02.07 Bacterial Role in Disease

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Included In This Lesson



  1. Bacterial Roles in Human Health

    1. Good Bacteria-
      1. 100 trillion bacterial cells live in and on us are essential to life
      2. Line our digestive track and aid in digestion and absorption
        1. Lactobacillus acidophilus in yogurt
        2. Bifidobacteria bifidum protects against unhealthy bacteria
        3. Streptococcus thermophilus can help prevent lactose intolerance
    2. Bad Bacteria
      1. Skin, sweat, tears, enzymes, stomach acid act as barriers to prevent bacterial infections
      2. Common bacterial Infections
        1. Streptococcus pneumoniae causes pneumonia
        2. Haemophilus influnzae causes meningitis
        3. Streptococcus pneumoniae causes pneumonia
        4. Group A Streptococcus causes strep throat
        5. Eschericia coli and Samonella cause food poisoning
    3. Bacterial Shapes- basic way to classify bacteria
      1. Rod- Bacillus
      2. Sphere-Coccus
      3. Spiral-Spirochaetes
    4. Gram Staining- mechanism used to distinguish two main types of bacteria
      1. Gram+ Retains purple crystal violet stain due to a thick peptidoglycan cell wall and no outer lipid membrane
        1. Ex. staphylococci, streptococci and listeria
      2. Gram – Does not retain violet stain and is light red/pink due to safranin counter stain.
        1. Ex. enterococci, Salmonella, Pseudomonas



Today we’re going to be talking about Bacterial Role in Disease.

In this lesson on Bacterial role in disease we will cover examples of good and bad bacteria, the three main shapes of bacteria and a method of testing for a class of bacteria known as gram staining and its significance,


If you were to compare the number of cells that make up the human body to the number of bacterial cells that live in and on the human body you would be amazed to find out that we are outnumbered. In fact about 90% of the cells that are in and on us are bacterial cells. That being said these microbes  must have a symbiotic role with us in order for them to exist in such large numbers. And indeed much of the bacteria that coexist within and around us are what we would consider good bacteria.  Bacteria that play a role in creating a biofilm on our skin that stimulate our immune system and keep harmful bacteria out. Gut bacteria aid in digestion and line the digestive tract and communicate with the body in ways that are thought to prevent diseases like diabetes, Crohn’s disease and ulcerative colitis. Some common examples of these are lactobacillus acidophilus (found in yogurt, bifidobacterium that protects against harmful bacteria and Streptococcus thermophilus that can prevent lactose intolerance.

And of course there are also bacteria that can do a lot of harm. These bad bacteria can put us in great danger upon entry.  Bad guys like Streptococcus pneumoniae named after the life threatening pneumonia that it can cause, Haemophilus influenzae that causes a life-threatening meningitis, Group A Streptococcus that causes strep throat or the gut wrenching food poison culprits like Escherichia coli and Salmonella.




One of the ways we categorize bacteria is by their shape. There are three many shapes that we divide them into to. Rod shaped bacteria are called Bacillus, sphere shaped bacteria are called Coccus and lastly the spiral shaped bacteria which are the spirochaetes. As you can see there are other shapes but these three are the main ones. Structure determines functions so these shapes have evolved to allow them to succeed in their environments.



Gram staining has been around a long time and most of us who have taken a bio or microbiology course are probably familiar with this staining technique  which puts most bacteria (there are exceptions) into one of two categories, Gram positive and gram negative.  If after staining the bacteria retains the purple crystal violet stain it is said to be a Gram positive bacteria. This is due to the bacteria havign  a thick peptidoglycan cell wall and no outer lipid membrane Ex of this are staphylococci, streptococci and listeria. In contrast,  Gram negative bacteria do  not retain violet stain and will instead turn out a light red/pink due to safranin counter stain. These bacteria have a thinner peptidoglycan wall and an outer lipid membrane surrounding it. Examples of Gram negative bacteria include enterococci, Salmonella, and Pseudomonas.  This stain was invented over 100 years ago and is still used, even clinically to verify a bacterial infection as well as quickly determine a fast course of antibiotics when time is of the essence (i.e. spinal tap checking for meningitis). So to reiterate Gram positive have a thick peptidoglycan wall and no outer membrane and Gram negative have a thin peptidoglycan cell wall and do have an outer lipid membrane.

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