At a family barbeque, you tell your family that you are taking microbiology this semester. Your aunt proceeds to gag and say how gross microbes are. How would you describe to your aunt three examples of human-associated microbes actually improving or maintaining human health?

Question

What will be an ideal response?

Answer 1

There are many ways in which human-associated microbes can benefit the body. For example, in the large intestine, prokaryotes break down complex polysaccharides that cannot be digested with human intestinal enzymes. Examples would include plant polysaccharides like cellulose and pectin. Also entering the colon are human-produced polysaccharides like mucins, which are released when small intestinal cells slough off. The main end products of polysaccharide fermentation are compounds like acetate, propionate, and butyrate. These compounds can be absorbed by the colon mucosa and used as sources of carbon and energy. The amount of energy the human body derives from the fermentation by bacteria has been estimated to about 15-20% of the total energy obtained from our diets. So, the colon can really be considered a second organ of digestion, but unlike the stomach, it consists mainly of prokaryotic cells.
People taking certain antibiotics that are particularly effective against colonic bacteria can experience dramatic decreases in the numbers of normally predominant anaerobes. Under such conditions, Clostridium difficile, which is normally present at low numbers, can overgrow other bacteria and produce toxins that cause severe damage to the lining of the colon. Death can result in the matter of a few days. This example highlights the fact that keeping our native microbiota intact is a big way we fight off pathogens, particularly because our native gut microbial communities are very diverse. Because of this, the community consumes nutrients that might otherwise support the growth of pathogenic species.
Other protective roles of human-associated microbes are also related to deterring the growth of disease-causing microbes. For example, Lactobacillus sp., which normally inhabits the female reproductive tract, defends the host against colonization of pathogens such as C. albicans. Evidence suggests that Lactobacillus reduces the adhesion of C. albicans to epithelial cells either by (a) outcompeting fungal cells for adhesion sites, such as cellular receptors to which Lactobacillus has higher affinity, or (b) by secreting biosurfactants such as surlactin that physically decrease fungal binding. Additionally, when L. acidophilus breaks down food in the intestine, (c) several substances are formed (such as lactic acid and hydrogen peroxide) that create an unfriendly environment for "bad" bacteria.