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 1 2 3 4 5 6 ... ... n CELL NUMBER: 1 2 4 8 16 32 64 ... ... AS BASE OF 2: 20 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· 2n log Nn = log N0 · n log 2 n Division rate v = 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 1O2Singlet 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: • attachment (adsorption), • penetration (injection), • protein and nucleic acid synthesis, • assembly and packaging, and • 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 restriction-modification 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 IIssDNA-> 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 RNA- polymerase -> reverse transcription -> pdsDNA
RNAMS2 – 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 1 Glands; minor colds Varicella; Lymphoma; Epstein-Barr (animal hosts)
Equine Encephalitis; Yellow fever; Rubella; Dengue; Rabies -ss Polio- ; Rhino- ; Hepatitis A ; Foot- and-Mouth + 5 4 Infuenza -ss 3 HIV; tumors 6 Respiratory & Enteric; Rota: Infant diarrhea Mumps; Measles