towards clinical diagnosis and therapeutic monitoring, and some other areas researched using SIFT-MS are summarised. Finally, extensions to current areas of application and indications of other directions in which SIFT-MS can be exploited for ambient analysis are alluded to.

Progress in SIFT-MS: breath analysis and other applications

The development of selected ion flow tube mass spectrometry, SIFT-MS, is described from its inception as the modified very large SIFT instruments used to demonstrate the feasibility of SIFT-MS as an analytical technique, towards the smaller but bulky transportable instruments and finally to the current smallest Profile 3 instruments that have been located in various places, including hospitals and schools to obtain on-line breath analyses. The essential physics and engineering principles are discussed, which must be appreciated to design and construct a SIFT-MS instrument. The versatility and sensitivity of the Profile 3 instrument is illustrated by typical mass spectra obtained using the three precursor ions H(3)O(+), NO(+) and O(2)(+)·, and the need to account for differential ionic diffusion and mass discrimination in the analytical algorithms is emphasized to obtain accurate trace gas analyses. The performance of the Profile 3 instrument is illustrated by the results of several pilot studies, including (i) on-line real time quantification of several breath metabolites for cohorts of healthy adults and children, which have provided representative concentration/population distributions, and the comparative analyses of breath exhaled via the mouth and nose that identify systemic and orally-generated compounds, (ii) the enhancement of breath metabolites by drug ingestion, (iii) the identification of HCN as a marker of Pseudomonas colonization of the airways and (iv) emission of volatile compounds from urine, especially ketone bodies, and from skin. Some very recent developments are discussed, including the quantification of carbon dioxide in breath and the combination of SIFT-MS with GC and ATD, and their significance. Finally, prospects for future SIFT-MS developments are alluded to.

Headspace measurements of irradiated in vitro cultured cells using PTR-MS

A pilot study was performed to evaluate a new concept for a radiation biodosimetry method. Proton transfer reaction-mass spectrometry (PTR-MS) was used to find out whether radiation induces changes in the composition of volatile organic compounds (VOCs) in the headspace of in vitro cultured cells. Two different cell lines, retinal pigment epithelium cells hTERT-RPE1 and lung epithelium cells A-549, were irradiated with gamma radiation at doses of 4 Gy and 8 Gy. For measuring the cell-specific effects, the VOC concentrations in the headspace of flasks containing cells plus medium, as well as of flasks containing pure medium were analyzed for changes before and after irradiation. No significant radiation-induced alterations in VOC concentrations in the headspace could be observed after irradiation.

Fast detection of volatile organic compounds from bacterial cultures by secondary electrospray ionization-mass spectrometry

We propose a novel application of secondary electrospray ionization-mass spectrometry (SESI-MS) as a real-time clinical diagnostic tool for bacterial infection. It is known that volatile organic compounds (VOCs), produced in different combinations and quantities by bacteria as metabolites, generate characteristic odors for certain bacteria. These VOCs comprise a specific metabolic profile that can be used for species or serovar identification, but rapid and sensitive analytical methods are required for broad utility. In this study, the VOC profiles of five bacterial groups from four genera, Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Salmonella enterica serovar Typhimurium, and Salmonella enterica serovar Pullorum, were characterized by SESI-MS. Thirteen compounds were identified from these bacterial cultures, and the combination of these VOCs creates a unique pattern for each genus. In addition, principal component analysis (PCA) was applied for the purpose of species or serovar discrimination. The first three principal components exhibit a clear separation between the metabolic volatile profiles of these five bacterial groups that is independent of the growth medium. As a first step toward addressing the complexity of clinical application, in vitro tests for mixed cultures were conducted. The results show that individual species or serovars in a mixed culture are identifiable among a biological VOC background, and the ratios of the detected volatiles reflect the proportion of each bacterium in the mixture. Our data confirm the utility of SESI-MS in real-time identification of bacterial species or serovars in vitro, which, in the future, may play a promising clinical role in diagnosing infections.

