Feasibility of self-collection of fecal specimens by randomly sampled women for health-related studies of the gut microbiome

Assessment of a Daily Diary Study Including Biospecimen Collections in a Sample of Sexual and Gender Minority Young Adults: Feasibility and Acceptability Study

BACKGROUND

Young sexual minority men (YSMM) engage in cardiometabolic risk behaviors (eg, substance use) at higher rates than their heterosexual counterparts. Theory and previous research suggest that these risk behaviors may stem, in part, from exposure to minority stress (ie, discrimination based on sexual identity and other identities such as race).

OBJECTIVE

This pilot study examined the feasibility and acceptability of a virtual 2-day daily diary study that examined daily experiences with discrimination, cardiometabolic risk behaviors (ie, sleep, physical activity, and substance use behaviors), and patterns of physiological stress and inflammation among YSMM aged 18 to 35 years.

METHODS

Participants (n=20) were recruited from the greater New York metropolitan area and engaged in a 2-day daily diary protocol wherein they provided web-based consent, took a web-based baseline survey, and then, starting the next day, provided 3 saliva samples a day for 2 consecutive days to measure salivary cortisol, engaged in 3 daily diaries per day, and provided 1 blood spot sample via the finger prick method to measure high-sensitivity C-reactive protein. At follow-up, participants were interviewed via videoconferencing to ascertain their experiences and feelings related to the study protocol. Qualitative analyses explored the feasibility and acceptability of the study protocol, and exploratory quantitative analyses explored the descriptive statistics and Pearson correlations among the main study variables of interest.

RESULTS

The retention rate was high (19/20, 95%) in our study sample. Qualitative analyses demonstrated that participants were willing to engage in similar, longer-term studies (eg, studies that include both week and weekend days) in the future and suggested the feasibility and acceptability of our study protocol among YSMM. However, participants noted several areas for improvement (eg, redundancy of survey items and difficulty pricking one's finger) that should be considered in future research. Preliminary quantitative analyses revealed a moderate negative correlation between everyday discrimination and mean cortisol levels (r=-0.51; P=.03). Furthermore, descriptive analyses suggest that that daily cortisol curves differ across races or ethnicities among YSMM. White and other-identified YSMM experienced the highest cortisol awakening response (mean 0.39, SD 0.21 µg/dL for White participants; mean 0.34, SD 0.34 µg/dL for others) with the steepest decline around bedtime (mean 0.05, SD 0.04 µg/dL for White participants; mean 0.09, SD 0.13 µg/dL for others) followed by a lower cortisol awakening response (mean 0.31, SD 0.11 µg/dL for Hispanic participants; mean 0.23, SD 0.15 µg/dL for Black participants) and a slower decline around bedtime (mean 0.10, SD 0.09 µg/dL for Hispanic participants; mean 0.03, SD 0.02 µg/dL for Black participants) among Hispanic and Black YSMM.

CONCLUSIONS

Overall, the results suggest that similar study protocols are feasible and acceptable among YSMM. Future research should highlight the pathways through which cardiovascular disease risk may arise among YSMM using longer-term study designs and more diverse study samples.

Emerging role of colorectal mucus in gastroenterology diagnostics

Colonoscopy is currently the gold standard for diagnosis of inflammatory bowel disease (IBD) and colorectal cancer (CRC). This has the obvious drawback of being invasive as well as carrying a small risk. The most widely used non-invasive approaches include the use of faecal calprotectin in the case of IBD and fecal immunochemical test in the case of CRC. However, the necessity of stool collection limits their acceptability for some patients. Over the recent years, there has been emerging data looking at the role of non-invasively obtained colorectal mucus as a screening and diagnostic tool in IBD and CRC. It has been shown that the mucus rich material obtained by self-sampling of anal surface following defecation, can be used to measure various biomarkers that can aid in diagnosis of these conditions.

Adaptation of Metabolomics and Microbiomic Research Protocols During the COVID-19 Pandemic

BACKGROUND

When the COVID-19 pandemic hit in 2020, researchers in the P30 Center for the Study of Symptom Science, Metabolomics, and Multiple Chronic Conditions at Emory University's Nell Hodgson Woodruff School of Nursing faced major challenges in recruitment and data collection because of limited access to the clinic and community facilities and the risk of COVID-19 exposure associated with in-person study contact.

