Introduction to microbiology (MIMM 211) Dr. Benoit Cousineau Lyman Duff Medical Building, room 617. (514) 398 – 8929 [email protected] Lee-Hwa Tai (TA) [email protected] Text : Prescott, Harley and Klein’s – Microbiology 7 th Edition by Willey, Sherwood and Woolverton. McGraw-Hill, New York.
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Introduction to microbiology (MIMM 211)
Dr. Benoit Cousineau Lyman Duff Medical Building, room 617. (514) 398 – 8929 [email protected]
Lee-Hwa Tai (TA) [email protected]
Text : Prescott, Harley and Klein’s – Microbiology 7 th Edition by Willey, Sherwood and Woolverton. McGraw-Hill, New York.
Computer support (1) : www.mcgill.ca/webct
• Course outline
o Important dates
o Midterm Wednesday October 21st. In class. Lectures 1-15 only.
• All lectures in PDF format.
• Audio and visual recording of lectures by Instructional Communication Centre.
Course evaluation :
1. Midterm Exam (50Q) 25% of course grade (L1-15)
2. Final Exam (150Q) 75% of course grade (L16-39)Multiple-choice questions.
What is microbiology? (1) Microbial biology, (2) biology of microorganisms, (3) study of microscopic organisms.
Chapter 1, the history and scope of microbiologyMicrobio : size of organisms studied (cannot be seen by the eye).
set of techniques used to study the organisms (microscope, grow pure cultures)
Microorganisms can be found everywhere in various conditions (-40 to 300 C). They are associated to humans (gut, skin). They can be useful to humans (food, biotechnology) or harmful (import historical impacts such as plague, malaria, AIDS).
Study perspective :
• Gross morphology;
• Fine structure;
• Nutrition;
• Reproduction;
• Physiology;
• Genetics;
• Classification;
• Evolution;
• Distribution;
• Intersections with other living things and environment.
Can be found in the 3 kingdoms of life : (1) bacteria, (2) archaea, (3) Eucarya.
Members of the microbial world
• Procaryotic or prokaryotic cell Pro = Before. Karyon = Nucleus. No enclosed DNA.
o Bacteria
o Archaea
• Eucaryotic or eukaryotic cell Eu = true.
o Algae
o Fungi
o Protozoa
• Non-living microorganisms
o Prokaryotic and eukaryotic viruses
Microorganisms share common threads of life
• Can grow (increase in size)
• Metabolism
o Need energy to grow (consume – transform – store)
o Consume nutrients
o Excrete wastes
• Motion (moving itself or having internal motion)
• Reproduction (create identical entities that are separate)
• Respond to stimuli (measure properties of their environment and act upon certain conditions)
The CELL is the basic structural unit of life. Microorganisms are usually unicellular.
Historical perspectives
• Lucretius and Girolamo Fracastoro suggested that invisible organisms cause disease.
• 1590-1608 : Johannes Jansen develops the first microscopes.
• 1665 : first description and depiction of a microorganism by Robert Hooke using a microscope : the reproductive structures of the microfungus Mucor.
• 1674 : discovery of bacteria and protozoa by Anthony van Leeuwenoek.
o Small hand-held microscopes (50x to 300x).
o Discovered invisible creatures he called animalcules.
o Found them everywhere (water, soil, teeth scrapings, excrements).
o Found that animalcules are alive.
Increase in numbers and move
“Appeared” in certain materials
Spontaneous generation (abiogenesis) : spontaneous formation of living things from inanimate matters.
• Aristotle, Descartes, Newton and other scientists believed in the spontaneous generation.
• Origin of many organisms was thought to be spontaneous: invertebrates, rats, flies, etc.
• In 1665, Francesco Redi showed that fly larvae can only develop in meat that fly can reach.
• Is it different for microorganisms? Does decomposition form microorganisms or do microorganisms cause decomposition?
