Bacterial Structure & Morphology
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Bacterial Structure & Morphology
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.
Cytoplasm
Ribosomes are sites of protein synthesis.
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
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
Cocci - Spherical
Clusters
Ex: Staphylococcus species
Chains
Ex: Streptococcus pyogenes
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.