Scientists studying Toxoplasma gondii, the parasite responsible for toxoplasmosis, found that bacteria present in the human gut help stimulate the body's defense mechanisms.
"While this is very early data, our results suggest that looking at the bacteria present in each patient's gut could help physicians understand their susceptibility to infectious diseases," said Dr Felix Yarovinsky from the University of Texas Southwestern.
"It also suggests the possibility of developing novel probiotic strategies for treating parasitic infections such as toxoplasmosis and cryptosporidiosis, a related disease caused by the parasite Cryptosporidium."
The UT Southwestern researchers published their findings in the new issue of Cell Host & Microbe.
Dr Yarovinsky told NutraIngredients that probiotics may occupy space in the intestine and thus reduce or prevent potentially pathogenic bacteria attaching to the intestinal wall.
“In our work we revealed a novel feature of the beneficial relationship between commensal microorganisms and the host immune system,” he said. “We found that gut commensal bacteria function as a molecular adjuvant, providing Toll-like receptor (TLR)-dependent immunostimulatory signals to dendritic cells, which are the active mechanism of defence.”
T gondii reportedly affects more than a billion people worldwide. While the humble pet cat is the primary host of the parasite, toxoplasmosis may also be passed through contaminated cat faeces. It can also be passed from mother to child in the womb. While many infants infected in the womb display no symptoms at birth, a small number may be born with serious eye and brain damage. Furthermore, the infant may develop symptoms later in life.
The researchers used mice which had an inactivated form of a specific immune protein, called toll-like receptor 11 (TLR-11), which plays a role in controlling the animals' immune response to the parasite. While TLR-11 is normally active in mice, humans do not have an active form of the receptor. It has therefore remained unclear quite how the human body ‘senses’ T gondii
Using the TLR-11 deficient mice as a model of the human immune response, Dr Yarovinsky and his team then infected the animals with T gondii.
Despiet a lack of their normal defence mechanism, the researchers noted that the animals’ immune system did react to T gondii infection.
The researchers found that the commensal – or good – bacteria in the gut activated their immune system, thereby inducing various inflammatory responses against the invading pathogen.
"This seems to be the first example of indirect pathogen recognition in vivo where activation of the immune system depends on indirect rather than direct sensing of a pathogen," said Dr Yarovinsky.
TLR-11: A blessing or a curse?
And the potential benefits of the gut microflora appear to outweigh the benefits of having a full-functioning TLR-11, said Dr Yarovinsky. In mice the receptor appears to do more harm than good.
TLR-11-deficient mice were able to mobilise enough signalling proteins, with the help of their commensal bacteria, to defeat the parasite, mice with the receptor activated too many signaling proteins and developed severe inflammation in their small intestines.
"We speculate that because commensal bacteria co-evolved with the host, they must have found this fine balance to induce the sufficient stimulatory effects of the immune system without causing illness or death," Dr Yarovinsky said. "The fact that commensal bacteria vary dramatically from person to person might explain why therapeutic outcomes vary so much."
The next step in the research is to determing which bacterial species confer the greatest benefits, said Dr Yarovinsky.
Source: Cell Host & Microbe
20 August 2009, Volume 6, Issue 2, Pages 187-196
“Gut Commensal Bacteria Direct a Protective Immune Response against Toxoplasma gondii”
Authors: A. Benson, R. Pifer, C.L. Behrendt, L.V. Hooper, F. Yarovinsky
To view the abstract, please click here.