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Bacterial Structure

Duration: 23:09

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Apr 19 2020, 6:35 am

Isn't clockwise movement tumbling instead of counterclockwise ?

Review:  Remember the structure of bacteria is slightly different between the gram +ive and gram -ive, and pleomorphic bacteria which don’t have a cell wall.

  • *Think of a bacterium as a tiny bean shaped globule floating about like a balloon filled with cytoplasm.  The wall of the capsule has many layers. The inside layer is a cytoplasmic membrane, made up of a lipid bilayer and penicillin binding proteins.

  • *Outside the plasma membrane, is a cell wall made up of multiple layers of peptidoglycans(PG),  the sugars  N-Acetylmuramic acid (NAM) and N-Acetylgalactosamine.

  • *In gram +ive bacteria there are up to 60 PG layers. In gram -ive, up to 3 layers of PG. In gram -ive, the space between the layers is called periplasmic space.

  • *Outside the PG layer is another lipid bilayer membrane which contains lipopolysaccharides (LPS) made up of two sugar molecules and a phosphate, and above that is a five sugar core, and above that is a chain of sugars of up to 25 units.  That is the outer membrane. (TC 5:32 You gotta see the pictures to understand it. Trust me. ed.)  

  • *Gram +ive have protein molecules or peptides coming out of the cell wall called teichoic acid (TA). Some are connected all the way through to the plasma membrane.  If they are connected to the lipids of the plasma membrane they are called  lipoteichoic acid (LTA). 

  • *In the case of gram -ive, it is LPS which induces the TNF and IL-1.

Additional Structures

  • *Bacterial motility is generated by  flagella connected through the cell walls to the inner membrane which use ATP for energy to move the bacteria.  Three types of movement depending on how the flagellum moves.  If clockwise, then the bacterium moves like a jet; counterclockwise causes tumbling motility; if flagellum is wrapped around the cell, then a rotary movement.

  • *Fimbria or Pili are small proteinaceous materials mostly on gram +ive (gram -ive have less), of two types: 1) sticking out like hair functioning to stick bacteria to surfaces and creating films; 2) sex pili which connects to another bacteria. Through the pilus there is an exchange of plasmids (circular DNA molecules present inside the bacteria) from one bacteria to another.  The gene that creates the sex pilus is on the surface of the plasmid.

  • *Bacteria with the ability to encode the sex pilus are called the F+(male) bacteria, and those that do not are called F- (female).  When a plasmid makes a sex pilus and the F+ genes are exchanged, the receiving bacteria become F+.

  • *The bacterial DNA is a double stranded chromosome the ends of which are tied, making a continuous loop


  • *Endotoxins are encoded by the primary DNA, however in gram+ive, secreted exotoxins are encoded by plasmids.

  • *The outside capsule of a bacteria is made up of negatively charged polysaccharide molecules. The function of the capsule is to: 

          1. prevent phagocytosis by repelling neutrophils which are also negatively charged, like two negative poles of a magnet; 

           2. Thus cause virulence by evading phagocytosis;

           3. For lab diagnosis the presence of the capsule can help with the Quellung test.

  • *Some bacteria can secrete a glycocalyx that makes biofilm.

  • *Spores are formed by some gram +ive bacteria. When spore-forming bacteria are dehydrated, the peptidoglycan layer compresses into a tight ball, squeezing out its cytoplasm along with ribosomes and protein granules, but retaining its plasmids and DNA. As the spore forms, an additional outer peptidoglycan layer forms.  The inner layer has good sugar crosslinks and the outer layer does not.  A final outside layer of keratin insulates the spore capsule. Spores are strong and can withstand heat, dehydration, chemicals and lots of abuse.  Once the nutritional status is restored however, the spore will regerminate into bacteria.

  • *The following are spore forming pathogens relevant to human disease:

               Bacillus (Aerobic) :  B. anthracis, B.cereus, B. subtilis

              Clostridium  (Anaerobic):  C. tetani, C. botulinum, C. perfringens, C. difficile

  More about those sugar crosslinks in the discussion of penicillins.  On to the next lesson!


Learning objectives of this video are the following: 

1. Illustrating the structure of a typical bacterium.

2. Understanding structure and functions of: 

  • Cell wall 
  • Cell membrane
  • Bacterial DNA and plasmids
  • Flagella 
  • Pilli 
  • Capsule
  • Glycocalyx

3. Differentiating features of gram positive and gram negative bacteria, with special focus on their cell walls. 

4. Understanding the concept of bacterial spores.

Presented by Dr. Mobeen Syed

Following answers are created by ChatGPT. Occasionally the answer may be harmful, incorrect, false, misleading, incomplete, or limited in knowledge of world. Please contact your doctor for all healthcare decisions. Also, double check the answer provided by the AI below.


In addition to the presenter, following authors may have helped with the content writing, review, or approval:

  • Dr. Mobeen Syed

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The DrBeen Corp designates this enduring material for a maximum of 0.5 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

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Dr. Mobeen Syed

Dr. Mobeen Syed

MD., MSc., MSc., BSc

Mobeen Syed is the CEO of DrBeen Corp, a modern online medical education marketplace. Mobeen is a medical doctor and a software engineer. He graduated from the prestigious King Edward Medical University Lahore. He has been teaching medicine since 1994. Mobeen is also a software engineer and engineering leader. In this role, Mobeen has run teams consisting of hundreds of engineers and millions of dollars of budgets. Mobeen loves music, teaching, and doing business. He lives in Cupertino CA.


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