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Bacterial Structure & Morphology

Notes

Bacterial Structure & Morphology

Sections




Overview

Cell wall

Plasmic/cytoplasmic membrane is deep to the cell wall.
It has an area of infolding. The membrane comprises a lipid bilayer that serves multiple functions, including transport of molecules into the cell, secretion of toxins and enzymes, and energy generation.

Nucleoid is the region with bacterial DNA; there is no nuclear membrane.

Outer capsule, which most often comprises gelatinous polysaccharides.
Variable; When present, the capsule contributes to the serologic type and enhances virulence.

Pili, aka, fimbriae, which are short filaments found primarily on gram-negative strains.
Variable; When present, they participate in bacterial attachment to host cells.
The sex pilus attaches donor and recipient bacteria during conjugation.

Flagella; variable. When present, propels the cell, and the number of flagella varies.

Be aware that different strains of bacteria have special anatomical and physiological traits, which we address elsewhere as these features relate to infectious disease.

Staining

Most bacteria associated with infectious disease can be categorized as gram-positive or gram-negative, which refers to whether they retain crystal violet stain, and depends on the composition of their cell walls.

Gram-positive

In a microscopic sample, we can see the bright purple stain – think Purple Positive.

Cell wall has thick layer of peptidoglyclan (aka, murein and mucopeptide)
It comprises a network of sugars and amino acids, and is the target of some antibiotic drugs.

Two acids that are present in the cell wall: teichoic acids, which attach to the peptidoglycan, and, lipoteichoic acid, which are attached to the cytoplasmic membrane.
These surface acids contribute to the cell wall structure and charge.

Gram-negative

In a microscopic sample, gram negative bacteria appear reddish-pink.

They have a thin peptidoglycan layer, which is covered by the outer membrane.

The outer membrane is unique to gram negative bacteria and comprises lipopolysaccharide, aka, endotoxin.

Endotoxin contributes to disease symptoms such as fever and shock.

Other stain types

Gram-staining is not appropriate for all bacterial strains.

We show an image of mycobacteria, which does not have a cell wall and is visualized with acid-fast staining methods.

We show Treponema pallidum, which is a bacterial strain that has very thin cell wall but can be seen with dark-field microscopy, as in our image, or fluorescent antibodies.

Some strains, such as the intracellular Chlamydiae, can be seen with hematoxylin and eosin (H&E) staining.

Morphology

The bacterial cell wall contributes to its morphology, which is also used for classification purposes.

Cocci - Spherical

Pairs
Ex: Pointed, like Streptococcus pneumoniae
Ex: Coffee-bean shaped, like Neisseria gonorrhoeae.

Bacilli - Rods

Rectangular ends
Ex: Bacillus anthracis

Rounded ends
Ex: Salmonella

Club-shaped
Ex: Corynebacterium diptheriae ("Coryne" means "club")

Fusiform-shaped
Ex: Fusobacterium nucleatum (associated with periodontal disease)

Bent/comma-shaped
Ex: Desulfovibrio desulfuricans (associated with the gut)

Coccobacilli - Intermediate

Short, rounded rods
Ex: Coxiella burnetii (Q fever)

Spirochetes - Spirals

Ex: Treponema pallidum (Syphilis)

Be aware that there is intertextual variation in regards to bacterial classification schema, and some authors describe more or fewer morphological types.

Full-Length Text

  • Here we will learn an overview of bacterial microbes in infectious disease.
  • To begin, let's draw some basic components of a representative bacteria cell.
    • Be aware that different strains of bacteria have special anatomical and physiological traits, which we address elsewhere as these features relate to infectious disease.
  • First, draw the bacterial cell wall;
  • Then, show that the plasma, aka, cytoplasmic membrane, is deep to the cell wall; notice that it has an area of infolding.
    • The membrane comprises a lipid bilayer that serves multiple functions, including transport of molecules into the cell, secretion of toxins and enzymes, and energy generation.
  • Show that, as its name suggests, the cytoplasmic membrane encloses the cytoplasm.
  • Within the cytoplasm, indicate ribosomes, which are sites of protein synthesis.
  • Draw the nucleoid, which comprises the bacterial DNA; there is no nuclear membrane.
  • Now, show that some bacteria produce an outer capsule, which most often comprises gelatinous polysaccharides.
    • When present, the capsule contributes to the serologic type and enhances virulence.
  • Draw some representative pili, aka, fimbriae, which are short filaments found primarily on gram-negative strains.
    • When present, they participate in bacterial attachment to host cells; the sex pilus attaches male and female bacteria during conjugation.
  • Finally, show that some bacteria have flagella, which propels the cell; the number of flagella varies.
  • Most bacteria associated with infectious disease can be categorized as gram-positive or gram-negative, which refers to whether they retain crystal violet stain, and depends on the composition of their cell walls.

