Infectious disease research has largely focused for the last 125 years on understanding the relationship between harmful bacteria pathogens and the animals or people they infect.
The success of this bug vs. host approach has led to enhanced under-standing of infectious diseases and has contributed to preventive interventions such as water sanitation, vaccination and antibiotics.
Indeed, few achievements of modern civilization rival the perhaps unglamorous successes of water and sewage hygiene in the prevention of bacterial disease and enhancing quality and length of life.
However, this traditional pathogen-centric view of biology and disease has also neglected the role of the “good bugs” in health.
Until recently, there were but limited means to study the complex ecosystem of healthy bacteria that live on and within animals.
The usual method to study bacteria requires culturing on agar plates, but only one percent of bacteria can be grown in laboratory settings this way. The remaining 99 percent were not identifiable and thus beyond the reach of scientific investigation until modern DNA technology became available.
Within the last 20 years, these DNA technologies and subsequent advances in computing power gave scientists the means to investigate the remaining 99 percent. The result is the growing field of microbiome research.
Capturing information about these complex bacterial communities has necessitated the development of new methods to interpret and analyze the massive volumes of data generated by this type of research.
Similar to the vastness of space, it is difficult to wrap one’s head around the enormity and complexity of these bacterial communities. A common point of reference is that there are more bacteria in your gut than the total number of people who have ever lived.
Microbial communities within an animal are constantly changing and adapting, just like the ecosystems we can see with the naked eye. They consist of complex interactions between plants, soil, animals and insects.
Animals have co-evolved with their resident bacteria communities, probably for mutual benefit.
Bacteria in and on their hosts provide protection from harmful bacteria and metabolize food. Perhaps an extreme example of this mutually beneficial relationship is the rumen in cattle.
This large second “stomach” is essentially a massive fermentation vat, which has allowed grass-eating ruminants to thrive on high-fibre diets that are unsuitable to many other animal species. The bacterial communities within the rumen degrade the food into a usable form.
Scientists are just beginning to understand of the role the microbiome plays in health and disease.
For example, disruption of the intestinal microbiome through abrupt changes in feed is suspected to cause several diseases, including colic in horses and certain clostridial diseases in calves and lambs.
Antibiotic treatments are greatly disruptive to these communities because they are incapable of distinguishing between good and bad bacteria. Some evidence suggests that intestinal bacteria never quite return to the composition they had before treatment.
Antibiotic treatment can select for resistance among good bacteria. When a nasty pathogenic species such as salmonella comes along, these resistance features can be promiscuously shared. This is one of the reasons for concern about widespread antibiotic use, particularly in animal feed.
Intestinal bacteria exert their influence directly on the intestine as well as throughout the body. Locally, they play an important role in regulating intestinal function.
Since the gut functions as the largest immune organ in the body, changes to gut bacteria affect the entire immune system.
There have been intriguing connections between the type of gut bacteria and several health conditions.
Specifically, studies have linked altered bacterial communities with obesity in mice and people. Transfers of these obesity-associated bacteria from obese to non-obese mice have led to the development of obesity among the recipients.
The ability to modify intestinal bacteria holds promise for treating a variety of diseases. In cattle medicine, this is already commonplace. Rumen transplants involve taking rumen juice from a healthy donor and instilling it into the rumen of an ill animal to get its rumen back in working order.
There is still lots to do and understand, but this age of microbiome research is an exciting one.