The human body's "good parasites"
The mutualistic microbiome: a population of organisms living inside us can be beneficial to our health
- Parasite, commensals, and mutualists
- Intestinal mutualists
- Symbionts and the immune system
- References and further reading:
Parasite, commensals, and mutualists
So-called "good parasites" in the human body have been linked to protection against a number of disease conditions, from colon cancer to obesity (1, 2). In addition, they perform general beneficial functions, such as synthesizing vitamins and helping to protect against infection from pathogenic microorganisms. In truth, however, these organisms living inside us are not, by definition, parasites. Symbionts, or organisms that live inside us or on us, can be divided into three types: parasites, which cause us harm, mutualists, which help us or do us good, and commensals, which do not necessarily do us good, but which do not hurt us. The "good parasites" are the mutualists.
One of the most important categories of mutualists is that of the intestinal bacteria. According to Dr. Elisabeth Bik of the Stanford University School of Medicine (3), the colon is colonized by more than a trillion organisms per gram of intestinal contents; the number of bacterial cells inside us is actually higher than the number of our own cells! These include the familiar E. coli, a natural gut-inhabiting bacterium, as well as bacteria commonly ingested in probiotic foods, such as Lactobacillus from yogurt. The vast majority of the bacteria fall within four phyla (major groups) of bacteria: Firmicutes (including Lactobacillus), Bacteroidetes, Actinobacteria, and Proteobacteria (including the familiar E. coli) (3). Methanobrevibacter smithii, an archaean (a microorganism that looks like a bacterium but which has a completely different ancestry), is another major intestinal resident in humans (4). Minority gut residents include the Fusobacteria, Cyanobacteria, Spirochaetes, and Verrucomicrobia.
Symbionts and the immune system
In addition to the clearly beneficial effects of these intestinal bacteria, there may also, paradoxically, be benefits to infection with disease organisms. Illustrating the axiom "what does not kill me makes me stronger", immunologists are increasingly finding that infection with organisms that are generally thought of as parasites can aid in the healthy development of the immune system. The prevention of infection with helminth parasites (worms) and other infectious organisms in developed countries has been linked to increased rates of allergy and asthma, and this is thought to be related to abnormal immune system development (5). It seems that such infections are actually required to correctly "wire" the immune system, and that without them, the immune system instead attacks benign substances such as pollen, resulting in allergic reactions. Infectious organisms that have been shown to provide protection against allergy include the nematode (roundworm) parasites Heligmosomoides polygyrus, Nippostrongylus brasiliensis, and species of Ascaris; the platyhelminth (flatworm) parasites Schistosoma mansoni and Schistosoma japonicum, bacteria such as Mycobacterium, Chlamydia, and Listeria, viruses, including Influenza virus and Respiratory syncytial virus, and even fungal pathogens, such as Aspergillus fumigatus (5).
It is becoming evident that the microbiome, the population of organisms that live inside us, is an important part of who we are, playing roles in gastrointestinal function, immunity, and other aspects of human health. Future molecular and genomic studies should lead to the further discovery of as-yet unknown mutualistic organisms.
References and further reading:
1. Tsai F, Coyle WJ. (2009)The microbiome and obesity: is obesity linked to our gut flora? Current Gastroenterology Reports: 11(4):307-13.
2. Kinross JM, von Roon AC, Holmes E, Darzi A, Nicholson JK. (2008) The human gut microbiome: implications for future health care. Current Gastroenterology Reports: 10(4):396-403.
3. Bik EM. (2009) Composition and function of the human-associated microbiota. Nutrition Reviews: 67:S164–S171.
4. Ley RE, Peterson DA, Gordon JI. (2006) Ecological and Evolutionary Forces Shaping Microbial Diversity in the Human Intestine. Cell: 124(4):837-848.
5. Roumier T, Capron M, Dombrowicz D, Faveeuw C. (2008) Pathogen induced regulatory cell populations preventing allergy through the Th1/Th2 paradigm point of view. Immunologic Research: 40(1):1-17.