Mutualism: Women and L. acidophilus

Introduction: 

As human beings, we have thousands of different species of bacteria on our bodies, not including the other types of microorganisms. With all the different bacterial species, it would seem that no single bacteria is an indispensable member of our microbiome, however this is not the case. Lactobacillus acidophilus is a species of bacteria that women rely on just as much as the bacteria do. The L. acidophilus reside (among other places in the body) on the human vagina and use the woman as a resource host in exchange for protection from other potentially more harmful micro-organisms.

Description of the Relationship:

The presence of bacteria on the vagina was discovered over 100 years ago and is now one of the most well known examples of mutualism between humans and bacteria that there is. It is considered mutualistic because both the woman and the bacteria necessarily rely on one another. The L. acidophilus, once introduced to the vagina,  begins to adhere to the vaginal epithelial cells and colonize. The bacterial colony consumes the natural sugars excreted by the skin and ferments them into lactic acid. This product lowers the pH to a level intolerable by most bacteria not of the Lactobacillus genus. During menstruation, many of the bacteria are killed, reducing them to an appropriate level.

Cost/Benefit Analysis:

The woman benefits from this mutualistic relationship by gaining protection from more harmful organisms that could potential cause a yeast, bladder, or urinary tract infection. This is at the cost of a slight decrease in reproductive fitness. L. acidophilus taking glucose molecules that the woman worked to obtain might not have a large negative affect on the woman by itself, but there are thousands of micro-organisms on and inside the human body that all contribute to a somewhat heavy, yet necessary burden. The bacteria (L. acidophilus) benefit from the relationship by gaining a steady food supply, and a habitable living place. This is at the cost a attack by the woman's defense system (daily hygiene, monthly menstruation, and immune response if levels get too high)

References:

1. "Lactobacillus acidophilus" University of Maryland Medical Center. http://www.umm.edu/altmed/articles/lactobacillus-acidophilus-000310.htm
2. "Methods for Quantitative and Qualitative Evaluation of Vaginal Microflora During Menstruation" Pub Med Central. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC238869/?tool=pmcentrez
3. "Bacteria Genomes - LACTOBACILLUS ACIDOPHILUS" EMBL-EBI. http://www.ebi.ac.uk/2can/genomes/bacteria/Lactobacillus_acidophilus.html
4. "Bugs Inside: What Happens When the Microbes That Keep Us Healthy Dissapear?" Scientific American. http://www.scientificamerican.com/article.cfm?id=human-microbiome-change
5. http://www.youtube.com/watch?v=zJC5738poZE