Meta-analysis of Soil and Dust Ingestion Studies.

The ingestion of soil and dust by children and adults is a potential source of exposure to environmental contaminants. To advance beyond the simple averaging of estimates used in the U.S. EPA's Exposure Factors Handbook (EFH), we describe a novel meta-analysis of all available studies that provided soil or dust ingestion estimates for children or adults conducted in the United States and Canada. Using meta-analytic techniques, we estimate the mean total soil plus dust ingestion rates and confidence intervals (CIs) for eleven age groups (0 - <1 month (m), 1 - <3 m, 3 - <6 m, 6 - <12 m, 1 - <2 years (y), 2 - <3 y, 3 - <6 y, 6 - <11 y, 11 - <16 y, 16 - <21 y, and 21+ y). These age groups were selected for consistency with the EFH update to Chapter 5 and the U.S. EPA's Age Grouping Guidance. For each age group, we calculated best estimates for the three main types of ingestion studies: tracer studies based on the aluminum tracer, biokinetic studies, and activity pattern (modeling) studies, as well as overall estimates for all three study types combined. Our meta-analysis combined study estimates using the alternative statistical approaches of the fixed effect method (inverse variance method, "I-V") and two random effects methods, DerSimonian and Laird's method of moments ("DSL") and the restricted maximum likelihood method ("MIXED"). For each approach, the mean total soil plus dust ingestion rate estimates for each study type generally aligned well with the EFH, ranging from 36-68 mg/day for infants, 56-72 mg/day for young children, and 12-32 mg/day for adolescents and adults. When all three study types were combined, the upper bounds of the 95% CI were generally the lowest for the I-V method and the highest for the MIXED method. The estimates produced here can be used for stochastic risk assessments and provide a better estimate of soil and dust ingestion rates across age groups.

Post-acute sequelae of SARS-CoV-2 infection (Long COVID) in older adults.

Long COVID, also known as PASC (post-acute sequelae of SARS-CoV-2), is a complex infection-associated chronic condition affecting tens of millions of people worldwide. Many aspects of this condition are incompletely understood. Among them is how this condition may manifest itself in older adults and how it might impact the older population. Here, we briefly review the current understanding of PASC in the adult population and examine what is known on its features with aging. Finally, we outline the major gaps and areas for research most germane to older adults.

Goals, Structure and Financing of Surgical Residency Training: A Subcommittee Report of the Blue Ribbon Committee II.

OBJECTIVE: As part of the Blue Ribbon Committee II, review current goals, structure and financing of surgical training in Graduate Medical Education (GME) and recommend needed changes. SUMMARY BACKGROUND DATA: Surgical training has continually undergone major transitions with the 80-hour work week, earlier specialization (vascular, plastics and cardiovascular) and now entrustable professional activities (EPAs) as part of competency based medical education (CBME). Changes are needed to ensure the efficiencies of CBME are utilized, that stable graduate medical education funding is secured, and that support for surgeons who teach is made available. METHODS: Convened subcommittee discussions to determine needed focus for recommendations. RESULTS: Five recommendations are offered for changes to GME financing, incorporation of CBME, and support for educators, students and residents in training. CONCLUSIONS: Changes in surgical training related to CBME offer opportunity for change and innovation. Our subcommittee has laid out a potential path forward for improvements in GME funding, training structure, compensation of surgical educators, and support of students and residents in training.

Breast Implant Associated Anaplastic Large Cell Lymphoma: Evidence-Based Consensus Conference Statement From The American Association of Plastic Surgeons.

BACKGROUND: In the absence of high-quality evidence, there is a need to provide guidelines and multidisciplinary consensus recommendations on Breast Implant-Associated Anaplastic Large Cell Lymphoma (BIA-ALCL). The purpose of this expert consensus conference was to evaluate the existing evidence regarding the diagnosis, and management of BIA-ALCL caused by textured implants. The aim is to provide evidence-based recommendations regarding the management and prevention of BIA-ALCL. METHODS: A comprehensive search was conducted in the MEDLINE, Cochrane Library, and Embase databases, supplemented by manual searches of relevant English language articles and "related articles" sections. Studies focusing on breast surgery and lymphoma associated with breast implants were included for analysis. Meta-analyses were performed and reviewed by experts selected by the American Association of Plastic Surgeons by a Delphi consensus method. RESULTS: 840 articles between January 2011 and January 2023 were initially identified and screened. Full-text of 188 articles were assessed. An additional 43 articles were excluded for focus, and 145 articles were included in the synthesis of results, with 105 of them being case reports or case series. The analysis encompassed a comprehensive examination of the selected articles to determine the incidence, risk factors, clinical presentation, diagnostic approaches, and treatment modalities related to BIA-ALCL. CONCLUSIONS: Plastic surgeons should be aware of the elevated risks by surface type, implement appropriate patient surveillance, and follow the recommendations outlined in this statement to ensure patient safety and optimize outcomes. Ongoing research on pathogenesis, genetic drivers, and preventative and prophylactic measures is crucial for improving patient care.

