For a new ‘respiratory’ virus, SARS-CoV-2 is well-travelled, not only across the world but also within the human body. As clinical experience has increased with the rise in the number of infected persons in many parts of the world, pathology from the virus has been observed to manifest from head to toe, with many organs affected.
The gut too is an area that is infected. While diarrhoea has been reported in about 10% of Covid patients, the virus has been recovered in the stool samples of half the patients studied. With high levels of the ACE 2 receptors, which are the cellular entry gateway to the virus, the gut lays out the welcome mat to the microbe and may be an important reservoir where it finds sanctuary and prolongs the infection. Faecal-oral transmission has also been proposed as a mode of infection, apart from the respiratory and conjunctival routes.
An emerging area of great interest is the role of the different types of intestinal bacteria (microbiota) whose genes collectively contribute to the gut ‘microbiome’. These represent a variable mix of multiple varieties of microbes that have a bearing on an individual’s state of good health or vulnerability to different diseases.
The microbiome is distinctive for each person but highly variable over time, depending on many of our habits but especially our diet. Since the genes of the microbes that cohabit our bodies far outnumber our own genes, the combination constitutes a ‘meta genome’ that influences our health.
Associations of distinctive microbiome patterns with different types of disorders have been reported in profusion in global scientific literature over the past two decades, making it one of the most exciting areas of research. Apart from disease associations, interventions to alter the composition of the microbiome are also being reported as ‘bacterial therapy’. The success of such therapy has been particularly impressive in the treatment of a dangerous bacterial infection with clostridium difficile.
Since it is now known that viruses can alter the composition of the gut microbiome and can in turn be influenced by the immunological processes potentiated by the microbial gut flora, its relationship with Covid-19 has evoked interest recently. Kourosh Kalantar-Zade and colleagues from the University of New South Wales, Australia, first hypothesised a role for using changes in human gut flora to diagnose and treat Covid-19.
They proposed the use of nanotechnology to create diagnostic tests based on breath tests which “can identify the presence of volatile organic compounds with the signatures of modulated microbiota”. Sounds strange? Well then, read a recent study from Hannover, Germany, which reported that trained dogs could differentiate between Covid and non-Covid patients with a sensitivity of 82.6% and specificity of 96.3%! You may also see a social media video about such dogs being employed at the Helsinki airport.
Then came a study from Hong Kong by Tao Zuo and colleagues who reported that faecal microbiomes of Covid-19 patients showed an increase in opportunistic pathogens and decrease in beneficial commensals that boost immunity. These changes, which also influenced the regulation of the ACE 2 receptors, correlated with the viral load in the stool samples. The authors suggested that strategies to alter the intestinal microbiota might reduce severity of the infection. On the other hand, they caution that inappropriate antibiotic therapy will harm the beneficial bacteria and cause problems.
An Italian study in July tested the therapeutic potential of protective microbiota in a trial of 70 patients and observed that oral ‘bacteriotherapy’ with a combination of beneficial bacteria was associated with reduced severity and faster recovery in Covid-19 patients. The authors state that the risk of developing severe respiratory failure was eight-fold less in patients receiving bacteriotherapy.
Kalantar-Zade suggests that the lower clinical severity and mortality of Covid-19 infections in India, other South Asian countries and South Africa may be due to higher dietary consumption of non-fermentable fibre that promotes healthier microbiota. He hypothesises that middle class Indians, who have access to processed foods, are more vulnerable to the severe manifestations of Covid-19, while poorer Indians who consume foods high in husk and non-fermentable fibre that produce a favourable pH in the colon are better protected. These foods protect the integrity of the gut epithelium by promoting the right microbiota, he says.
Other components of the Indian diet too may promote healthy gut flora. Lactobacillus is known to be a beneficial microbiota that protects the gut. It is abundant in curds (yogurt), a regular part of traditional Indian diets. While all the studies of microbiota in Covid-19 must be regarded as preliminary, the relationship between diet, gut flora and immunity is by now so well-established that it is a line of scientific enquiry worth pursuing. We need a buddy bug to beat a bad bug. (The author has written Make Health in India: Reaching a Billion Plus. Views are personal.)
Dr K Srinath Reddy
Cardiologist, epidemiologist and President, PHFI (ksrinath.reddy@phfi.org)