Transcript Slide 1

Microbial Growth
( and growth control)
• Cell division and chromosome
replication are coordinately regulated, and
the
• Fts proteins are the keys to these
processes.
• The protein FtsZ defines the division
plane in prokaryotes, while
• Mre proteins help define cell shape.
New cell wall is synthesized during bacterial
growth by inserting new glycan units into
preexisting wall material. A hydrophobic
alcohol called bactoprenol facilitates transport
of new glycan units through the cytoplasmic
membrane to become part of the growing cell
wall. Transpeptidation bonds the precursors
into the peptidoglycan fabric.
GENERATIONS: 0
CELL NUMBER: 1
AS BASE OF 2: 20
1 2 3 4 5 6 ... ... n
2 4 8 16 32 64 ... ...
21 22 23 24 25 26... ... 2n
if at the time 0, the population consists of N0 bacteria, then
after n generations, the number of bacteria Nn will be:
Nn = N0 · 2 n
log Nn = log N0 · n log 2
Division rate v =
n
t
• Continuous culture devices (chemostats)
are a means of maintaining cell populations
in exponential growth for long periods.
• In a chemostat, the rate at which the
culture is diluted governs the growth rate,
and the population size is governed by the
concentration of the growth-limiting nutrient
entering the vessel.
• Organisms with cold temperature optima are
called psychrophiles, and the most extreme
representatives inhabit permanently cold
environments.
• Psychrophiles have evolved biomolecules
that function best at cold temperatures but
that can be unusually sensitive to warm
temperatures.
• Organisms with growth temperature optima
between 45 and 80°C are called
thermophiles, while
• Those with optima greater than 80°C are
called hyperthermophiles. These organisms
inhabit hot environments up to and including
boiling hot springs, as well as undersea
hydrothermal vents that can have
temperatures in excess of 100°C.
• Thermophiles and hyperthermophiles
produce heat-stable macromolecules.
• The acidity or alkalinity of an environment
can greatly affect microbial growth. Some
organisms have evolved to grow best at low or
high pH, but most organisms grow best
between pH 6 and 8.
•The internal pH of a cell must stay relatively
close to neutral even though the external pH is
highly acidic or basic.
• Water activity becomes limiting to an
organism when the dissolved solute
concentration in its environment increases. To
counteract this situation . . . .
• organisms produce or accumulate
intracellular compatible solutes that function to
maintain the cell in positive water balance.
• Some microorganisms have evolved to grow
best at reduced water potential, and some
even require high levels of salts in order to
grow.
Active antibacterial forms of oxygen
1O
2
Singlet Oxygen
Several toxic forms of oxygen can be formed
in the cell, but enzymes are present that can
neutralize most of them. Superoxide in
particular seems to be a common toxic
oxygen species.
• The three key processes of macromolecular
synthesis are
(1) DNA replication;
(2) transcription (the synthesis of RNA from a DNA
template); and
(3) translation (the synthesis of proteins using
messenger RNA as template).
• Although the basic processes are the same in
both prokaryotes and eukaryotes, the organization
of genetic information is more complex in
eukaryotes.
• Most eukaryotic genes have both coding regions
(exons) and noncoding regions (introns).
• DNA is a double-stranded molecule that forms a
helical configuration and is measured in terms of
numbers of base pairs.
• The two strands in the double helix are
antiparallel, but inverted repeats allow for the
formation of secondary structure.
• The strands of a double-helical DNA molecule can
be denatured by heat and allowed to re-associate
following cooling.
• Both strands of the DNA helix serve as
templates for the synthesis of two new strands
(semiconservative replication).
• The two progeny double helices each
contain one parental strand and one new
strand.
• The new strands are elongated by addition
to the 3’-end. DNA polymerases require a
primer, which is composed of RNA.
• The very long DNA molecule can be packaged
into the cell because it is supercoiled.
• In prokaryotes this supercoiling is brought about
by enzymes called topoisomerases.
• In eukaryotic chromosomes, DNA is wound
around proteins called histones, forming structures
called nucleosomes.
• DNA gyrase is a key enzyme in prokaryotes,
introducing negative supercoils to the DNA. Reverse
gyrase introduces positive supercoiling.
• Viruses are genetic elements that depend on
cellular host but replicate independently of cell’s
chromosome(s).
• A virion is the extracellular form of a virus and
contains either an RNA or a DNA genome.
• The virus genome is introduced into a new host
cell by infection.
• The virus redirects the host metabolism in order
to support virus replication.