OBJECTIVES

The purpose of this article is to (a) describe how a cadre of pilot/supplement principal investigators adapted their studies to allow for safe and trustworthy data collection during the COVID-19 pandemic (March 2020 through date of publication) and (b) discuss steps that facilitated the technical aspects of remote data collection, especially involving biological specimens.

RESULTS

Four pilot studies and two administrative supplements within the center-all at different stages of execution-adopted various alternative remote recruitment, enrollment, and data and specimen collection approaches to continue their research endeavors in a way that maximized the safety of both the research participants and the research teams.

DISCUSSION

The article concludes with a discussion on the importance of a participant-centered approach when using remote methods, actions, or steps initiated to facilitate the technical aspects of remote data collection and reflections on the continued use of remote research strategies beyond the COVID-19 pandemic.

Carriage of ESBL-producing Enterobacterales in wastewater treatment plant workers and surrounding residents - the AWARE Study

To investigate whether wastewater treatment plant (WWTP) workers and residents living in close proximity to a WWTP have elevated carriage rates of ESBL-producing Enterobacterales, as compared to the general population. From 2018 to 2020, we carried out a cross-sectional study in Germany, the Netherlands, and Romania among WWTP workers (N = 344), nearby residents (living ≤ 300 m away from WWTPs; N = 431) and distant residents (living ≥ 1000 m away = reference group; N = 1165). We collected information on potential confounders via questionnaire. Culture of participants' stool samples was performed with ChromID®-ESBL agar plates and species identification with MALDI-TOF-MS. We used logistic regression to estimate the odds ratio (OR) for carrying ESBL-producing E. coli (ESBL-EC). Sensitivity analyses included stratification by country and interaction models using country as secondary exposure. Prevalence of ESBL-EC was 11% (workers), 29% (nearby residents), and 7% (distant residents), and higher in Romania (28%) than in Germany (7%) and the Netherlands (6%). Models stratified by country showed that within the Romanian population, WWTP workers are about twice as likely (aOR = 2.34, 95% CI: 1.22-4.50) and nearby residents about three times as likely (aOR = 3.17, 95% CI: 1.80-5.59) to be ESBL-EC carriers, when compared with distant residents. In stratified analyses by country, we found an increased risk for carriage of ESBL-EC in Romanian workers and nearby residents. This effect was higher for nearby residents than for workers, which suggests that, for nearby residents, factors other than the local WWTP could contribute to the increased carriage.

Opioid agonist and antagonist use and the gut microbiota: associations among people in addiction treatment

Murine models suggest that opioids alter the gut microbiota, which may impact opioid tolerance and psychopathology. We examined how gut microbiota characteristics related to use of opioid agonists and antagonists among people receiving outpatient addiction treatment. Patients (n = 46) collected stool samples and were grouped by use of opioid agonists (heroin, prescription opioids), antagonists (naltrexone), agonist-antagonist combinations (buprenorphine-naloxone), or neither agonists nor antagonists within the month before enrollment. We sequenced the V4 region of the 16S rRNA gene using Illumina MiSeq to examine how alpha diversity, enterotypes, and relative abundance of bacterial genera varied by opioid agonist and antagonist exposures. Compared to 31 participants who used neither agonists nor antagonists, 5 participants who used opioid agonists (without antagonists) had lower microbiota diversity, Bacteroides enterotypes, and lower relative abundance of Roseburia, a butyrate producing genus, and Bilophila, a bile acid metabolizing genus. There were no differences in gut microbiota features between those using agonist + antagonists (n = 4), antagonists only (n = 6), and neither agonists nor antagonists. Similar to murine morphine exposure models, opioid agonist use was associated with lower microbiota diversity. Lower abundance of Roseburia and Bilophila may relate to the gut inflammation/permeability and dysregulated bile acid metabolism observed in opioid-exposed mice.