To illustrate, we'll begin with the gram-positive cell wall; in a microscopic sample, we can see the bright purple stain – think Purple Positive.

  • First, indicate the lipid bilayer of the cytoplasmic membrane, and show the cytoplasm deep to it.
  • Then, show that the cell wall of gram positive bacteria is thick with peptidoglyclan (aka, murein and mucopeptide); it comprises a network of sugars and amino acids, and is the target of some antibiotic drugs.
  • Show that, when present, the capsule is the outermost layer.
  • Then, indicate two acids that are present in the cell wall: teichoic acids, which attach to the peptidoglycan, and, lipoteichoic acid, which are attached to the cytoplasmic membrane.
    • These surface acids contribute to the cell wall structure and charge.

Next, let's show the cell wall of a gram-negative bacterium; notice in our microscopic sample that gram negative bacteria appear reddish-pink.

  • Show the cytoplasmic membrane, then,
  • Indicate the periplasmic space.
  • Above this, show the thin peptidoglycan layer, which is covered by the outer membrane, and,
  • The outer membrane, which is unique to gram negative bacteria and comprises lipopolysaccharide, aka, endotoxin.
    • Endotoxin contributes to disease symptoms such as fever and shock.
  • Gram-staining is not appropriate for all bacterial strains;
    • For example, we show an image of mycobacteria, which does not have a cell wall and is visualized with acid-fast staining methods.
    • Another example is Treponema pallidum, a bacterial strain that has very thin cell wall but can be seen with dark-field microscopy, as in our image, or fluorescent antibodies.
  • Some strains, such as the intracellular Chlamydiae, can be seen with hematoxylin and eosin (H&E) staining.

The bacterial cell wall contributes to its morphology, which is also used for classification purposes;

  • Create a table to show some major morphological types and examples of each.

First, we'll learn some variations of the cocci, which are spherical in shape;

  • Show that these can be arranged in clusters, as the ubiquitous Staphylococcus species do;
  • Or, they can form chains, as Streptococcus pyogenes;
  • Others come in pairs, and can be pointed, like Streptococcus pneumoniae, or coffee-bean shaped, like Neisseria gonorrhoeae.
  • Next, indicate that Bacilli means "rod-shaped"; variations include rectangular ends, as in Bacillus anthracis, and rounded ends, as in Salmonella.
  • Other bacilli forms include the following:
    • Club-shaped, such as Corynebacterium diptheriae; ("Coryne" means "club").
    • Fusiform-shaped, such as Fusobacterium nucleatum, which is associated with periodontal disease;
    • Or bent and comma-shaped, as in Desulfovibrio desulfuricans, which is associated with the gut flora.
  • Then, indicate that the coccobacilli are intermediate between the cocci and bacilli – for example, Coxiella burnetii, which causes Q fever, look like short, rounded rods.
  • Finally, show that the spirochetes are spiral shaped, as in the Treponema pallidum, which causes syphilis.

Be aware that there is intertextual variation in regards to bacterial classification schema, and some authors describe more or fewer morphological types.

References

  • Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
  • Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016)

Images:

  • Treponema (Centers for Disease Control)
  • Streptococcus pneumoniae (Centers for Disease Control)
  • Salmonella typhimurium (Wikipedia; Volker Brinkmann, Max Planck Institute for Infection Biology, Berlin, Germany; A Novel Data-Mining Approach Systematically Links Genes to Traits. PLoS Biology Vol. 3/5/2005, e166 doi:10.1371/journal.pbio.0030166)
  • Mycobacteria (CDC/Dr. George P. Kubica - phil.cdc.gov CDC-PHIL ID #5789)
  • Fusobacterium nucleatum (J. Michael Miller, Ph.D.,(D)ABMM of National Center for Zoonotic, Vector-borne, and Enteric Diseases.)
  • Desulfovibrio desulfurians (Wikipedia, public domain. https://commons.wikimedia.org/wiki/File:Desulfovibrio_desulfuricans.jpg)
  • Coxiella burnetii (http://www3.niaid.nih.gov/biodefense/public/images.htm Biodefense Image Library)
  • Diptheria (Centers for Disease Control; https://www.cdc.gov/diphtheria/about/photos.html)
  • Chlamydiae (https://visualsonline.cancer.gov/details.cfm?imageid=2331)
  • Bacillus anthracis (Wikipedia; Author BruceBlaus).