Discrimination of cancerous and non-cancerous cell lines by headspace-analysis with PTR-MS

Proton transfer reaction mass spectrometry (PTR-MS) has been used to analyze the volatile organic compounds (VOCs) emitted by in-vitro cultured human cells. For this purpose, two pairs of cancerous and non-cancerous human cell lines were selected:1. lung epithelium cells A-549 and retinal pigment epithelium cells hTERT-RPE1, cultured in different growth media; and 2. squamous lung carcinoma cells EPLC and immortalized human bronchial epithelial cells BEAS2B, cultured in identical growth medium. The VOCs in the headspace of the cell cultures were sampled: 1. online by drawing off the gas directly from the culture flask; and 2. by accumulation of the VOCs in PTFE bags connected to the flask for at least 12 h. The pure media were analyzed in the same way as the corresponding cells in order to provide a reference. Direct comparison of headspace VOCs from flasks with cells plus medium and from flasks with pure medium enabled the characterization of cell-line-specific production or consumption of VOCs. Among all identified VOCs in this respect, the most outstanding compound was m/z = 45 (acetaldehyde) revealing significant consumption by the cancerous cell lines but not by the non-cancerous cells. By applying multivariate statistical analysis using 42 selected marker VOCs, it was possible to clearly separate the cancerous and non-cancerous cell lines from each other.

Background levels of hydrogen cyanide in human breath measured by infrared cavity ring down spectroscopy

Hydrogen cyanide (HCN) in breath has been suggested as a diagnostic tool for cyanide poisoning and for cyanide-producing bacterial infections. To distinguish elevated levels of breath HCN, baseline data are needed. Background levels of HCN were measured in mixed exhaled air from 40 healthy subjects (26 men, 14 women, age 21-61 years; detection limit: 1.5 ppb; median: 4.4 ppb; range <1.5-14 ppb) by near-infrared cavity ring down spectroscopy (CRDS). No correlation was observed with smoking habits, recent meals or age. However, female subjects had slightly higher breath levels of HCN than male subjects. CRDS has not previously been used for this purpose.

Monitoring respiratory disease severity in cystic fibrosis

Measurements of disease severity provide a guide for the physician to tailor therapies, for the patient and family to gauge progress, and are required for clinical trials. For many respiratory diseases, including cystic fibrosis, sensitive, noninvasive measurements are few, and some of those that are available are applicable only to certain subgroups of patients or lack sufficient sensitivity. We discuss currently available measurements in 4 groups: physiology, infection, inflammation, and radiology. For each group we highlight strengths and weaknesses, ask how we could improve upon these, and provide details of alternative methods.

Bugs, biofilms, and resistance in cystic fibrosis

Bacteria infect the respiratory tract early in the course of cystic fibrosis disease, often fail to be eradicated, and together with an aggressive host inflammatory response, are thought to be key players in the irreversible airway damage from which most patients ultimately die. Although incompletely understood, certain aspects of the cystic fibrosis airway itself appear to favor the development of chronic modes of survival, in particular biofilm formation; this and the development of antibiotic resistance following exposure to multiple antibiotic courses lead to chronic, persistent infection. In addition to the common cystic fibrosis pathogens, such as Staphylococcus aureus, Haemophilus influenzae, and Pseudomonas aeruginosa, several newer species are becoming more common. Furthermore, new molecular techniques have led to the identification of multiple different organisms within respiratory secretions, many of which are not cultured with conventional tools. Future work should aim to develop clinically applicable methods to identify these and to determine which have the potential to impact pulmonary health. We outline the basic tenets of infection control and treatment.

Volatile compounds characteristic of sinus-related bacteria and infected sinus mucus: analysis by solid-phase microextraction and gas chromatography-mass spectrometry

Volatile compounds from human breath are a potential source of information for disease diagnosis. Breath may include volatile organic compounds (VOCs) originating in the nasal sinuses. If the sinuses are infected, disease-specific volatiles may enter exhaled air. Sinus infections are commonly caused by several known bacteria. We examined the volatiles characteristic of infectious bacteria in culture using solid-phase microextraction to collect and gas chromatography-mass spectrometry as well as gas chromatography with flame photometric detection to separate and analyze the resulting VOCs. Infected sinus mucus samples were also collected and their VOCs examined. Similar characteristic volatiles were seen from both cultures of individual "pure" bacteria and several mucus samples. However, the relative amounts of characteristic VOCs from individual bacteria differ greatly between cultures and sinus mucus. New compounds, not seen in culture were also seen in some mucus samples. Our results suggest an important role for growth substrate and environment. Our data further suggests that in some sinus mucus samples identification of bacteria-specific volatiles is possible and can suggest the identity of an infecting organism to physicians. Knowledge of these bacteria-related volatiles is necessary to create electronic nose-based, volatile-specific sensors for non-invasive examination for suspected sinus infection.