Stool sampling and DNA isolation kits affect DNA quality and bacterial composition following 16S rRNA gene sequencing using MiSeq Illumina platform

Many studies correlate changes in human gut microbiome with the onset of various diseases, mostly by 16S rRNA gene sequencing. Setting up the optimal sampling and DNA isolation procedures is crucial for robustness and reproducibility of the results. We performed a systematic comparison of several sampling and DNA isolation kits, quantified their effect on bacterial gDNA quality and the bacterial composition estimates at all taxonomic levels. Sixteen volunteers tested three sampling kits. All samples were consequently processed by two DNA isolation kits. We found that the choice of both stool sampling and DNA isolation kits have an effect on bacterial composition with respect to Gram-positivity, however the isolation kit had a stronger effect than the sampling kit. The proportion of bacteria affected by isolation and sampling kits was larger at higher taxa levels compared to lower taxa levels. The PowerLyzer PowerSoil DNA Isolation Kit outperformed the QIAamp DNA Stool Mini Kit mainly due to better lysis of Gram-positive bacteria while keeping the values of all the other assessed parameters within a reasonable range. The presented effects need to be taken into account when comparing results across multiple studies or computing ratios between Gram-positive and Gram-negative bacteria.

Mammographic breast density and its association with urinary estrogens and the fecal microbiota in postmenopausal women

BACKGROUND

Breast density, as estimated by mammography, is a strong risk factor for breast cancer in pre- and postmenopausal women, but the determinants of breast density have not yet been established. The aim of this study was to assess if urinary estrogens or gut microbiota alterations are associated with mammographic density in postmenopausal women.

METHODS

Among 54 cancer-free, postmenopausal controls in the Breast and Colon Health study, we classified low- versus high-density women with Breast Imaging Reporting and Data System (BI-RADS, 5th edition) mammographic screening data, then assessed associations with urinary estrogens and estrogen metabolites (determined by liquid chromatography/tandem mass spectrometry), and fecal microbiota alpha and beta diversity (using Illumina sequencing of 16S rRNA amplicons).

RESULTS

Multiple logistic regression revealed no significant association between breast density and fecal microbiota metrics (PD_tree P-value = 0.82; un-weighted and weighted UniFrac P = 0.92 and 0.83, respectively, both by MiRKAT). In contrast, total urinary estrogens (and all 15 estrogens/estrogen metabolites) were strongly and inversely associated with breast density (P = 0.01) after adjustment for age and body mass index.

CONCLUSION

Mammographic density was not associated with the gut microbiota, but it was inversely associated with urinary estrogen levels.

IMPACT

The finding of an inverse association between urinary estrogens and breast density in cancer-free women adds to the growing breast cancer literature on understanding the relationship between endogenous estrogens and mammographic density.

Postmenopausal breast cancer and oestrogen associations with the IgA-coated and IgA-noncoated faecal microbiota

Background:

The diversity and composition of the gut microbiota may affect breast cancer risk by modulating systemic levels of oestrogens and inflammation. The current investigation tested this hypothesis in postmenopausal women by identifying breast cancer associations with an inflammation marker, oestrogen levels, and faecal microbes that were or were not coated with mucosal immunoglobulin A (IgA).

Methods:

In this population-based study, we compared 48 postmenopausal breast cancer cases (75% stage 0–1, 88% oestrogen-receptor positive) to 48 contemporaneous, postmenopausal, normal-mammogram, age-matched controls. Microbiota metrics employed 16S rRNA gene amplicon sequencing from IgA-coated and -noncoated faecal microbes. High-performance liquid chromatography/mass spectrometry (HPLC/MS) and radioimmunoassay were used to quantify urine prostaglandin E metabolite (PGE-M), a possible marker of inflammation; urine oestrogens and oestrogen metabolites were quantified by HPLC/MS-MS.

Results:

Women with pre-treatment breast cancer had non-significantly elevated oestrogen levels; controls’ (but not cases’) oestrogens were directly correlated with their IgA-negative microbiota alpha diversity (P=0.012). Prostaglandin E metabolite levels were not associated with case status, oestrogen levels, or alpha diversity. Adjusted for oestrogens and other variables, cases had significantly reduced alpha diversity and altered composition of both their IgA-positive and IgA-negative faecal microbiota. Cases’ faecal microbial IgA-positive imputed Immune System Diseases metabolic pathway genes were increased; also, cases’ IgA-positive and IgA-negative imputed Genetic Information Processing pathway genes were decreased (P⩽0.01).

Conclusions:

Compared to controls, breast cancer cases had significant oestrogen-independent associations with the IgA-positive and IgA-negative gut microbiota. These suggest that the gut microbiota may influence breast cancer risk by altered metabolism, oestrogen recycling, and immune pressure.

