Bacterial Structure, Growth &

Metabolism

Diagrammatic structure of a generalized bacterium

The cell envelope is composed of •cell membrane – phospholipid bilayar; acts a permeability barrier•cell wall•outer membrane (ONLY IN GRAM NEGATIVE BACTERIA).

Bacterial cell wall Unique to bacteria

consists of the peptidoglycan layer & attached structures

Forms a strong mesh around the bacterial cells

Present in both Gram positive & Gram negative bacteria Absent in Mycoplasma & Chlamydia

Important for Maintaining the cell's shape

the rigid wall compensates for the flexibility of the phospholipid membrane

Countering the effects of osmotic pressure Providing attachment sites for bacteriophages via the

teichoic acids on the outer surface of the wall Providing a rigid platform for surface appendages e.g

flagella & pili

Peptidoglycan: Basic structure Glycan backbone: alternating

monosaccharide subunits N-acetyl muramic acid

(NAM) & N-acetyl glucosamine (NAG)

Peptide side chains - made up of D- and L- amino acids Sometimes diaminopimelic

acid Peptide side chains cross link

NAM subunits Peptide bridges

cross-link the peptide side chainsMuramic acid, D-amino acids & diamionpimelic acid are unique to bacteria

Comparison of Gram positive & Gram negative cell envelopes

Gram -ve: -thin peptidoglycan-Outer membrane lipopolysaccharide Braun lipoproteinPorin channels

Gram +ve: -thick peptidoglycan-Lipotechoic & Teichoic acid

Comparison of Gram-Positive and Gram-Negative Bacteria

Principle of Gram stain

Gram positive bacteria stain bluish-purpleGram negative

bacteria appear pinkish.

Because of the thick peptidoglycan cell wall, Gram postive bacteria retain the Crytal violet dye after acetone decolourisation

Because of thin peptidoglycan cell wall, Gram negative bacteria lose the crystal violet after acetone decolourisation & take up the safranin counterstain

Bacterial surface structures

Capsules Fimbriae (pili) Flagellae

Bacterial capsule Found surrounding outside of cell

envelope not essential to cell viability some bacteria may produce a capsule,

others do not It may be thick or thin May be closely or loosely associated

Called slime layer when it is not well defined

Usually polysaccharide in nature

Functions of capsule:-important virulence factor protects cell from desiccation and toxic

materials promote the concentration of nutrients

at bacterial cell surface play a role in adherence protects bacteria from phagocytosis

(killing) capsular material is antigenic and helps

in serological identification

Bacterial Surface Structures: The flagella

Long, filamentous appendages Responsible for bacterial

movement Differ in their number and

arrangement in the cell single polar flagella – monotrichous single flagella at two different poles –

amphitrichous

two or more at one pole – lophotrichous

arising over the entire cell surface - peritrichous

Bacterial Surface Structures: Fimbriae (pili)

smaller appendages found on the surface of many bacteria Shorter than flagella,

thread-like Composed of a protein

fimbrillin or pilin Aid bacteria to adhere to

one another – important virulence factor

Serve as attachment site for attachment to host cell surface important for pathogenecity

Sex pili involved in specific pair

formation for exchange of genetic material during conjugation

Endospores Dormant form of bacterial cell

The actively growing form is called vegetative

Produced by certain bacteria under adverse conditions (especially inadequate nutrients) Spores are resistant to adverse

conditions high temperatures & organic solvents

Survive under adverse conditions Re-germinate when conditions are

favourable Commonly found in the genera

Bacillus (e.g. Anthrax) and Clostridium (both Gram positive)

Endospore formation

1. Bacterial chromosomes replicates

2. Small amount of cytoplasm gathers with it

3. Cell membrane grows to seal off the developing spore

4. Once the walls of the spore are complete, spore is released

Endospores are difficult to eliminate

Most resistant living things known Contain dipicolinic acid which helps

stabilise their proteins and DNA Makes it difficult to eliminate them from

contaminated materials Living spores have been recovered from

Egyptian mummies Can resist boiling for 2hours Can survive in 70% ethanol for years Highly resistant to radiation

Bacterial requirements for growth Nutrients

All bacteria obtain energy by oxidising preformed organic molecules (carbohydrates, lipids and protein) from their environment

Metal Ions especially Iron They secrete small molecules called siderophores Siderophores bind iron which are then internalized into

the bacterial cells via receptors Ability to compete for iron is an impt virulence factor

Energy : ATP from metabolism of organic molecules

Optimal temperature Many human bacteria are Mesophiles (grow at

human body temp) Some bacteria are pyschrophiles (survive at temp

close to freezing) & some are thermophiles (survive at temp close to boiling)

Optimal pH : Many grow best at neutral pH ; Some can survive/grow in acid / alkali

Oxygen (or absence)

Environmental (O2) requirement for bacterial growth

Obligate Aerobes Have an absolute or obligate requirement for oxygen e.g.

Pseudomonas

Obligate Anaerobes Cannot grow if any oxgen is present e.g. Bacteroides

Facultative anaerobes Grow better if there is oxygen present but will also grow

if oxygen is absent. Facultative: means the organism is flexible in its oxygen

requirement E.g. Escherichia coli

Aerotolerant anaerobes Also called obligate fermenters; They are indifferent to

oxygen as they do not use it to transform energy. E.g. Streptococcus pyogenes; Lactobacillus

Microaerophilic Require small amounts of oxygen for aerobic respiration

(2-10%); Higher concentrations of oxygen are inhibitory E.g. Campylobacter & Helicobacter

Types of media In the laboratory we can grow

bacteria on solid media (agar plates) or in liquid media (e.g. broth)

Categories of media Complex media

Differential media

Selective media

TYPES OF MEDIA: DEFINITIONS

Complex media/ Enriched media : Contains a variety of ingredients for general bacterial growth e.g. Nutrient agar & Blood agar

Selective media: Substances e.g. antibiotics have been added suppress the growth of other organisms and only the desired organism will grow

Differential media: Contain substances that will change in a recognizable way for particular bacteria

Kinetics of bacterial growth Phases of growth

Lag phase ( cells adjusting to new environment

Exponential phase (cells growing exponentially) Exponential growth via binary fission Generation or doubling time (from 20 min to 24 h

Stationary phase ( rate of cell growth=rate of cell death)

Death phase (cells dying due to lack nutrition in media)