# Normal Microbiota Our **normal microbiota**, or **normal flora** are the collection of [[microbe#Symbiotic Relationships|symbiotic]] bacteria that lives in and on our body that can be bennificial to them as well as to us. ## Locations The common locations where normal flora live on our bodies are the skin, mouth, nose, throat, large intestine, urethra, and vagina. ### Examples skin | nose | throat | mouth | large intestine | urethra | vagina -----------------|------------------|------------------|-------------|-----------------|--------------|-------- •Staphylococcus<br>•Propionibacterium|•Corynebacterium<br>•Staphylococcus|•Corynebacterium<br>•Haemophilus<br>•Streptococcus|•Actinomyces|•Bacteroides<br>•Escherichia<br>•Klebsiella<br>•Lactobacillus |•Bacteroides<br>•Escherichia |•Lactobacillus| Notes: A certain strain of Streptococcus is the one that causes Strep throat. A certain strain of Corynebacterium can cause diptheria (through [[lysogenic phage infection|lysogenic conversion]].) A certain strain of Haemophilus can cause a disease Actinomyces can cause a mouth infection if there is poor dental hygene. ## Normal Microbiota in the Immune System The normal microbiota on our skin and mucous membranes play an important role in the [[innate immune system#First Line Defense|first line defense of our innate immune system]]. As we have evolved to our symbiotic relationship, our bodies also have ways of recognizing normal microbiota and not attacking them. ### Antimicrobial Activity of the Normal Microbiota Firstly, they compete with pathogens for space and resources. Secondly, they can actuually release [[antimicrobial substances]] that can kill off pathogens. ### Our Immune System Protects Normal Microbiota #### Differentiating Between Normal Microbiota and Pathogens Our bodies have evolved to recognize and protect our normal microbiota. One way of doing this is the cells of our [[intestines]] can differentiate between normal microbiota and pathogens, because of the placement of their [[toll-like receptor|toll-like receptors]]. Their TLRs are located inside their cytoplasm *and* on the basolateral side, but not on the "top" side. As normal microbiota just live on the top, and do not enter or pass through the intestinal cells, they do not engage those TLRs. #### Normal Microbiota Can Activate Regulatory T-Cells [[Regulatory T-cells]] are responsible for turning down the immune response, so it is beneficial for our helpful bacteria to be able to activate the T$_{regs}$ for their own protection. They can activate the T$_{regs}$ directly or indirectly. ##### Activating Regualtory T-Cells Driectly Normal microbiota can engage a [[regulatory T-cells|regulatory T-cell's]] [[toll-like receptor]] and then stay engaged continiously. This tells the T$_{reg}$ that they are not dangerious and they can activate. ##### Activating Regulatory T-Cells Indirectly Normal microbiota can signal to [[macrophages]], who can then signal to [[helper T-cells]], who can then in turn signal to activate the [[regulatory T-cells]]. ## The Old Friends Hypothesis The **[[old friends hypothesis]]** is a hypothesis on how exposure to the friendly bacteria—the potiential normal microbiota of our systems—helps our systems learn how to not over-react to harmless substances or microbes. ___