Informing future research for carriage of multiresistant Gram-negative bacteria: problems with recruiting to an English stool sample community prevalence study

OBJECTIVES

This study aims to highlight problems with recruiting to an English stool sample community prevalence study. It was part of a larger cross-sectional research to determine the risk factors for the presence of extended-spectrum beta-lactamase and carbapenemase-producing coliforms in stool samples of the asymptomatic general English population.

SETTING

Four National Health Service primary care trusts (PCTs) of England representing a different section of the population of England: Newham PCT; Heart of Birmingham Teaching PCT; Shropshire County PCT; and Southampton City PCT.

PARTICIPANTS

Sixteen general practices across the four PCTs were purposefully selected. After stratification of GP lists by age, ethnicity and antibiotic use, 58 337 randomly selected patients were sent a postal invitation.Patients who had died, moved to a different surgery, were deemed too ill by their General Practitioner or hospitalised at the time of mailing were excluded.

RESULTS

Stool and questionnaire returns varied by area, age, gender and ethnicity; the highest return rate of 27.3% was in Shropshire in the age group of over 60 years; the lowest, 0.6%, was in Birmingham in the age group of 18-39 years. Whereas only 3.9%(2296) returned a completed questionnaire and stool sample, 94.9% of participants gave permission for their sample and data to be used in future research.

CONCLUSION

Researchers should consider the low stool specimen return rate and wide variation by ethnicity and age when planning future studies involving stool specimen collection. This is particularly pertinent if the study has no health benefit to participants. Further research is needed to explore how to improve recruitment in multicultural communities and in younger people.

[Design of a genomic, environmental, microbial and metabolomic study on celiac disease: an approach to the future of personalized prevention of celiac disease]

Over recent years we have seen rising many clinical and scientific innovations about celiac disease (CE), however the most important innovation that will contribute to change the future of the research and clinic in this field is the natural history of the disease. For many years it has been though that a genetic predisposition and the exposure to gluten were necessary and sufficient to develop CE. Recent studies, however, suggest that the loss of tolerance to gluten may occur in any moment of life upon certain conditions. Furthermore, several environmental factors known to play a role in shaping the intestinal microflora have also been considered related to the development of CE. Delivery mode, the infant diet and the use of antibiotics are included among these factors. To this day no large scale studies have determined if and how the microbiome composition and its metabolomic profile may influence the loss of tolerance to gluten and the consequent development of CE. In this paper we describe a prospective, multi-centric and longitudinal study on infants at risk for CE that will use different techniques to better understand the role of the microbome during the first steps in the development of the autoimmune disease.

Intricacies of assessing the human microbiome in epidemiologic studies

PURPOSE

In the past decade, remarkable relationships have been documented between dysbiosis of the human microbiota and adverse health outcomes. This review seeks to highlight some of the challenges and pitfalls that may be encountered during all stages of microbiota research, from study design and sample collection, to nucleic acid extraction and sequencing, and bioinformatic and statistical analysis.

METHODS

Literature focused on human microbiota research was reviewed and summarized.

RESULTS

Although most studies have focused on surveying the composition of the microbiota, fewer have explored the causal roles of these bacteria, archaea, viruses, and fungi in affecting disease states. Microbiome research is in its relatively early years and many aspects remain challenging, including the complexity and personalized aspects of microbial communities, the influence of exogenous and often confounding factors, the need to apply fundamental principles of ecology and epidemiology, the necessity for new software tools, and the rapidly evolving genomic, technological, and analytical landscapes.

CONCLUSIONS

Incorporating human microbiome research in large epidemiologic studies will soon help us unravel the intricate relationships that we have with our microbial partners and provide interventional opportunities to improve human health.

Characterization of the gut microbiome in epidemiologic studies: the multiethnic cohort experience

PURPOSE

The development of next-generation sequencing and accompanying bioinformatics tools has revolutionized characterization of microbial communities. As interest grows in the role of the human microbiome in health and disease, so does the need for well-powered, robustly designed epidemiologic studies. Here, we discuss sources of bias that can arise in gut microbiome research.

METHODS

Research comparing methods of specimen collection, preservation, processing, and analysis of gut microbiome samples is reviewed. Although selected studies are primarily based on the gut, many of the same principles are applicable to samples derived from other anatomical sites. Methods for participant recruitment and sampling of the gut microbiome implemented in an ongoing population-based study, the Multiethnic Cohort (MEC), are also described.

