IV. Bacterial Structure and Growth

A. Bacterial Cells: An Overview

B. Bacterial Cell Structures

C. Factors that Influence Bacterial Growth

IV. A. Bacterial Cells: An Overview

• Shapes & Arrangements– Round Bacteria

Coccus Staphylococcus

Diplococcus Tetrad

Streptococcus Sarcina

– Rod-shaped Bacteria

Bacillus Streptobacillus

Diplobacillus Coryneform bacteria

IV. A. Bacterial Cells: An Overview

• Shapes & Arrangements (cont.)– Curved & Spiral Bacteria

Vibrio

Spirillum

Spirochaete

IV. A. Bacterial Cells: An Overview

• Sizes– Typically ~ 0.1 - 20 m (with some exceptions)

– Typical coccus: ~ 1 m (eg Staphylococcus)

– Typical short rod: ~ 1 x 5 m (eg E. coli)

– Barely within the best resolution of a good compound light microscope

IV. A. Bacterial Cells: An Overview

IV. B. Bacterial Cell Structures

1.     Capsules

2.     Cell Wall

3.     Plasma Membrane

4.     Cytoplasm & Cytoplasmic Inclusions

5.     Ribosomes

6.     Bacterial DNA

7.     Pili

8.     Flagella

9.     Spores

IV. B. 1.     Capsules

• Species and strain specific

• Structure – Polysaccharide or polypeptide layer outside cell

wall

– May be tightly or loosely bound

– Detected by negative staining techniques

IV. B. 1.     Capsules (cont.)

• Functions – Attachment

– Resistance to desiccation

– Nutrient Storage

– Evasion of phagocytosis

eg. in Streptococcus pneumoniae

S strain is encapsulated & virulent

R strain is nonencapsulated & nonvirulent

IV. B. 2.     Cell Wall

• Gram Staining– Method developed by Gram in 1888

– Gram-positive cells stain purpleGram-negative cells stain pink

– Later, it was discovered that the major factor determining Gram reactions is the bacterial cell wall structure

– “Gram-positive” & “Gram-negative”These terms can mean either:Staining results, or Types of cell wall structure

IV. B. 2.     Cell Wall

• Peptidoglycan Structure– Composition

• A Polysaccharide

• Composed of alternating units ofN-acetylglucosamine (NAG) andN-acetylmuramic acid (NAM)

– Peptide Crosslinking BetweenNAM units

– Much thicker and more crosslinking in Gram-positive than in Gram-negative Bacteria

IV. B. 2.     Cell Wall

• Gram-positive Cell Wall– Thick Layer of Highly Crosslinked

Peptidoglycan

– Teichoic Acid Strands

IV. B. 2.     Cell Wall

• Gram-negative Cell Walls– Outer Membrane

• Lipopolysaccharide Layercontaining Lipid A

• Phospholipid Layer

• Outer Membrane Proteins

– Thin Layer of Peptidoglycanwith no teichoic acid

– Periplasmic Space

IV. B. 2.     Cell Wall

• Variations on Cell Wall Architecture– Acid-fast Cell Walls

• Similar to Gram-positive structure, buthave Mycolic Acid: A waxy lipid

• Require special acid-fast staining technique

• Includes Mycobacterium and Nocardia

IV. B. 2.     Cell Wall

• Variations on Cell Wall Architecture (cont.)– Mycoplasmas

• Bacteria that are naturally have no cell walls

• Includes Mycoplasma and Ureaplasma

– Archaeobacteria• Have unusual archaeobacterial cell walls

with no peptidoglycan

• Have unusual metabolisms

• Share a more recent common ancestor with eukaryotes than with eubacteria (“true bacteria”)

IV. B. 3.     Plasma Membrane

• Structure– Phospholipid Bilayer with Associated

Proteins

• Functions – Maintain Cell Integrity

– Regulate Transport

– Specialized Functions in Bacteria

IV. B. 4.Cytoplasm & Cytoplasmic Inclusions

• Composition: – Viscous aqueous suspension of proteins,

nucleic acid, dissolved organic compounds, mineral salts

• Cytoplasmic Inclusions:– Metachromatic Granules (Phosphate)

