Cell Morphology

• In microbiology, the term morphology means cell shape.
• A bacterium that is spherical or ovoid in morphology is called a coccus (plural, cocci).
• A bacterium with a cylindrical shape is�called a rod or a bacillus.
• Some rods twist into spiral shapes and�are called spirilla. �

• The cells of many prokaryotic species remain�together in groups or clusters after cell division, and the arrangements are often characteristic of certain genera.
• some cocci form long chains (for example, the bacterium Streptococcus), others occur in three-dimensional cubes (Sarcina), and still others in grapelike clusters (Staphylococcus). �

Size of Bacteria

�• Unit of measurement in bacteriology is the micron (micrometre, µm)

�• Bacteria of medical importance (0.2 – 1.5 µm) in diameter (3– 5 µm) in length �

Prokaryotic Cell Organization

The Cell Wall of Bacteria:

• Species of Bacteria can be divided into two major groups, called gram-positive and gram-negative.
• The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane.
• It is one of the most important procaryotic structures for several reasons:
• it helps determine the shape of the cell;
• it helps protect the cell from osmotic lysis;
• it can protect the cell from toxic substances; and
• in pathogens, it can contribute to pathogenicity.
• The procaryotic cell wall also is the site of action of several antibiotics.

Peptidoglycan

• The rigid layer, called peptidoglycan,
• is a polysaccharide composed of two sugar�derivatives—N-acetylglucosamine and N-acetylmuramic acid— and
• a few amino acids, including L-alanine, D-alanine, D-glutamic acid, and either lysine or the structurally similar amino acid analog,�diaminopimelic acid (DAP). �

• Peptidoglycan can be destroyed by certain agents.
• One such agent is the enzyme lysozyme, a protein that cleaves the β-1,4-glycosidic bonds between N-acetylglucosamine and N-acetylmuramic acid in peptidoglycan, thereby weakening the wall; water can then enter the cell and cause lysis.
• Lysozyme is found in animal secretions including tears, saliva, and other body fluids, and functions as a major line of defense against bacterial infection.

Cell Walls of Archaea

• Peptidoglycan is absent from the cell walls of Archaea.
• a variety of chemistries are found in the cell walls of Archaea, including polysaccharides, proteins, and glycoproteins.
• The cell walls of certain methanogenic Archaea contain similar to peptidoglycan, a polysaccharide called pseudomurein
• some other Archaea lack pseudomurein and have polysaccharide walls composed of polymers of glucose, glucuronic acid, galactosamine uronic acid, and acetate

Pseudomurein

Capsules, Slime Layers, and S-Layers

• Some prokaryotes have a layer of material lying outside the cell wall, usually are composed of polysaccharides.
• When the layer is well organized and not easily washed off, it is called a capsule, It is called a slime layer when it is a zone of diffuse, unorganized material that is removed easily.
• When the layer consists of a network of polysaccharides extending from the surface of the cell, it is referred to as the glycocalyx

Pili and Fimbriae

• Many procaryotes have short, fine, hairlike appendages that are thinner than flagella. These are usually called fimbriae (s., fimbria).
• They are slender tubes composed of helically arranged protein subunits and are about 3 to 10 nm in diameter and up to several micrometers long.
• They can aid in attachment to objects, and also are required for the twitching motility.�

• Many bacteria have about 1-10 sex pili (s., pilus) per cell.
• These are hairlike structures that differ from fimbriae in the following ways.
• Pili often are larger than fimbriae (around 9 to 10 nm in diameter).
• genetically determined by plasmids and are required for conjugation

Flagella and Motility

• Most motile procaryotes move by use of flagella (s., flagellum), threadlike locomotor appendages extending outward from the�plasma membrane and cell wall
• Bacterial flagella are slender, rigid structures, about 20 nm across and up to 15 or 20 m long. ��

• Gram negative

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Gram negative

Gram positive

• (1) The longest and most obvious portion is the flagellar filament, which extends from the cell surface to the tip.
• (2) A basal body is embedded in the cell; and
• (3) a short, curved segment, the flagellar�hook, links the filament to its basal body and acts as a flexible coupling.
• The filament is a hollow, rigid cylinder constructed of subunits of the protein flagellin

Endospore

• A number of gram-positive bacteria can form a special resistant, dormant structure called an endospore.
• Endospores develop within vegetative bacterial cells of several genera: Bacillus and Clostridium (rods), Sporosarcina (cocci).
• These structures are extraordinarily resistant to environmental stresses such as heat, ultraviolet radiation, gamma radiation, chemical disinfectants, and desiccation

Endospore Structure

• The spore often is surrounded by a thin, delicate covering called the exosporium.
• A spore coat lies beneath the exosporium, is composed of several protein layers, and may be fairly thick.
• It is impermeable to many toxic molecules and is responsible for the spore’s resistance to chemicals.
• The cortex, which may occupy as much as half the spore volume, rests beneath the spore coat.
• It is made of a peptidoglycan that is less cross-linked than that in vegetative cells.
• The spore cell wall (or core wall) is inside the cortex and surrounds the protoplast or spore core.
• The core has normal cell structures such as ribosomes and a nucleoid, but is metabolically inactive

• As much as 15% of the spore’s dry weight consists of dipicolinic acid complexed with�calcium ions
• Calcium does aid in resistance to wet heat, oxidizing agents, and sometimes dry heat
• calcium-dipicolinate stabilizes the spore’s nucleic acid
• specialized small, acid-soluble DNA-binding proteins (SASPs), are found in the endospore
• protect against enzymes and chemicals such as hydrogen peroxide.
• Finally, spores contain some DNA repair enzymes.

Ribosomes

• Ribosomes are very complex structures made of both protein and ribonucleic acid (RNA)
• site of protein synthesis;
• molecular chaperones - polypeptide in folding to its proper shape
• Procaryotic ribosomes are called 70S ribosomes

THE NUCLEOID

• the nuclear body, chromatin body, nuclear region
• single circle of double-stranded deoxyribonucleic acid (DNA),
• linear DNA chromosome and some, such as Vibrio cholerae and Borrelia burgdorferi
• Bacteria do not use histone proteins

(a) The DNA double helix of most procaryotes is in the shape of a closed circle. (b) The circular DNA strands, �

PLASMIDS

• extrachromosomal DNA molecules
• small, double-stranded DNA molecules that can exist independently of the chromosome.
• circular and linear plasmids
• carry genes that confer a selective advantage
• Single-copy plasmids or Multicopy plasmids
• plasmids are able to integrate into the chromosome and are thus replicated with the chromosome episomes.
• The loss of a plasmid is called curing.

PLASMIDS…..

• Classified in terms of their mode of existence, spread, and function.
• Conjugative plasmids - They have genes for the construction of hairlike structures called pili and can transfer copies of themselves to other bacteria during conjugation.
• Resistance factors (R factors, R plasmids) - confer antibiotic resistance on the cells
• Col plasmids contain genes for the synthesis of bacteriocins known as colicins, which are directed against E. coli.
• Virulence plasmids encode factors that make their hosts more pathogenic.
• Metabolic plasmids carry genes for enzymes that degrade substances such as aromatic compounds (toluene), pesticides (2,4-dichlorophenoxyacetic acid), and sugars (lactose).