RESULTS

Variation in methodologies can influence the results of human microbiome studies. To help minimize bias, techniques such as sample homogenization, addition of internal standards, and quality filtering should be adopted in protocols. Within the MEC, participant response rates to stool sample collection were comparable to other studies, and in-home stool sample collection yields sufficient high-quality DNA for gut microbiome analysis.

CONCLUSIONS

Application of standardized and quality controlled methods in human microbiome studies is necessary to ensure data quality and comparability among studies.

Assessment of cytology and mucin 2 in colorectal mucus collected from patients with inflammatory bowel disease: Results of a pilot trial

BACKGROUND AND AIM

Non-invasive diagnosis of colorectal disease remains problematic, fecal biomarkers presenting the only current option. Colorectal mucus is the diagnostically informative element of stool samples, but its separation from stool is difficult. We aimed to: (i) test a novel method of non-invasive colorectal mucus sampling in a pilot clinical trial; (ii) evaluate sampling method acceptance by study participants; (iii) characterize the collected material cytologically; and (iv) assess feasibility of quantitative protein analysis in the samples.

METHODS

A total of 141 patients with IBD (58), IBS (50), and healthy controls (33) participated in the study. Samples rich in colorectal mucus were self-collected by swabbing the anal area immediately following defecation. Collected samples were examined cytologically and subjected to quantitative analysis for total protein and mucin 2 (MUC2).

RESULTS

The novel sampling technique was assessed as "good" or "adequate" by 96% of study participants. A total of 55% of the collected samples were free of fecal contamination. Cytology showed large numbers of well preserved inflammatory cells in IBD cases. Total protein values varied in all groups, being affected by fecal contamination. MUC2 levels were similar among all IBD-free individuals (control and IBS groups) and elevated in IBD patients (p < 0.001). MUC2 measurement applied as a test for IBD detection provided sensitivity = 72.4% and specificity = 86.7%.

CONCLUSIONS

A novel non-invasive method for collecting human colorectal mucus has been successfully tested. The method was very well accepted by trial participants. The results have proven high quality of collected samples for both cytological investigation and diagnostic biomarker analysis.

Celiac Disease Genomic, Environmental, Microbiome, and Metabolomic (CDGEMM) Study Design: Approach to the Future of Personalized Prevention of Celiac Disease

In the past it was believed that genetic predisposition and exposure to gluten were necessary and sufficient to develop celiac disease (CD). Recent studies however suggest that loss of gluten tolerance can occur at any time in life as a consequence of other environmental stimuli. Many environmental factors known to influence the composition of the intestinal microbiota are also suggested to play a role in the development of CD. These include birthing delivery mode, infant feeding, and antibiotic use. To date no large-scale longitudinal studies have defined if and how gut microbiota composition and metabolomic profiles may influence the loss of gluten tolerance and subsequent onset of CD in genetically-susceptible individuals. Here we describe a prospective, multicenter, longitudinal study of infants at risk for CD which will employ a blend of basic and applied studies to yield fundamental insights into the role of the gut microbiome as an additional factor that may play a key role in early steps involved in the onset of autoimmune disease.

Feasibility of a birth cohort study dedicated to assessing acute infections using symptom diaries and parental collection of biomaterials

Background

A birth cohort dedicated to studying infections in early childhood may be assisted by parental recording of symptoms on a daily basis and a collection of biomaterials. We aimed at testing the feasibility of this approach for use in a long-term study focusing on infections in children in Germany.

Methods

Parents of 1- to 3-year-old children (n = 75) were recruited in nursery schools. They were asked to complete a symptom diary on a daily basis and to take monthly and symptom-triggered nasal swabs and stool samples from their child over the study period of three months. Feasibility was measured by means of the return proportions of symptom diaries and bio samples; acceptance was assessed by a questionnaire delivered to participants at the end of the study.

Results

The majority of the participants filled in the symptom diary during the three months study for 75 or more days (77.3 %), and provided the monthly nasal swabs (62.7 %) and stool samples (65.3 %). The time needed for the tasks was acceptable for most participants (symptom diary: 92.3 %, nasal swabs: 98.5 %, stool samples: 100.0 %). In 64.3 % of the symptom-triggered nasal swabs, respiratory viruses were found compared to 55.5 % in throat swabs taken by health-care professionals within the “ARE surveillance Lower Saxony”, a special project by the Governmental Institute of Public Health of Lower Saxony to investigate causal pathogens for acute respiratory infections in children.

