As the world approaches a dark milestone of three million deaths from COVID-19, a new preprint research paper has been published on bioRxiv* server indicates that the presence of intestinal bacteria in plasma may be an indicator of progressive disease. In patients with pre-existing comorbidities, COVID-19 is associated with more severe disease.
The intestines are a well-established route of infection and are the target of viral damage by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19. This supports the clinical observation that about half of patients with COVID-19 show gastrointestinal (GI) symptoms.
Hospitalized patients with critical COVID-19 also often have bowel complications. In addition to the above, venous or arterial thromboembolism of mesenteric vessels and small bowel ischemia have been reported, especially in long-term hospitalized patients.
Objectives and details of the study
The current study, based in Birmingham, Alabama, USA, aimed to capture the presence of plasma biomarkers that indicate a violation of the intestinal epithelial barrier and the presence of intestinal microbes in plasma.
Unfortunately, both could not be tested on the same subset of patients due to the small volume of plasma available.
Of the 30 patients included in the study with confirmed SARS-CoV-2 infection, all had diarrhea and nausea, along with fever and shortness of breath. The median age was 63 years. Only two patients had critical illness.
Over a third had diabetics, and half had blood clots. Of the 30, 23 patients were overweight. Five patients were fatal at the hospital.
Lymphopenia and anemia were reported in half and two-thirds of patients, along with high monocyte counts. Neutrophils are also elevated, in 60% of men compared to 45% of women. This is significant because they are the first to react to any infection.
Total leukocyte counts were elevated in about 40% of individuals with COVID-19, but platelet abnormalities were noted in only 17% of subjects. Only two patients showed high levels of natriuretic peptide in the brain (BNP), probably due to heart failure.
The C-reactive peptide, an inflammatory marker, was elevated in all patients, with six patients showing levels consistent with severe inflammation. Eight subjects showed high levels of ferritin, half of which had a level of indication of inflammation.
Almost all patients had high fasting glucose and lactate dehydrogenase (LDH) levels. About two-thirds had anemia. Elevated troponin-I levels, indicating heart injury, were found in 80% of men, compared to only one woman.
Intestinal microbes in plasma
The 14 plasma samples sent for assessment for the presence of bacteria gave over 150,000 sequencing readings, with the signal indicating a strong presence of bacteria in two-thirds of the samples. The total microbial population was comparable between patients with COVID-19.
Using a polymerase chain reaction, a dysbiosis index was obtained to measure the abundance of bacterial groups in each sample. All nine samples that indicated the presence of bacteria showed the same three main species, Proteobacteria,, Firmicutes, i Actinobacteria, with one patient showing unknown bacteria in a significant number among all 14.
These are the same ones found in healthy plasma.
He was the richest Proteobacteria, while Bacteroids were present in very limited numbers. Among the two patients with a fatal outcome of COVID-19, the number of Firmicutes was low. Perhaps an abundance of this type may be a biomarker for disease severity.
Gram-negative bacteria and lipopolysaccharides (LPS), which is the major endotoxin derived from the cell wall of these bacteria, are higher in the plasma samples of patients with COVID-19.
Violation of the intestinal barrier
The presence of intestinal microbes in the plasma may suggest deficiencies in the intestinal epithelial barrier, allowing bacteria to migrate through epithelial cells into systemic blood vessels. This is an important component of systemic inflammation and underlies the progression of COVID-19 in these patients.
As a marker of intestinal permeability, levels of protein-2 (FABP2) that bind fatty acids were measured, because it is a protein found in intestinal epithelial cells and binds free fatty acids, cholesterol and retinoids. As such, its increase in plasma indicates damage to the intestinal mucosa.
As expected, FABP2 levels were high in the plasma of patients with COVID-19 compared to healthy subjects.
Intestinal microbial peptides in plasma are toxic because they trigger inflammatory pathways and lead to systemic damage. As a measure of this phenomenon, the researchers observed higher levels of peptidoglycan (PGN) and plasma LPS in COVID-19, almost twice as high as in healthy controls.
What are the implications?
The translocation of intestinal microbes, which are usually found only in the feces, into the systemic circulation is a fundamental determinant of immune function and metabolism. The presence of intestinal microbes in the plasma can trigger and also worsen the signaling pathways of inflammation in the body.
Inflammation is crucial for the pathogenesis of severe and critical COVID-19. The findings of this study may support the theory that this is affected by the movement of bacteria in the gut into the body’s circulation in these patients. This, in turn, could be due to higher intestinal permeability due to epithelial barrier dysfunction.
It has been found that the release of the virus in the feces is maintained for up to a month after the symptoms of the lungs subside, suggesting that viral colonization of the intestine may take longer than the airways.
Patients with COVID-19 in this sample are likely to be diabetic and obese compared to controls. In such patients there are commensal bacteria Lactobacillus are less abundant, and this reduction was found in a small group of nine patients tested at the start of hospitalization in this study.
Most deaths from COVID-19 are due to sepsis. In this study, an abundance of multiple pathogenic species such as Acinetobacter i Pseudomona was larger in the intestines. Even after the infection resolved, the dysbiosis persisted, indicating that the intestines may suffer the consequences of this disease in the long run.
Plasma metabolism is associated with intestinal microbiome in the pathogenesis of many diseases. Failure of the intestinal barrier leads to the detection of bacterial metabolic products in plasma, in conditions such as ulcerative colitis.
The study suggests, “Lintestinal eaky and microbial dysbiosis may contribute to cytokine storm in patients with severe COVID-19. “
* Important notice
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered definitive, guide clinical practice / health-related behavior, or treat it as established information.