Learning from the early careers of master clinicians.

BACKGROUND: Master clinicians are recognized as multidimensional experts in clinical medicine. Studying their formative clinical activities could generate insights to guide medical trainees and early career clinicians. OBJECTIVES: To investigate which early career activities were adopted more commonly by master clinicians than their matched peers and to characterize master clinicians' early career activities across institutions and specialties. SUBJECTS AND METHODS: We surveyed master clinicians at seven medical centres about their early career activities. For master clinicians in the Department of Medicine (DOM), we also surveyed matched internist peers. RESULTS: Of 150 master clinician respondents, 65% were internists (DOM); 35% practiced in other specialties. Compared to their internist peers, there was a trend toward internist master clinicians reading more about their patients' conditions (6.0 vs. 4.8 h per week), reading more case reports (4.0 vs. 2.1 per month), engaging in more frequent teaching duties and devoting less time to research. CONCLUSIONS: The early career activities identified in this study can be adopted by clinicians pursuing clinical excellence and promoted by training programs that seek to foster life-long learning.

Advancing Methodologies Used in Trace Element-Based Mass Balance Studies to Separately Estimate Soil and Dust Ingestion Rates for Children.

Historically, soil ingestion rate estimates were based on trace element-based mass balance (MB) study results. These were used in assessing exposures and health risks for children residing in Superfund or chemically contaminated communities. However, soil and dust can have considerable differences with respect to their sources, chemical, physical, and toxicological characteristics. Unfortunately, the MB approach is incapable of disentangling dust ingestion rates from soil ingestion rates. Alternative methods, such as activity pattern and biokinetic modeling techniques, have also been used to predict soil and dust ingestion rates. The results from these studies differed from those obtained from the MB studies. This research evaluated the MB methodology and formulated a physical model which characterized the environmental and behavioral determinants of soil and dust ingestion exposures by children. This new approach explicitly separates outdoor soil exposures from the indoor tracked-in soil portion of the dust and total dust exposures by utilizing information from five key MB studies along with new information derived from the SHEDS-Soil/Dust time-activity pattern-based modeling runs. Application of this new hybrid methodology showed that the predicted mean soil ingestion rates are 30%-70% less than the "total soil" ingestion rates obtained from the selected MB studies. In contrast, most of the predicted dust ingestion rate estimates were typically greater than the predicted soil ingestion rates. Moreover, the predicted total soil plus dust ingestion rates were found to be mostly higher (by ≤ 60%) than the MB-based "total soil" ingestion rates. Except for one study these results were higher than the results produced by the stand-alone SHEDS-Soil/Dust model runs. Across the MB studies analyzed, predicted outdoor soil ingestion rate contributions to "total soil" ingestion rates varied between 29% and 70% while the tracked-in soil portion of the indoor dust ingestion rates varied between 30% and 71%.

EEPD1 promotes repair of oxidatively-stressed replication forks.

Unrepaired oxidatively-stressed replication forks can lead to chromosomal instability and neoplastic transformation or cell death. To meet these challenges cells have evolved a robust mechanism to repair oxidative genomic DNA damage through the base excision repair (BER) pathway, but less is known about repair of oxidative damage at replication forks. We found that depletion or genetic deletion of EEPD1 decreases clonogenic cell survival after oxidative DNA damage. We demonstrate that EEPD1 is recruited to replication forks stressed by oxidative damage induced by H2O2 and that EEPD1 promotes replication fork repair and restart and decreases chromosomal abnormalities after such damage. EEPD1 binds to abasic DNA structures and promotes resolution of genomic abasic sites after oxidative stress. We further observed that restoration of expression of EEPD1 via expression vector transfection restores cell survival and suppresses chromosomal abnormalities induced by oxidative stress in EEPD1-depleted cells. Consistent with this, we found that EEPD1 preserves replication fork integrity by preventing oxidatively-stressed unrepaired fork fusion, thereby decreasing chromosome instability and mitotic abnormalities. Our results indicate a novel role for EEPD1 in replication fork preservation and maintenance of chromosomal stability during oxidative stress.