Conclusions

The parental assessment of symptoms and collection of biomaterials in a birth cohort dedicated to studying infections appears feasible in a middle class German population. The success of the study will depend on the ability to maintain these activities over a long time period.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-1189-0) contains supplementary material, which is available to authorized users.

Fecal microbiota analysis: an overview of sample collection methods and sequencing strategies

Despite huge interest, there are still no universally accepted standards to conduct clinical studies in the field of gut microbiota analysis. Stool material is frequently used as a proxy of gut microbiota, but many different protocols can be used for collection and DNA extraction. Whereas 16S rRNA encoding gene amplification and sequencing has been widely used to study the composition of bacterial populations, it is now being challenged by the random, shotgun approach that brings far more information, although at a higher cost. In this review we give an overview of existing methods and important points to consider when conducting gut microbiota studies, with the objective to provide recommendations to those who would like to conduct such research.

Investigation of the association between the fecal microbiota and breast cancer in postmenopausal women: a population-based case-control pilot study

We investigated whether the gut microbiota differed in 48 postmenopausal breast cancer case patients, pretreatment, vs 48 control patients. Microbiota profiles in fecal DNA were determined by Illumina sequencing and taxonomy of 16S rRNA genes. Estrogens were quantified in urine. Case-control comparisons employed linear and unconditional logistic regression of microbiota α-diversity (PD_whole tree) and UniFrac analysis of β-diversity, with two-sided statistical tests. Total estrogens correlated with α-diversity in control patients (Spearman Rho = 0.37, P = .009) but not case patients (Spearman Rho = 0.04, P = .77). Compared with control patients, case patients had statistically significantly altered microbiota composition (β-diversity, P = .006) and lower α-diversity (P = .004). Adjusted for estrogens and other covariates, odds ratio of cancer was 0.50 (95% confidence interval = 0.30 to 0.85) per α-diversity tertile. Differences in specific taxa were not statistically significant when adjusted for multiple comparisons. This pilot study shows that postmenopausal women with breast cancer have altered composition and estrogen-independent low diversity of their gut microbiota. Whether these affect breast cancer risk and prognosis is unknown.

Associations of the fecal microbiome with urinary estrogens and estrogen metabolites in postmenopausal women

CONTEXT

The gut microbiota may influence the risk of breast cancer through effects on endogenous estrogens.

OBJECTIVE

The objective of the study was to investigate whether urinary estrogens and estrogen metabolites are associated with the diversity and composition of the fecal microbiome.

DESIGN AND SETTING

This was a cross-sectional study among women enrolled in Kaiser Permanente of Colorado.

PARTICIPANTS

A total of 60 women drawn from a random sample of healthy postmenopausal women (aged 55-69 y), without current or recent use of antibiotics or hormone therapy and no history of cancer or gastrointestinal disease participated in the study. OUTCOME MEASURES AND METHODS: Creatinine-standardized urinary estrogens (estrone and estradiol) and 13 hydroxylated estrogen metabolites were measured in spot urines by liquid chromatography-tandem mass spectrometry. The fecal microbiome was assessed using pyrosequencing of 16S rRNA amplicons. General linear models were used to test for associations of diversity and composition of the fecal microbiome with parent estrogen (estrone + estradiol), total estrogens, and estrogen metabolites and the ratio of estrogen metabolites to parent estrogen, which has been predictive of postmenopausal breast cancer risk in previous studies.

RESULTS

The ratio of metabolites to parents was directly associated with whole-tree phylogenetic diversity (R = 0.35, P = .01). Relative abundances of the order Clostridiales (R = 0.32, P = .02) and the genus Bacteroides (R = -0.30, P = .03) were also correlated with the ratio of metabolites to parents. Associations were independent of age, body mass index, and study design factors.

CONCLUSIONS

Our data suggest that women with a more diverse gut microbiome exhibit an elevated urinary ratio of hydroxylated estrogen metabolites to parent estrogen. Further research is warranted to confirm and relate these findings to clinical disease.