To identify factors that regulate gut microbiota density and the impact of varied microbiota density on health, we assayed this fundamental ecosystem property in fecal samples across mammals, human disease, and therapeutic interventions. Physiologic features of the host (carrying capacity) and the fitness of the gut microbiota shape microbiota density. Therapeutic manipulation of microbiota density in mice altered host metabolic and immune homeostasis. In humans, gut microbiota density was reduced in Crohn’s disease, ulcerative colitis, and ileal pouch-anal anastomosis. The gut microbiota in recurrent Clostridium difficile infection had lower density and reduced fitness that were restored by fecal microbiota transplantation. Understanding the interplay between microbiota and disease in terms of microbiota density, host carrying capacity, and microbiota fitness provide new insights into microbiome structure and microbiome targeted therapeutics.Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Summary 34To identify factors that regulate the absolute microbiota and the impact of varied microbiota density on health, we assayed 35 gut microbiota density across mammals, disease, and therapeutic interventions. Physiologic features of the host (carrying 36 capacity) and the fitness of the gut microbiota shape microbiota density. Therapeutic manipulation of microbiota density in 37 mice altered host metabolic and immune homeostasis. In humans, gut microbiota density was reduced in Crohn's disease, 38 ulcerative colitis, and ileal pouch-anal anastomosis. The gut microbiota in recurrent Clostridium difficile infection had lower 39 density and reduced fitness that were restored by fecal microbiota transplantation. Understanding the interplay between 40 microbiota and disease through the conceptual framework of microbiota density, host carrying capacity, and microbiota fitness 41 could provide biomarkers to identify candidates for microbiota therapeutics and monitor their response.

Heroin-induced leukoencephalopathy (HLE) is a rare but potentially debilitating and sometimes fatal neurological disorder. Despite the widely practiced heroin use via different routes and modalities, the syndrome is said to be rare and mostly associated with inhaling rather than injecting or snorting practices. We reviewed the literature to address the latest diagnostic, therapeutic, and prognostic measures related to the condition. Here, we present a case of a 35-year-old male who admitted to inhaling heroin 18 days ago and has been experiencing ongoing neurological symptoms for the past 17 days. Imaging was consistent with extensive white matter disease at multiple levels and different anatomical regions. Although there is no known cure for HLE, the patient benefited, somewhat, from antioxidants and physical rehabilitation.

Diabetic ketoacidosis is typically associated with type I diabetes mellitus, but it can be associated with type II diabetes mellitus under the conditions of extreme stress or as a presenting manifestation of ketosis-prone type II diabetes mellitus. A 38-year-old prediabetic male presented to the emergency room with hyperglycemia six weeks after recovery from coronavirus disease 2019 (COVID-19) pneumonia. Laboratory results showed severe hyperglycemia, metabolic acidosis, positive ketones in urine and blood, and elevated fasting C- peptide level. COVID-19 polymerase chain reaction (PCR) was negative, and immunoglobulin G (IgG) antibodies were positive. The workup was completely unremarkable for acute infection. Hemoglobin A1C increased from 6.1% to 10.8% within six weeks. The mechanism by which COVID-19 infection may trigger the onset of full-blown diabetes mellitus remains unknown. Viral infection may cause the direct destruction of pancreatic beta cells or trigger the changes in the body that induce the state of insulin resistance. Antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection may cross-react or interfere with the functioning of endogenous insulin. The association between type II diabetes and COVID-19 infections needs additional investigations to ascertain the exact mechanism by which COVID-19 infection triggers the onset of full-blown diabetes mellitus.

Rhabdomyolysis has many causes; however, hypothyroidism is a rare cause of such a condition. Usually, management is similar in many cases, but some exceptions do exist, especially in the case of hypothyroidism. Thus, we reviewed the literature to investigate further precipitant factors, clinical presentations, complications, management, and prognoses. We report a 19-year-old male with a history of hypothyroidism who was brought in for questionable suicidal ideation. Although asymptomatic, he was found to have an acute kidney injury (AKI). Further investigations revealed significantly elevated levels of creatine kinase (CK) and thyroid-stimulating hormone (TSH) in the setting of medication non-compliance. Management with intravenous (IV) fluids and thyroid hormone replacement resulted in an improvement in AKI and CK levels.