Bacterial Structure & Morphology

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

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

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.

Bacterial Structure & Morphology

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