BRCA1 mediates protein homeostasis through the ubiquitination of PERK and IRE1.

Tumors with BRCA1 mutations have poor prognoses due to genomic instability. Yet this genomic instability has risks and BRCA1-deficient (def) cancer cells must develop pathways to mitigate these risks. One such risk is the accumulation of unfolded proteins in BRCA1-def cancers from increased mutations due to their loss of genomic integrity. Little is known about how BRCA1-def cancers survive their genomic instability. Here we show that BRCA1 is an E3 ligase in the endoplasmic reticulum (ER) that targets the unfolded protein response (UPR) stress sensors, Eukaryotic Translation Initiation Factor 2-alpha Kinase 3 (PERK) and Serine/Threonine-Protein Kinase/Endoribonuclease Inositol-Requiring Enzyme 1 (IRE1) for ubiquitination and subsequent proteasome-mediated degradation. When BRCA1 is mutated or depleted, both PERK and IRE1 protein levels are increased, resulting in a constitutively activated UPR. Furthermore, the inhibition of protein folding or UPR signaling markedly decreases the overall survival of BRCA1-def cancer cells. Our findings define a mechanism used by the BRCA1-def cancer cells to survive their increased unfolded protein burden which can be used to develop new therapeutic strategies to treat these cancers.

Model-based predictions of soil and dust ingestion rates for U.S. adults using the stochastic human exposure and dose simulation soil and dust model.

BACKGROUND: Adults can be exposed to chemicals through incidental ingestion of soil and dust, either through hobbies, occupations, or behaviors that increase contact with soil or dust (e.g., cleaning or renovating). However, few data describing these ingestion rates are available. OBJECTIVE: Our objective was to use the Stochastic Human Exposure and Dose Simulation Soil and Dust (SHEDS-Soil/Dust) model to estimate distributions of soil and dust ingestion rates for adults (≥21 years old) with varying degrees of soil and dust contact. METHODS: We parameterized SHEDS-Soil/Dust to estimate soil and dust ingestion rates for several categories of adults: adults in the general population; adults with moderate (higher) soil exposure (represented by hobbyists, such as gardeners, with increased soil contact); adults with high soil exposure (represented by occupationally exposed individuals, such as landscapers); and individuals who have high dust exposure (e.g., are in contact with very dusty indoor environments). RESULTS: Total soil plus dust ingestion for adults in the general population was 7 mg/day. Hobbyists or adults with moderate soil exposure averaged 33 mg/day and occupationally exposed individuals averaged 123 mg/day. Total soil plus dust ingestion for adults in the high dust exposure scenario was 25 mg/day. Results were driven by time spent in contact with soil and, thus, warmer seasons (e.g., summer) were associated with higher ingestion rates than colder seasons (e.g., winter). SIGNIFICANCE: These results provide modeled estimates of soil and dust ingestion rates for adults for use in decision making using real-world exposure considerations. These modeled estimates suggest that soil and dust ingestion is a potential concern for adults who spend a higher amount of time interacting with either soil or dusty environments. IMPACT STATEMENT: The parameterization of real-world scenarios within the application of SHEDS-Soil/Dust model to predict soil and dust ingestion rates for adults provides estimates of soil and dust ingestion rates useful for refining population-based risk assessments. These data illuminate drivers of exposure useful for both risk management decisions and designing future studies to improve existing tracer methodologies.

Evaluation of Variability in Operative Efficiency in Plastic Surgery Procedures.

BACKGROUND: As the cost of healthcare rises, it is imperative to assess value delivered to patients and impact on the economic viability of institutions. We aimed to characterize plastic surgery operative time management and identified areas for efficiency improvement. METHODS: Procedures from a US academic plastic surgery division from September 2017 to August 2018 were reviewed. Times were categorized into preparation (patient in room to incision), procedure (incision to closure), exit (closure to patient exiting room), and turnover (patient out of room to next patient in room). Median and interquartile ranges were calculated. Procedures were classified by relative value units (RVUs) for comparison of procedure complexities and resources. Components were plotted against RVUs; r2 values were calculated. RESULTS: We analyzed 522 cases; 69 were excluded for missing data, primary surgeon not a plastic surgeon, emergent cases, or burn procedures; a total of 453 cases were analyzed. Median and interquartile range (in minutes) for preparation was (34, 18 minutes; 23% of total), procedure (53, 75 minutes; 36% of total), exit (30, 27 minutes; 20% of total), and turnover (30, 26 minutes; 20% of total). Normalized to RVUs, preparation demonstrated the most variability (r2 = 0.19), followed by exit (r2 = 0.38), and procedure (r2 = 0.57). Average work RVUs per month was 678.1 ± 158.7. Average work RVUs per OR hour was 7.2. CONCLUSIONS: The largest component with greatest variability was preparation for surgery in the OR. Improved efficiency by decreasing variability increases the value of healthcare delivered to patients and OR throughput.

