Chapter 4
Bacterial Structure and Growth
Bacteria
• Bacteria can be classified by their morphology
(shape), stain reactions, atmospheric
requirements, growth needs, organization,
physical and biochemical composition and
antigenic structure.
Morphology
Shapes and Arrangements (Pg 122 – 124)
• Three common shapes
– Bacilli (rod)
• Long as 20 µm or as short as 0.5 µm
– Cocci (sphere)
• Kokkos meaning “berry”
• 0.5 to 1.0 µm
– Spirochetes (spiral)
• 1 to 100 µm in length
Structure of Bacteria Pg. 122 to 139
Flagella
• Motility – is the independent movement of an
organism.
• In bacteria movement is carried out by flagella.
• Each flagella is composed of a filament, hook
and basal body.
Flagella
• The long filament is composed of a helical protein
strand.
• Subunits of the protein filaments are called flagellin.
• The filament is anchored into the cell membrane by
hook which is attached to the basal body.
• Basal body is made up of 20 protein that forms a
central rod and closing rings.
• There can be two to four rings making up the basal
body.
• Range in length 10 to 20 um and 20nm thick
– To view need to stain
Arrangement of Flagella
• Monotrichous – single flagellum at one end.
• Lophotrichous – two or more flagella at one end.
• Amphitrichous – single or group of flagella at both
ends.
• Peritrichous – flagella over the whole cell surface.
Movement
• Flagella spin counter-clockwise to move the
bacteria forward.
• The flagella reverse (Clock-wise) to move the
bacteria back.
• The backward movement is more like a tumble that
helps change direction not backup the bacteria.
• Most movement is stimulated by chemicals and
light and is known as taxis
– Light taxis
– Chemotaxis
Pili (Fimbriae)
• Structures used for attachment to specific surfaces.
• Many Gram negative bacteria have pili.
• Short hair like fibers.
• Made up of protein subunits called pilin.
• Have protein adhesion located at the tip of the pili that
bind to different animal tissue.
• Neisseria gonorrhoeae pili attach to urogenital tract (or
toilet seat).
Conjugation Pili
• True pili is used in conjugation (transfer of
DNA from one bacteria to another).
• It is a tube that attaches to a bacteria.
• The DNA is moved down the tube from the
donor to the receiver.
Capsule (Glycocalyx)
• Many bacteria have capsules.
• Bacteria will secrete layer of polysaccharides
and proteins.
• The capsule serves as a buffer between the
bacteria and the environment to prevent
dehydration.
• It also helps to attach the cell to each other and
to host (V. cholerae to intestinal wall).
• Protects some bacteria from being engulfed by
white blood cells.
Cell Wall
• All bacteria except for Mycoplasma have a cell wall.
• A cell wall protects the bacteria and gives it its shape.
• Major role is to prevent lysis (bursting) due to osmotic
pressure.
• Cells have more dissolved material inside them then is
found in the surrounding environment.
• This hypertonic condition would cause water to move
into the cell an increase the pressure inside.
• Without the cell wall the bacteria would burst.
Cell Wall
• Eubacteria cell walls are made up of a network
of peptidoglycan chains.
• Peptidoglycan is made up of alternating units of
two amino containing sugars.
– N-acetylglucosamine (NAG)
– N-acetylmuramic acid (NAM)
• This carbohydrate backbone is held together by
side chains of four amino acids.
• Plus polypeptide cross-linkage
Cell Wall Gram-Positive Bacteria
• Gram-positive bacteria have thick layered
peptidoglycan wall.
• 60 to 90 % made up of peptidoglycan.
• Peptidoglycan layer also has a anionic
polysaccharide called teichoic acid.
• Helps to link peptidoglycan layers together.
• Penicillin interferes with the construction of the
peptidoglycan layer.
Insert Picture of Peptidoglycan
Cell Wall of Gram-Negative Bacteria
• Gram-negative bacteria have a multilayered cell
wall.
• No teichoic acid.
• Peptidoglycan layer much thinner.
• Peptidoglycan makes only 10% total of cell wall.
Gram-negative Cell Wall Layers
• Outer membrane
– Made up of bilayer.
– Inner layer of phospholipids.
– Outer layer of lipopolysaccharides (LPS).
– Contains proteins porins
• Porins form pores in the outer membrane.
• Allows small molecules to move through the cell wall.
• Inner layer is made up of peptidoglycan.
