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)