– Starch Granules

– Lipid Droplets

– Sulfur Granules

IV. B. 5.     Ribosomes

• Suspended in Cytoplasm

• Sites of Protein Synthesis

IV. B. 6.     Bacterial DNA

• Chromosomal DNA

• Plasmid DNA –R-Plasmids

–F-Plasmids

IV. B. 7.     Pili

• Hair-like structures on cell surface

• Functions –Attachment

–Conjugation

IV. B. 8.     Flagella

• Function– Motility

Almost all motile bacteria are motile by means of flagella

– Motile vs. nonmotile bacteria

• Structure– Filament

Composed of the protein flagellin

– Hook & Rotor AssemblyPermits rotational "spinning" movement

IV. B. 8.     Flagella

• Mechanism of Motility– “Run and Tumble” Movement

controlled by the direction of the flagellar spin

– Counterclockwise spin = Straight RunClockwise spin = Random Tumble

IV. B. 8.     Flagella

• Chemotaxis– Response to the concentration of chemical

attractants and repellants

– As a bacterium approaches an attractant:the lengths of the straight runs increase

– As a bacterium approaches a repellant:the lengths of the straight runs decrease

IV. B. 9.     Spores• Function

– To permit the organism to survive during conditions of desiccation, nutrient depletion, and waste buildup

– Bacterial spores are NOT a reproductive structure, like plant or fungal spores

• Occurrence – Produced by very few genera of bacteria

– Major examplesBacillus Clostridium

IV. B. 9.     Spores

• Significance in Medicine & Industry – Spores are resistant to killing

– Cannot be killed by 100°C (boiling)

– Requires heating to 120°C for 15-20 min (autoclaving or pressure cooking)

IV. B. 9.     Spores

• Sporulation – The process of spore formation

– Governed by genetic mechanism

– A copy of the bacterial chromosome is surrounded by a thick, durable spore coat

– This forms an endospore within a vegetative cell

– When the vegetative cell dies and ruptures, the free spore is released

IV. B. 9.     Spores

• Spore Germination – When a spore encounters favorable growth

conditions

– The spore coat ruptures and a new vegetative cell is formed

IV. C. Factors that Influence Bacterial Growth

• Growth vs. Survival– Bacteria may tolerate or survive under more

extreme conditions than their growth conditions

IV. C. Factors that Influence Bacterial Growth

• Nutrient Requirements– Energy Source

Most bacteria are chemotrophs; a few are phototrophs

– Carbon SourceMost bacteria are heterotrophs; a few are autotrophs

– Nitrogen, Phosphate, Sulfur, Trace Minerals

IV. C. Factors that Influence Bacterial Growth

• Nutrient Requirements (cont.)– Special Requirements

examples: amino acids and enzyme cofactors (vitamins) Fastidious bacteria: Strains that are difficult or impossible to culture due to special growth requirements

IV. C. Factors that Influence Bacterial Growth

• Temperature– Psychrophiles

Grow at ~0°C - 20°C

– Mesophiles Grow at ~20°C - 45°C

– Moderate Thermophiles Grow at ~45°C - 70°C

– Extreme Thermophiles Grow at ~70°C - 100°C

IV. C. Factors that Influence Bacterial Growth

• pH– Acidophiles

Grow at ~pH 1.0 - pH 6.0

– Neutrophiles Grow at ~pH 6.0 - pH 8.5

– Alkalophiles Grow above pH 8.5

IV. C. Factors that Influence Bacterial Growth

• Oxygen – Strict aerobes (Obligate aerobes)

Use oxygen for respiration in their metabolismRequire the presence of a normal oxygen concentration (~20%) for growth

– Strict anaerobes (Obligate anaerobes) Oxygen is a poison for these microbes Cannot grow at all in the presence of oxygen

IV. C. Factors that Influence Bacterial Growth

• Oxygen (cont.)– Aerotolerate anaerobes

Do not use oxygen, but oxygen is not a poison for these Can grow equally well with or without oxygen

– Facultative anaerobes Use oxygen for respiration, but can also grow without oxygen Grow better with oxygen that without oxygen

IV. C. Factors that Influence Bacterial Growth

• Oxygen (cont.) – Microaerophiles

Require low concentrations (~5% - 10%) of oxygen for growth