Quality Improvement Intervention Bundle Using the PUPPIES Acronym Reduces Pressure Injury Incidence in Critically Ill Patients.

OBJECTIVE: To assess whether a quality improvement bundle focusing on prevention is effective in reducing pressure injury (PI) incidence or costs or delaying PI onset. METHODS: A combined retrospective/prospective cohort study was performed at an academic tertiary care ICU on all patients admitted with a length of stay longer than 48 hours and Braden scale score of 18 or less. Following retrospective data collection (preintervention), a multimodal quality improvement bundle focusing on PI prevention through leadership initiatives, visual tools, and staff/patient education was developed, and data were prospectively collected (postintervention). RESULTS: Statistical and cost analyses were performed comparing both cohorts. A total of 930 patients met the study inclusion criteria (preintervention, n = 599; postintervention, n = 331). A significant decrease in PI incidence was observed from preintervention (n = 37 [6%]) to postintervention (n = 7 [2%], P = .005). This led to a predicted yearly cost savings of $826,810. Further, a significant increase in time to PI occurrence was observed from preintervention (mean, 5 days) to postintervention (mean, 9 days; P = .04). Staff were compliant with the bundle implementation 80% of the time. CONCLUSIONS: Implementation of the quality improvement bundle focused on multimodal PI prevention in critically ill patients led to a significant reduction in PI incidence, increased time to PI occurrence, and was cost-effective.

Model based prediction of age-specific soil and dust ingestion rates for children.

BACKGROUND: Soil and dust ingestion can be a primary route of environmental exposures. Studies have shown that young children are more vulnerable to incidental soil and dust ingestion. However, available data to develop soil and dust ingestion rates for some child-specific age groups are either lacking or uncertain. OBJECTIVE: Our objective was to use the Stochastic Human Exposure and Dose Simulation Soil and Dust (SHEDS-Soil/Dust) model to estimate distributions of soil and dust ingestion rates for ten age ranges from infancy to late adolescents (birth to 21 years). METHODS: We developed approaches for modeling age groups previously not studied, including a new exposure scenario for infants to capture exposures to indoor dust via pacifier use and accounting for use of blankets that act as a barrier to soil and dust exposure. RESULTS: Overall mean soil and dust ingestion rates ranged from ~35 mg/day (infants, 0-<6 m) to ~60 mg/day (toddlers and young children, 6m-<11 yr) and were considerably lower (about 20 mg/day) for teenagers and late adolescents (16-<21 y). The pacifier use scenario contributed about 20 mg/day to the median dust ingestion rate for young infants. Except for the infant age groups, seasonal analysis showed that the modeled estimates of average summer mean daily total soil plus dust ingestion rates were about 50% higher than the values predicted for the winter months. Pacifier use factors and carpet dust loading values were drivers of exposure for infants and younger children. For older children, influential variables included carpet dust loading, soil adherence, and factors that capture the frequency and intensity of hand-to-mouth behaviors. SIGNIFICANCE: These results provide modeled estimates of children's soil and dust ingestion rates for use in decision making using real-world exposure considerations. IMPACT STATEMENT: The parameterization of scenarios to capture infant soil and dust ingestion and the application of SHEDS-Soil/Dust to a broader age range of children provides additional estimates of soil and dust ingestion rates that are useful in refining population-based risk assessments. These data illuminate drivers of exposure that are useful to both risk management applications and for designing future studies that improve upon existing tracer methodologies.

Diacylglycerol Lipase-ß Knockout Mice Display a Sex-Dependent Attenuation of Traumatic Brain Injury-Induced Mortality with No Impact on Memory or Other Functional Consequences.