Pseudomonas aeruginosa AlgR controls cyanide production in an AlgZ-dependent manner

Pseudomonas aeruginosa is an opportunistic pathogen that causes chronic infections in individuals suffering from the genetic disorder cystic fibrosis. In P. aeruginosa, the transcriptional regulator AlgR controls a variety of virulence factors, including alginate production, twitching motility, biofilm formation, quorum sensing, and hydrogen cyanide (HCN) production. In this study, the regulation of HCN production was examined. Strains lacking AlgR or the putative AlgR sensor AlgZ produced significantly less HCN than did a nonmucoid isogenic parent. In contrast, algR and algZ mutants showed increased HCN production in an alginate-producing (mucoid) background. HCN production was optimal in a 5% O2 environment. In addition, cyanide production was elevated in bacteria grown on an agar surface compared to bacteria grown in planktonic culture. A conserved AlgR phosphorylation site (aspartate at amino acid position 54), which is required for surface-dependent twitching motility but not alginate production, was found to be critical for cyanide production. Nuclease protection mapping of the hcnA promoter identified a new transcriptional start site required for HCN production. A subset of clinical isolates that lack this start site produced small amounts of cyanide. Taken together, these data show that the P. aeruginosa hcnA promoter contains three transcriptional start sites and that HCN production is regulated by AlgZ and AlgR and is maximal under microaerobic conditions when the organism is surface attached.

Hydrogen cyanide as a biomarker for Pseudomonas aeruginosa in the breath of children with cystic fibrosis

HYPOTHESIS

Hydrogen cyanide (HCN) is emitted by Pseudomonas aeruginosa (PA) in vitro. We hypothesized that exhaled HCN could be measured using Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) and that concentrations would be higher in children with cystic fibrosis (CF) and PA infection than in children with asthma.

METHODS

Children aged 7-17 years with CF (n = 16) or asthma (n = 21) attending outpatient clinics provided breath samples between July and December 2007. HCN was measured using the SIFT-MS Profile 3 instrument. FeNO was measured with a Sievers NOA 280i analyzer. Baseline inter-group differences between HCN and FeNO concentrations were compared using the Mann-Whitney U test. Children were invited to re-attend fortnightly. Breath samples, spirometry, growth and clinical status were measured at each visit.

RESULTS

There were significant baseline differences in exhaled HCN and FeNO concentrations between the two groups. Children with CF had higher median HCN concentrations than those with asthma: 13.5 parts per billion (ppb) (IQR 8.1-16.5) versus 2.0 ppb (IQR 0.0-4.8) (P < 0.001). Children with CF had lower median FeNO levels compared to children with asthma: 13.4 ppb (IQR 8.9-17.6) versus 57.9 ppb (IQR 34.0-85.7) (P < 0.001). Intra-subject variability was high and significant changes in HCN concentrations were not observed related to changes in lung function or clinical status.

CONCLUSION

This study provides proof of principle that HCN is detectable in the breath of children with CF and is elevated compared to children with asthma. Further studies are required to capture data from acutely unwell children and more accurately delineate responses to treatment.

Analysis of breath, exhaled via the mouth and nose, and the air in the oral cavity

Analyses have been performed, using on-line selected ion flow tube mass spectrometry (SIFT-MS), of the breath of three healthy volunteers, as exhaled via the mouth and the nose and also of the air in the oral cavity during breath hold, each morning over a period of one month. Nine trace compounds have been quantified and concentration distributions have been constructed. Of these compounds, the levels of acetone, methanol and isoprene are the same in the mouth-exhaled and the nose-exhaled breath; hence, we deduce that these compounds are totally systemic. The levels of ammonia, ethanol and hydrogen cyanide are much lower in the nose-exhaled breath than in the mouth-exhaled breath and highest in the oral cavity, indicating that these compounds are largely generated in the mouth with little being released at the alveolar interface. Using the same ideas, both the low levels of propanol and acetaldehyde in mouth-exhaled breath appear to have both oral and systemic components. Formaldehyde is at levels in mouth- and nose-exhaled breath and the oral cavity that are lower than that of the ambient air and so its origin is difficult to ascertain, but it appears to be partially systemic. These results indicate that serious contamination of alveolar breath exhaled via the mouth can occur and if breath analysis is to be used to diagnose metabolic disease then analyses should be carried out of both mouth- and nose-exhaled breath to identify the major sources of particular trace compounds.