Introduction to microbiology (MIMM 211) Course 2
Historical Perspectives
• John Needham boiled mutton broth, put it in a flask and sealed it. Microorganisms appeared and he suggested that his experiments were supporting spontaneous generation.
• Lazzaro Sapllanzani put broth in a flask, sealed it and boiled it. No microorganisms appeared and he claimed that he disproved spontaneous generation.
• Supporters of the spontaneous generation suggested that spontaneous generation probably required air.
• In 1859, the French Academy of science sponsored a competition to prove/disprove this theory.
• Louis Pasteur, in 1861, boiled meat broth in a flask and curved its neck. Air could freely enter but not dust. No microorganisms grew. This experiment disproved spontaneous generation. Microbiology became an experimental science as opposed to an observational science. (Pasteur’s swan-necked flasks)
• In 1877, John Tyndall supported Pasteur’s finding by demonstrating that dust carries microorganisms.
o Let dust settled in a closed box containing tubes filled with broth.
o Interior of the bax coated with glycerin : traps dust particles.
o Air could enter but dust was trapped. No microorganisms.
• Role of microorganisms in disease
o Lucretius and Fracastoro suggested that invisible organisms cause disease.
o People believed that diseases were caused by (1) supernatural forces, (2) poisonous vapors called miasmas, (3) imbalances between the four humors of the body (blood, phlegm, yellow bile, black bile).
o Agostino Bassi first showed that a microorganism (fungus) can cause disease in silkworms. Beginning of the “germ theory of disease”.
o In 1845, Berkeley showed that the great potato blight of Ireland was caused by a fungus.
o Pasteur showed that silkworms were parasitized by a protozoan in the French silk industry. They then started to raise caterpillars from eggs produced by healthy moths.
o In 1867, Joseph Lister heat sterilized his instruments and used phenol on surgical dressings and on patient wounds to prevent infections.
Influenced by Pasteur’s work : showed that heat and phenol could kill.
Highly successful in preventing infections.
o In 1876, Robert Koch demonstrated that a bacterium caused anthrax.
Injected healthy mice with material from diseased animals. Mice became ill.
Transferred anthrax through a series of 20 mice. They all got sick.
Incubated a piece of spleen from the 20th mouse in beef broth (bacteria grew producing endospores).
Isolated endospores from the bacterial culture were injected into healthy mice (developed anthrax).
o This lead to Koch’s Postulates:
The microorganism must be present in every case of the disease but absent from healthy organisms.
The suspected microorganism must be isolated and grown in a pure culture.
The same disease must result when the isolated microorganism is inoculated into a healthy host.
The same microorganism must be isolated again from the diseased host.
o Development of techniques to study pathogens
Robert Koch wanted to isolate suspected pathogens.
• Difficult to work with liquid (mixed) cultures.
• Grow bacteria on a solid surface (clonal cultures leading to colonies).
• He tried on slices of boiled potatoes (bad growth).
• He tried on solidified culture media (media + gelatin).
o Gelatin is degraded by microorganisms.
o Gelatin melts at 37, optimal growth temperature.
• Fannie Hesse (Koch’s assistant’s wife) suggested agar (seaweed powder).
• Richard Petri (Koch’s assistant) developed the Petri dish.
• To culture and isolate human pathogens, Koch developed media similar to body fluids (meat extracts and protein digests).
• Using these techniques and media Koch isolated tuberculosis, numerous other human pathogens were rapidly isolated in different laboratories.
• Beginnings of immunological studies
o Edward Jenner immunized people against smallpox using the cowpox virus (fluid from cowpox blisters).
He heard that dairymaids were protected from smallpox.
o Louis Pasteur created the first attenuated vaccine.
Grew pure culture of pathogens (anthrax, Bacillus anthracis).
Attenuate them; 42-43 degrees with potassium bichromate.