Gram-Negative Cell Wall LPS
• Lipopolysaccharide (LPS) are not found in any
other organism.
• The lipid portion (lipid A) is found in the outer
membrane bi-layer.
• Lipid A portion is a endotoxin
– When the bacteria die they breakdown and the LPS is
released and causes fever and circulatory collapse.
• The polysaccharide is a O-polysaccharide
– Used to identify bacteria ie O157:H7
Bacterial Cell Membrane
• Boundary between cytoplasm and external
environment.
• Act as a permeability barrier to hold bacterial
cytoplasm inside the cell.
• Also acts to transport nutrients in and waste out
of the cell.
• Made up of 40% phospholipids and 60%
proteins.
• Phospholipids are arranged in two parallel layers
(bi-layer).
Cell Membrane
• Proteins in the membrane can be used for
synthesis of the membrane.
• Also, some of these proteins are used for
energy production (ATP).
• Other proteins act as transporters of
charged molecules in and out of the
bacteria.
Cytoplasm (Pg 135 to 136)
• Cytosol a gelatinous mass of proteins,
amino acids, sugars, nucleotides, salts,
vitamins and ions.
• Structures
– Ribosomes hundreds of thousands in bacteria
– Made of RNA and protein
– Used for protein synthesis
Cytoplasm
• Inclusion bodies
– Store nutrients or monomers for bacterial structures
– Metachromatic granules – phosphate depots
– Magnetosome crystals of iron-containing molecules
magnetite.
• Help align bacteria to magnetic fields
• Nucleoid – chromosome region
– No membrane DNA aggregates and no ribosomes
are found in that area.
• Plasmids – small circular DNA
– Can be one or more
– 5-100 genes
– Important function in causing disease
DNA Plasmids
• Some plasmids carry genes for drug
resistance.
• R-plasmids (R = “resistance”).
• These plasmids can be transferred
between bacteria during conjugation.
Endospores
• A few Gram-positive bacteria produce a highly
resistant structures called endospores (spores).
• Bacillus and Clostridium Genuses are of major
concern in human bacterial infections and are
spore produces.
• Endospores will be produced by these bacteria
due to nutritional or environmental pressures.
• Endospores are one of the most resistant living
things known.
Endospores
• Little water and undergo few chemical reactions.
• Have some ribosomes and enzymes.
• Has an unique organic substance called dipicolinic
acid.
• Dipicolinic acid helps stabilize proteins and DNA.
• Helps to make spores viable at 100ºC for 2 hr.
Endospore
Diseases Caused by Spore Forming Bacteria
• Anthrax caused by Bacillus anthracis.
• Botulism, gas gangrene and tetanus caused by
different Clostridium species.
• Clostridium difficile when you take antibiotics this
bacteria can overpopulate your intestine or colon and
can cause colitis.
Bacterial Reproduction (Pg 141 – 142)
• Almost all bacteria reproduce by binary fission.
• In binary fission:
– Cell elongates and DNA replicates.
– The two DNA strands are attached to the cell
membrane and separated by new membrane growth.
– The new membrane pinches inward.
– The cell membrane thickens.
– The cell wall divides.
Generation Time
• Generation Time - Is the interval of time
between successive binary fission of a cell.
• Escherichia coli generation time 20 minutes.
• Staphylococcus aureus 30 minutes.
• Mycobacterium tuberculosis 15 hours.
• Treponema pallidum (Syphilis) 33 hours.
How Many E. coli Would There Be After 2 Days?
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E. coli generation time is every 20 minutes
3 generations per hour
48 hour period
N(t) =N(0)2n N(48) = (1) x 2144
N(48) = 2144
2144 = 2.2 x1043
109 = 1,000,000,000
Each bacteria is 10-12 gms
The total bacterial mass from 2.2 x1043 = 1031
gms.
• 454 gms = 1 pound
• The mass would be
24,000,000,000,000,000,000,000,000 tons
• or 400 x the mass of the earth
Bacterial Growth (Pg 143 – 149)
• Four distinctive phases
Lag phase
Logarithmic phase
Stationary Phase
Decline Phase
Lag Phase
• Bacteria adapts to its new environment.
• There is no cell division
• Bacteria:
– Grow in size
– Store nutrients
– Synthesize enzymes
Logarithmic Phase (Log)
• Bacteria enter an active phase of growth.
• Exponential growth occurs by binary fission.
• In infection disease symptoms develop in animals or
humans when the bacteria are in log phase.
