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The Anatomy of Bacterial Organisms

Bacteria come in all shapes and sizes. In combination with a well-prepared gram stain, we can further characterize bacteria based on their size, shape, and arrangement. The figure to the right demonstrates the many different shapes and grouping of various bacteria.

 

External Structures

Bacteria can have a number of structures on their cell surface that can impact the way the bug looks and functions. These include methods for a bacteria to move, attach to surfaces, replicate or infect a host.

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FOUND IN ALL BACTERIA:

  • Cytoplasm

  • Ribosomes

  • Plasma membrane

  • Bacterial chromosome/ Nucleoid

 

EXTERNAL STRUCTURES (in some):

  • Flagellum

  • Pilus (Pilli)

  • Capsule

  • Cell wall

  • Plasmid

  • Mesosome

  • Glycocalyx

 

Cell Wall Composition

GRAM-POSITIVE BACTERIA

(If you haven't read it yet, check out the Gram Stain Explained page). We briefly discussed how gram staining is used to stain bacteria either purple or pink based on differences in their cell wall composition. Mainly, these include variations in the cell wall thickness and differing glycosaminopeptide and lipoprotein compositions.

 

The outermost layer is the cell wall (synonymous with the peptidoglycan (PG) layer) which forms a rigid framework of cross linked PGs.

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Now we will look further into the cell wall properties that cause the bacteria to stain in such a pattern.

 

GRAM-NEGATIVE BACTERIA

Gram positive organisms have a very thick outer peptidoglycan cell wall that is 10-15x thicker than gram neg bacteria. Lipoteichoic acid can be found on this outer surface and is a source of antigenicity for gram pos organisms. The thick PG layer creates a physical permeability barrier that restricts/retards the diffusion of the crystal violet stain and resists the decolorization phase, thus staining the organism purple. In contrast, the gram negative organisms have a "leaky" outer lipid membrane and only a thin PG layer. Thus the crystal violet stain is removed in the decolorization step and will then stain pink with the counterstain- safranin.

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Gram negative organisms, unlike their gram pos counterparts, have an outer membrane composed of lipoproteins, phospholipids and lipopolysaccharides (LPS- an endotoxin unique to gram neg bacteria). Beneath the outer membrane is a thin peptidoglycan cell wall (much thinner compared to the PG wall of gram pos bacteria) which overlies another (inner) plasma membrane. The space between the PG layer and the inner membrane is referred to as the periplasmic space. Beta-lactamase (an enzyme used to break down beta lactam antibiotics) can be found in this space which is one such mechanism of antibiotic resistance.

 

SUMMARY

Gram POSITIVE organisms= PURPLE

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  • Gram-positive organisms have a VERY THICK CELL WALL composed of PEPTIDOGLYCANS & (LIPO)TEICHOIC ACID.​

    • 10-15x thicker than gram neg organisms.

    • The thick PEPTIDOGLYCAN layer holds on to the purple crystal violet dye and is not easily washed out during the decoloring step. This causes gram pos organisms to stain PURPLE

  •   INNER CELL MEMBRANE 

    • Contain fewer lipids compared to gram neg organisms.

Gram NEGATIVE organisms= PINK

  • Cell surface characteristics (from external layer to inner membrane):

    • OUTER CELL MEMBRANE 

      • LIPID RICH- LIPOPOLYSACCHARIDES (ENDOTOXINS), lipoproteins and phospholipids.

      • Relatively "leaky" and allows the crystal violet dye to wash off during the decoloring step.

    • THIN CELL WALL (peptidoglycan layer)

      • Much thinner than the gram pos peptidoglycan layer.

    • PERIPLASMIC SPACE

      • Some organisms contain beta-lactamases in the periplasmic space (between inner and outer membranes) which causes resistance to beta-lactam drugs.

    • INNER CELL MEMBRANE

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Continue to the next topic to see how bacteria are classified according to their gram staining and morphologic characteristics!

MICRO & IMMUNO 

INDEX

NEXT TOPIC

Bacteria Classification by Gram Stain & Morphology

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The Principles of Gram Staining 

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