Cyanide produced by human isolates of Pseudomonas aeruginosa contributes to lethality in Drosophila melanogaster

Some Pseudomonas aeruginosa strains are cyanogenic, and cyanide may contribute to the bacterium's virulence. Using human isolates of P. aeruginosa, we have shown that Drosophila melanogaster suspended above cyanogenic strains become motionless and develop bradycardia and that flies injected with cyanogenic bacterial strains die more rapidly than those injected with noncyanogenic strains. Flies exposed to cyanogenic strains had high cyanide and low adenosine triphosphate (ATP) concentrations in body extracts, and treatment with a cyanide antidote equalized survival of flies injected with cyanogenic and noncyanogenic strains. P. aeruginosa PAO1 strain with a mutation in the hydrogen cyanide synthase gene cluster was much less toxic to flies than the parental cyanogenic strain or 2 knock-in strains. Transgenic flies overexpressing rhodanese, which detoxifies cyanide by converting it to thiocyanate, were resistant to cyanide and the increased virulence of cyanogenic strains. We conclude that D. melanogaster is a good model for studying cyanide produced by P. aeruginosa.

Selected ion flow tube mass spectrometry for on-line trace gas analysis in biology and medicine

Selected ion flow tube mass spectrometry, (SIFT-MS), is a technique for simultaneous real-time quantification of several trace gases in air and exhaled breath. It relies on chemical ionization of the trace gas molecules in air/breath samples introduced into helium carrier gas, using H(3)O(+), NO(+) and O(2)(+) reagent (precursor ions). Reactions between the precursor ions and the trace gas molecules proceed for an accurately defined time, the precursor and product ions being detected and counted by a downstream mass spectrometer. Absolute concentrations of trace gases in single breath exhalation can be determined by SIFT-MS down to parts-per-billion (ppb) levels, obviating sample collection into bags or onto traps. Calibration using chemical standards is not required, as the concentrations are calculated using the known reaction rate constants and measured flow rates and pressures. SIFT-MS has been used for many pilot investigations in several areas of research, especially as a non-invasive breath analysis tool to investigate physiological processes in humans and animals, for clinical diagnosis and for therapeutic monitoring. Examples of the results obtained from several such studies are outlined to demonstrate the potential of SIFT-MS for trace gas analysis of air, exhaled breath and the headspace above liquids.

Bronchoalveolar lavage examined by solid phase microextraction, gas chromatography–mass spectrometry and selected ion flow tube mass spectrometry

Samples (210 in total) of broncholaveolar lavages (BALs), obtained from patients hospitalized with pneumonia in various departments of two hospitals, were analysed using the method of solid phase microextraction-gas chromatography (SPME-GC) with FID detection. Up to 20% (9% unequivocally, 11% probably) of these samples was found to contain volatile fatty acids (VFAs) in the series from acetic acid to heptanoic acid. Importantly, the presence of these acids indicates the presence of fermenting anaerobic bacteria, which were not detected by the conventional microbiological examination. Other compounds, namely the heptanol and cyclohexanone, were also detected by this method in some samples. Cyclohexanone occurred almost exclusively in samples from patients receiving intensive care with mechanical ventilation, and is suspected to originate from plastic parts of ventilators. Selected representative samples were also analysed using further methods, namely gas chromatography-mass spectrometry (GC-MS) of native and silylated samples, and selected ion flow tube mass spectrometry (SIFT-MS). These methods confirmed the identities of above mentioned compounds, and detected numerous other compounds tentatively identified as various alcohols, aldehydes, ketones, esters and hydrogen cyanide, HCN. Most of these compounds occurred in small amounts and their origin and diagnostic significance remains uncertain, except, that is, for the HCN, which indicates the presence of Pseudomonas aeruginosa.

The analysis of 1-propanol and 2-propanol in humid air samples using selected ion flow tube mass spectrometry

Following the observation that propanol is present in the breath samples of cystic fibrosis (CF) patients infected by Pseudomonas aeruginosa (PA), a study of the reactions of H(3)O(+), NO(+) and O(2) (+.) with 1-propanol and 2-propanol has been conducted using selected ion flow tube mass spectrometry (SIFT-MS). In this study the number and the distribution of the product ions from NO(+) reactions with the two propanol isomers under humid air conditions were able to differentiate between the two isomers. The reaction mechanisms and the structures of the product ions for these reactions, especially those with H(3)O(+) and NO(+), have been proposed. As an example, 2-propanol was shown to be present in a breath sample from one CF patient infected with PA, and also in a PA isolate from another CF patient grown on Pseudomonas-selective media. The results of this study allow an analytical procedure to be advanced for the analysis of the two propanol isomers, which can also be utilised in other applications.