So-called “good parasites” in the human body have been linked to protection against a number of disease conditions, from colon cancer to obesity. They also perform beneficial functions like synthesizing vitamins and helping to protect against infection from pathogenic microorganisms. 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. The colon is colonized by more than a trillion organisms per gram of intestinal contents. In fact, there are more bacterial cells than human cells in our bodies! 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): Firmicutes (including Lactobacillus), Bacteroidetes, Actinobacteria, and Proteobacteria (including the familiar E. coli). Methanobrevibacter smithii, an archaean (a microorganism that looks like a bacterium but is not a true bacterium), is another major intestinal resident in humans. Minority gut residents include the fusobacteria, cyanobacteria, spirochaetes, and verrucomicrobia. Probiotic supplements can help you boost many of these good symbionts.
In addition to these directly beneficial bacteria, there may also, paradoxically, be benefits to infection with true parasites that cause disease. Illustrating the axiom “what does not kill me makes me stronger”, immunologists have found that infection with 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 lead to abnormal development of the immune system. It seems that such infections are integral to the natural “wiring” of the immune system. Without them, immune systems can develop inappropriately, attacking benign substances such as pollen—these are 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.
Clearly, 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 further discovery of as-yet unknown mutualistic organisms.
References and further reading:
Tsai F, Coyle WJ. (2009)The microbiome and obesity: is obesity linked to our gut flora? Current Gastroenterology Reports: 11(4):307-13.
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.
Bik EM. (2009) Composition and function of the human-associated microbiota. Nutrition Reviews: 67:S164–S171.
Ley RE, Peterson DA, Gordon JI. (2006) Ecological and Evolutionary Forces Shaping Microbial Diversity in the Human Intestine. Cell: 124(4):837-848.
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.
Lice, Mites, and Worms, Oh My! Metazoan Parasites of Children in New Orleans