Every month I showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is between humans and a large group of bacteria present in your mouth.
According to the International Agency for Research on Cancer, cancer of the mouth, including lip cancer, is estimated to be the 16th most common cancer worldwide. Carcinogenesis (production of cancer cells) in the mouth is frequently related to tobacco smoking and although the oral cavity of a smoker is directly exposed to cigarette smoke, the incidence rate of oral cancer is relatively low among other tobacco-related cancers.
Several researchers at Tsurumi University (in Yokohama, Japan) have postulated this is due to a high concentration of the chemical hydrogen sulfide, the chemical most familiar for smelling like rotten eggs. Studies have indicated a relationship between garlic consumption and the reduction of several types of cancer and cardiovascular diseases. Garlic-derived chemicals are converted into hydrogen sulfide within human red blood cells and this may be the mechanism explaining the observed reduction in cardiovascular disease.
Hydrogen sulfide plays an important role in many aspects of normal human physiology and is produced in significant amounts in various tissues. For example, our brain has been reported to have a concentration of hydrogen sulfide ranging from 50 to 160 μM (micromolars), much higher than ambient concentrations in the air you breathe (<0.01 μM).
Hydrogen sulfide in the oral cavity is produced by many members of your normal oral bacteria, particularly anaerobic sulfate-reducing bacteria (bacteria that get their energy by working with sulfur instead of oxygen). There are many different groups of anaerobic bacteria found within the healthy mouth of humans: Actinomyces, Arachnia, Bacteroides, Bifidobacterium, Eubacterium, Fusobacterium, Lactobacillus, Leptotrichia, Peptococcus, Peptostreptococcus, Propionibacterium, Selenomonas, Treponema, and Veillonella. These groups of bacteria are the main cause of halitosis, better known as bad breath. The concentration of hydrogen sulfide in gingival crevicular fluids (the fluid between your gums and teeth), where there are high concentrations of these bacteria, can be as high as 1900 μM. This high level of hydrogen sulfide hydrogen sulfide can cause apoptosis (programed cell death) in gingival (gum) cancer cells found within the mouth, but has nearly no effect on normal, healthy gum cells. It is still unknown as to how gum cancer cells are particularly sensitive to hydrogen sulfide, but this may lead to some interesting and stinky treatments for cancer later.
In return for producing these helpful chemicals, we give them housing and plenty of sulfides to work with. So while these sulfide-reducing bacteria may be ruining your breath, they are working hard to produce anticancer compounds.
To read the original scientific article from the journal Cancer Cell and Microenvironment click here.
Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!