The Good-Natured Commensal

My neighbour, Mrs. Louis, practices excessive cleaning to the point where she even washes her tomatoes with detergent before using it in her cooking. She has always been afraid of germs and the potential harm they could inflict upon her and her family. She dusts, sweeps, mops and vacuums her house at least twice a day to satisfy her urge for extreme cleanliness. 

Mrs. Louis, is a prime example of “germaphobe” (phobia towards germs) but she could really be suffering from obsessive-compulsive disorder, but that’s a story for another day. Many do not realize the importance of bacteria and its role in the environment and most importantly, on our very own body. We are all aware that human skin is one of the largest organ where it plays an important role in acting as a physical barrier mainly to prevent the entry of pathogens but at the same time, our skin is also home to countless commensal bacteria.

When I was trying to explain to Mrs. Louis and her daughter, Mary, whom I am greatly fond of, the ever-eager Mary asked “What is the difference between pathogens and commensals when they are both bacteria?” Mrs. Louis then added “Aren’t all bacteria bad for human health?”

The area size of human skin is nearly 1.8 m² and approximately 1 million of bacteria is estimated to reside on skin area that is only 1 cm x 1 cm whereas the total number of bacteria that can inhabit the whole of human skin at any time is estimated to be around ~10¹⁰. Therefore, the group of bacteria that makes the human skin its home without causing any harm is the commensal and this mutual relationship has co-evolved together to gain mutual benefit. Meanwhile, pathogens, on the other hand, describes infectious group of bacteria that may cause illness to humans by gaining illegal entry into the cellular environment.

Skin commensals, also known as skin microbiota, have been claimed to compete for metabolites with the pathogens. Bacteroides thetaiotaomicron, a type of gut bacteria, competes for carbohydrate consumption with the gut pathogen, Citrobacter rodentium, and this is an excellent example of nutrient competition among commensals and pathogens. Human microbiota stimulates host immune system against the invading pathogens, is another example of indirect pathway partaken by the commensals to compete against the opportunistic pathogens.

Mary looked at me while batting her eyes. “Isn’t the surface of skin just full of sweat? Then how can this condition support the growth of commensals?” asked Mary with full of interest. “Oh, then isn’t our sweat salty too? So does that mean only certain types of bacteria reside on our skin surface?” Mary asked once more while pointing her delicate finger to the sweat-laden arms of her mother, who was then unknowingly scrubbing the clean-looking part of the kitchen counter with a white towel while listening to our conversation earnestly.

            Human skin is acidic, cool and dry besides being often covered with a layer of sweat. Therefore, the skin is generally inhospitable and deficient in nutrients, where the only source of nutrient comes from sebum, skin stratum corneum, peptides and lipids. Additionally, sweat too contains antibacterial molecules including antimicrobial peptides (AMPs) and free fatty acids that represent a natural class of antibiotic-like components that forms protective response against invasive pathogens. However, there are predominantly symbiotic bacterial species such as Staphylococcus sp. which tolerates high concentration of salt and may also utilize urea that is present in the sweat as its nutrient source. Hence, commensal bacteria may have actually evolved to contribute to the overall good health of the human.