• Viruses are classified by replication strategy as
well as by type of host.
• In the virion of the naked virus, only nucleic acid
and protein are present, with the nucleic acid on the
inside; the whole unit is called the nucleocapsid.
• Enveloped viruses have one or more lipoprotein
layers surrounding the nucleocapsid.
• The nucleocapsid is arranged in a symmetric
fashion, with a precise number and arrangement of
structural subunits surrounding the virus nucleic acid.
• Although viruses are metabolically inert, in some
viruses, one or more key enzymes are present within
the virion.
• Viruses attack: Bacteria, Plants, and
Animals
• Bacterial viruses (bacteriophage or phage)
have proved useful as model systems
because the host cells are easy to grow and
manipulate in culture.
• Viruses can only replicate in certain types of
cells or in whole organisms (specificity).
• Many animal and plant viruses can be
grown in cultured cells.
• Although it requires only a single virion to
initiate an infectious cycle, not all virions are
equally infectious.
• Virus infectivity is measured by the plaque
assay.
• Plaques are clear zones that develop on
lawns of host cells.
• Theoretically, each plaque is due to infection
by a single virus particle. The virus plaque is
analogous to the bacterial colony.
The virus life cycle can be divided into
five stages:
1. attachment (adsorption),
2. penetration (injection),
3. protein and nucleic acid synthesis,
4. assembly and packaging, and
5. virion release.
• The attachment of a virion to a host cell
is a highly specific process involving
complementary receptors on the
surface of a susceptible host cell and its
infecting virus.
• Resistance of the host to infection by
the virus can involve restrictionmodification systems that recognize and
destroy double-stranded foreign DNA.
• Before replication of viral nucleic acid can occur,
new virus proteins are needed. These are encoded
by messenger RNA molecules transcribed from the
virus genome.
• In some RNA viruses, the viral RNA itself is the
mRNA.
• In others, the virus genome is a template for the
formation of viral mRNA, and
• in certain cases, essential transcriptional enzymes
are contained in the virion.
• Lysogeny is a state in which viral genes
become integrated into the host chromosome
and lytic events are repressed.
• Viruses capable of inducing the lysogenic
state are called temperate viruses.
• In lysogeny, the virus genome becomes a
prophage; however, lytic events can be
induced by certain environmental stimuli.
• Lambda is a double-stranded DNA temperate
phage.
• Regulation of lambda lytic versus lysogenic
events is under the control of several promoters
and regulatory proteins.
• Repression of lambda lytic events is caused by
the cI protein (the lambda repressor), while
activation of lytic events is under control of the Cro
protein.
• Although the genome of lambda is linear, the
genome circularizes inside the cell, where DNA
synthesis occurs by a rolling circle mechanism.
Baltimore Virus Classification System
Class
I dsDNA -> host RNA-polymerase -> mRNA
II
ssDNA -> ds DNA -> mRNA
III
dsRNA (Replicase in virion) -> mRNA
IV
+ss RNA -> translation -> viral Replicase -> mRNA
V
–ssRNA (Replicase in v.) -> mRNA
VI
+ssRNA (Reverse transcript in v.) -> ss -> dsDNA ->
mRNA
VII
pdDNA (DNA-RT-polymerase in v.) -> host RNApolymerase -> reverse transcription -> pdsDNA
RNA MS2 –
conjungative plasmid-gene
overlap; In E. coli - 3.6kb
фX174 - Circular, rolling
cycle - 5.4kb
T7- linear - 40kbp
92% protein, concatamers
Lambda - 48.5kbp,
temperate
T4- linear - 170kbp
135 proteins
Animal viruses
Virion or nucleocapsid penetrate
DNA-polymerase dependent viruses – in nucleus
ssRNA+ - in cytoplasm
Retroviruses
viroids
prions
Infections:
Lytic
Persistent
Latent
Oncogenesis
Transformation
Benign
Malignant
Metastasis
7
2
Hepatitis-B
(animal hosts)
1
Papilloma tumors
Smallpox;
Cowpox;
Myxomatosis
1
Glands; minor colds
1
Varicella;
Lymphoma;
Epstein-Barr
(animal hosts)
Polio- ; Rhino- ;
Hepatitis A ;
Foot- and-Mouth
4
+
Equine Encephalitis;
Yellow fever;
Rubella;
Dengue;
Rabies -ss
5
Infuenza -ss
3
HIV; tumors
6
Respiratory &
Enteric; Rota:
Infant diarrhea
Mumps; Measles