Background: The endogenous cannabinoid system modulates inflammatory signaling in a variety of pathological states, including traumatic brain injury (TBI). The selective expression of diacylglycerol lipase-ß (DAGL-ß), the 2-arachidonylglycerol biosynthetic enzyme, on resident immune cells of the brain (microglia) and the role of this pathway in neuroinflammation, suggest that this enzyme may contribute to TBI-induced neuroinflammation. Accordingly, we tested whether DAGL-ß-/- mice would show a protective phenotype from the deleterious consequences of TBI on cognitive and neurological motor functions. Materials and Methods: DAGL-ß-/- and -ß+/+ mice were subjected to the lateral fluid percussion model of TBI and assessed for learning and memory in the Morris water maze (MWM) Fixed Platform (reference memory) and Reversal (cognitive flexibility) tasks, as well as in a cued MWM task to infer potential sensorimotor/motivational deficits. In addition, subjects were assessed for motor behavior (Rotarod and the Neurological Severity Score assays) and in the light/dark box and the elevated plus maze to infer whether these manipulations affected anxiety-like behavior. Finally, we also examined whether brain injury disrupts the ceramide/sphingolipid lipid signaling system and if DAGL-ß deletion offers protection. Results: TBI disrupted all measures of neurological motor function and reduced body weight, but did not affect body temperature or performance in common assays used to infer anxiety. TBI also impaired performance in MWM Fixed Platform and Reversal tasks, but did not affect cued MWM performance. Although no differences were found between DAGL-ß-/- and -ß+/+ mice in any of these measures, male DAGL-ß-/- mice displayed an unexpected survival-protective phenotype, which persisted at increased injury severities. In contrast, TBI did not elicit mortality in female mice regardless of genotype. TBI also produced significant changes in sphingolipid profiles (a family of lipids, members of which have been linked to both apoptotic and antiapoptotic pathways), in which DAGL-ß deletion modestly altered levels of select species. Conclusions: These findings indicate that although DAGL-ß does not play a necessary role in TBI-induced cognitive and neurological function, it appears to contribute to the increased vulnerability of male mice to TBI-induced mortality, whereas female mice show high survival rates irrespective of DAGL-ß expression.

Seizing opportunity: Diverse early-entry nursing students' perceptions of seeking a PhD.

BACKGROUND: The aging and retirement of the current nursing professoriate and the increasing numbers of nurses pursuing practice doctorates has precipitated decreasing numbers of nurses, specifically diverse nurses pursuing a research doctorate, thus limiting the development of nursing science. PURPOSE: To describe factors influencing decisions about entering a PhD program from the perspectives of early-entry PhD nursing students. METHOD: A qualitative descriptive design using semistructured interviews to explore the perceptions of making the decision to pursue a PhD in nursing of the students who participated in two early-entry mentoring programs. FINDINGS: A model, entitled "Seizing Opportunity" was developed from the findings about the process of students deciding to pursue a PhD. DISCUSSION: Motivators and detractors that can help nursing educators understand how to successfully recruit diverse PhD students were uncovered. Providing knowledge and mentoring for early entry students can increase the numbers of diverse students pursuing a PhD in nursing.

A state-of-the-science review of polychlorinated biphenyl exposures at background levels: relative contributions of exposure routes.

Exposure to polychlorinated biphenyls (PCBs) can occur through multiple routes and sources, including dietary intake, inhalation, dermal contact, and ingestion of dust and soils. Dietary exposure to PCBs is often considered the primary exposure route for the general population; however, recent studies suggest an increasing contribution from indoor inhalation exposure. Here, we aim to estimate the relative contribution of different PCB exposure pathways for the general population, as well as for select age groups. We conducted a targeted literature review of PCB concentrations in environmental media, including indoor and outdoor air, indoor dust, and soils, as well as of total dietary intake. Using the average concentrations from the studies identified, we estimated PCB exposure through different routes for the general population. In addition, we assessed exposure via environmental media for select age groups. We identified a total of 70 studies, 64 that provided background PCB concentrations for one or more of the environmental media of interest and 6 studies that provided estimates of dietary intake. Using estimates from studies conducted worldwide, for the general population, dietary intake of PCBs was the major exposure pathway. In general, our review identifies important limitations in the data available to assess population exposures, highlighting the need for more current and population-based estimates of PCB exposure, particularly for indoor air and dietary intake.

ExpoKids: An R-based tool for characterizing aggregate chemical exposure during childhood.