– Toxin production causes tissue damage.
– Fever, coughing.
• However, bacteria are most susceptible to antibiotics in
this phase because they are producing new cell walls.
Stationary Phase
• Point where death rate equals reproduction.
• Immune system is killing large numbers.
• Nutrients are running low.
• Waste products accumulate.
• Oxygen may be running out.
Death Phase
• Dying cells exceeds reproduction.
• Some cells, Bacillus and Clostridium, may
have produced endospores and will enter
dormancy.
• Others will end here.
• Bacterial Growth
Physical Requirements for Bacterial Growth
• Temperature
– Each bacteria has an optimal growing
temperature.
– Also, a 30ºC range at which growth can occur.
– Reduced growth is the result of slower
reaction times for enzymes.
– Three Groups
• Psychrophiles
• Thermophiles
• Mesophiles
Temperature
• Psychrophiles
– Psychro = “Cold” Philes = “Loving”
– Optimum growth rate 15ºC
– Can grow at 0ºC - 20ºC
– Unsaturated fats in phospholipids so they stay
fluid at lower temperatures.
• Thermophiles
– Optimum temperature 60ºC
– Growth at 40ºC - 70ºC
– Compost heaps, hot tubs, hot springs
– Highly saturated fats to stabilize structures
and heat stable enzymes.
Temperature
• Mesophiles
– Middle temperature range 20ºC - 42ºC
– Includes the pathogens
– Get slow growth with some at low
temperatures 5ºC
– Not true psychrophiles so referred to as
psychrotrophic
– Psychrotrophic – grow at 0ºC but prefer to
grow in the range of 20ºC - 42ºC
• Campylobacter jejuni can grow at low
temperatures
Oxygen
• Aerobes – need a good supply of oxygen.
• Microaerophiles – live in areas of low oxygen.
• Anaerobes – do not and can not use oxygen.
– Aerotolerant – can grow with oxygen present.
– Obligate anaerobes are killed by any oxygen.
– H2S not H2O
– Clostridium species botulism gangrene.
• Facultative - grow in the presence or reduced oxygen.
– Facultative aerobe prefers anaerobic conditions.
– Facultative anaerobe prefers aerobic conditions.
• Capnophilic – low oxygen but high CO2
– Streptococcus are capnophilic
pH
• In most bacterial cytoplasm is neutral pH 7.0.
• Blood and tissue of humans is 7.2 to 7.4 which is
ideal for bacteria.
• Minimum to maximum pH range is between 3 pH
units.
• Some can survive 2.0 pH or lower (acidic).
• Vibrio cholerae pH 2.0 to pH 9.5
Acidophiles
• Acidophiles are bacteria that can tolerate acidic
conditions.
• Valuable to the food industry Lactobacillus and
Streptococcus coverts milk to butter milk and cream to
sour cream.
• Extreme acidophiles are found in the archaea
bacteria.
• Most bacteria however do not survive in low pH
Measuring Bacterial Growth (Pg 155 to 156)
• Spectrophotometer measures turbidity (cloudiness).
– A spectrophotometer measures amount of light scatter (optical
density, OD).
• Direct microscope count place culture on a specially
designed slide (Petroff-Hausser counting chamber).
• Most probable number count (MPN) estimate number of
cells by the amount of gas they produce through fermentation
(breakdown of sugar).
• Total Plate Count bacterial culture is diluted and the different
dilutions are plated and counted for colonies.
– Colony forming units (CFU) we will do this next week in
hand scrub experiment
Summary of Key Concepts
• The Shape and Arrangements of Bacteria
– Bacilli a cylindrical shape
– Cocci form a variety of arrangements
– Spirals and other shapes also exist
The Structure of Bacteria
• Bacterial flagella provide motility.
• Pili are structures used for attachment.
• The capsule (glycocalyx) is a sticky layer coating
many bacteria.
• The cell wall provides shape and protection.
• The cell membrane is a permeability barrier.
• The cytoplasm is the center of biochemical activity.
• Endospores are designed for dormancy
• Bacterial reproduction
– Bacteria reproduction by binary fission
• Bacterial growth
– A bacterial growth curve illustrates the
dynamics of growth
– Temperature is one of the most important
factors governing growth
– Oxygen can support or hinder growth
– Most bacteria prefer to grow at a neutral pH
• Culture media and growth measurements
– Population growth can be measured in
several ways