BACKGROUND: Aggregate exposure, the combined exposures to a single chemical from all pathways, is a critical children's health issue. OBJECTIVE: The primary objective is to develop a tool to illustrate potential differences in aggregate exposure at various childhood lifestages and the adult lifestage. METHODS: We developed ExpoKids (an R-based tool) using oral exposure estimates across lifestages generated by US EPA's Exposure Factors Interactive Resource for Scenarios Tool (ExpoFIRST). RESULTS: ExpoKids is applied to illustrate aggregate oral exposure, for ten media, as average daily doses (ADD) and lifetime average daily doses (LADD) in five graphs organized across seven postnatal childhood lifestages and the adult lifestage. This data visualization tool conveys ExpoFIRST findings, from available exposure data, to highlight the relative contributions of media and lifestages to chemical exposure. To evaluate the effectiveness of ExpoKids, three chemical case examples (di[2-ethylhexyl] phthalate [DEHP], manganese, and endosulfan) were explored. Data available from the published literature and databases for each case example were used to explore research questions regarding media and lifestage contributions to aggregate exposure. SIGNIFICANCE: These illustrative case examples demonstrate ExpoKids' versatile application to explore a diverse set of children's health risk assessment and management questions by visually depicting specific media and lifestage contributions to aggregate exposure.

Insurance Denials in Reduction Mammaplasty: How Can We Serve Our Patients Better?

BACKGROUND: Reduction mammaplasty is the standard of care for symptomatic macromastia. The process of requesting insurance coverage for reduction mammaplasty is cumbersome and potentially controversial, and insurance policies vary significantly. The goal of our analysis is to identify trends in insurance coverage rates, assess for the presence of disparities, and propose ways to increase chances of successful preauthorization. METHODS: The authors performed a retrospective review of preauthorizations for reduction mammaplasty at a single institution from 2012 to 2017. Insurance company names were deidentified. Preauthorization denial rates were assessed by year, insurance carrier, and reason for denial. Multiple regression analysis was performed to identify predictors for predetermination denial by insurance companies. RESULTS: Among 295 preauthorizations, 212 were approved (72 percent) and 83 were denied (28 percent), among which 18 were appealed, 13 successfully. Rates of insurance denials have been increasing steadily, from 18 percent to 41 percent. Medicaid had the lowest denial rates (9.3 percent), whereas private carriers denials ranged from 21.4 to 62.1 percent. In terms of reason for denial, 30 percent were because of contract exclusion, 39 percent were because of inadequate documentation or not meeting medical criteria, and 12 percent were because of inadequate predicted resection weight. Certain private insurance carriers were the only independent predictors of predetermination denial. CONCLUSIONS: Rate of preauthorization denials is high and has been increasing steadily. Insurance criteria remain arbitrary. A proper documentation and appeal process by the plastic surgeon may improve rates of insurance approval. Although resection weight does not correlate with symptom relief, predicted breast tissue resection weight continues to be critical for insurance approval.

Multiple enzymes can make hydrogen sulfide from cysteine in Treponema denticola.

Treponema denticola is a spirochete that is involved in causing periodontal diseases. This bacterium can produce H2S from thiol compounds found in the gingival crevicular fluid. Determining how H2S is made by oral bacteria is important since this molecule is present at high levels in periodontally-diseased pockets and the biological effects of H2S can explain some of the pathologies seen in periodontitis. Thus, it is of interest to identify the enzyme, or enzymes, involved in the synthesis of H2S by T. denticola. We, and others, have previously identified and characterized a T. denticola cystalysin, called HlyA, which hydrolyzes cysteine into H2S (and pyruvate and ammonia). However, there have been no studies to show that HlyA is, or is not, the only pathway that T. denticola can use to make H2S. To address this question, allelic replacement mutagenesis was used to make a deletion mutant (ΔhlyA) in the gene encoding HlyA. The mutant produces the same amount of H2S from cysteine as do wild type spirochetes, indicating that T. denticola has at least one other enzyme that can generate H2S from cysteine. To identify candidates for this other enzyme, a BLASTp search of T. denticola strain 33520 was done. There was one gene that encoded an HlyA homolog so we named it HlyB. Recombinant His-tagged HlyB was expressed in E. coli and partially purified. This enzyme was able to make H2S from cysteine in vitro. To test the role of HlyB in vivo, an HlyB deletion mutant (ΔhlyB) was constructed in T. denticola. This mutant still made normal levels of H2S from cysteine, but a strain mutated in both hly genes (ΔhlyA ΔhlyB) synthesizes significantly less H2S from cysteine. We conclude that the HlyA and HlyB enzymes perform redundant functions in vivo and are the major contributors to H2S production in T. denticola. However, at least one other enzyme can still convert cysteine to H2S in the ΔhlyA ΔhlyB mutant. An in silico analysis that identifies candidate genes for this other enzyme is presented.