They did not cause disease but conferred immunity.
o DE Salmon and Theobald Smith found that killed microbial cells were effective as vaccines.
o Emil von Behring discovered humoral immunity. Antibodies could be produced in blood against the diphtheria toxin and could neutralize the toxin and be protective against infection.
o Elie Metchnikoff discovered cellular immunity: phagocytes could engulf disease-causing bacteria.
o Rabies vaccine and the Pasteur Institute in Paris
Attenuated by growth in an abnormal host
• H Brains and spinal cords from dead rabbits were dried.
• Joseph Meister had been bitten by a rabid dog. He received 13 injections from Pasteur over a 10-day period. The boy survived.
• Industrial microbiology and microbial ecology
o Pasteur discovered that microorganisms are responsible for fermentation (chemical instability of sugar)
Beet sugar to alcohol produced less alcohol and sour taste.
Yeast was being replaced by lactic acid bacteria.
o Sergey Winogradsky oxidated iron, sulfur and ammonia by soil bacteria to obtain energy; transformation of carbon dioxide to organic matter.
o Beijerinck isolated nitrogen-fixing bacteria and sulfate-reducing bacteria.
o Both Wino and Beij introduced the enrichment-culture technique and use of selective media.
Classifying organisms : Phylogeny vs Taxonomy
• Taxonomy
o Artificial classification of organisms.
o Solely based on visible similarities.
o Still used to name organisms.
• Phylogeny
o Natural classification of organisms.
o Reflects evolutionary relatedness between organisms.
Phylogeny (evolutionary relationship between organisms)
• Before the concept of evolution, organisms were grouped based on morphological similarities, no relationship between the groups.
• Many fossils of animals and plants were found and used to suggest the appearance of different groops, built evolutionary family trees.
• Microorganisms were left out until late 60’s.
o No microbial fossils.
o Very similar shaped.
• Use ubiquitous gene sequence to compare m-o and construct a universal phylogenetic tree of life.
Evolution of early schemes for classifying microbes
• Tradition early schemes (before 1866)
o Plants and animals
• Ernest Haeckel’s proposal (1866)
o Plants, animals, microorganisms
• Edouard Chatton (1937)
o Eucaryotes and procaryotes
• Roger Stainer and CB …
o
• Five-kingdom scheme of Robert Whittaker (1959)
o Monera, Protista, Fungi, Plantae and Animalia
• Three-kingdom of Carle Woese and colleagues (1977). They used a molecular approach (16S ribosomal RNA gene was found in every organism).
o Bacteria, Archaea and eucaryotes
o These three groups are distant.
o Length of branches depends on the distance between two species.
Endosymbiotic theory of evolution
• Procaryotes do not contain organelles (mitochondria, chloroplasts);
• Almost all eucaryotes have organelles.
o Plants have both mito and chloro;
o All other eucaryotes only have mito.
• Mitochondria and chloroplasts resemble prokaryotes
o Approximately the same size (1*10^-6 m)
o The only organelles containing DNA (active genome)
o Mitochondrial and chloroplast genomes are circular.
o They contain 70S ribosomes (80S in eucaryotes)
o Double membranes resemble cytoplasmic and outer membranes of gram-negative bacteria. However they do not have the cell envelope.
o They multiply and divide by binary fission.
• Lynn Margulis proposed in 1981 the endosymbiotic theory:A primitive eukaryotic cell engulfed an ancient prokaryote to create the first eukaryotic cell.
o First prokaryote appeared 3.5 billion years ago
o First eukaryote appeared only 1 billion years ago
o The two cells continued to evolve in a mutually beneficial symbiotic relationship for 1 billion years.
o The prokaryotic cell benefited from having a sheltered environment rich in nutrients.
o The eukaryotic cell benefited from containing an organism that produced energy (ATP) by respiration (mitochondria) or by photosynthesis (chloroplast).
o Mitochondria are the descendants of an alpha proteobacteria (gram-negative, Rickettsia prowazekii)
RP is an obligate intracellular parasite.
o Chloroplasts are the descendants of a cyanobacteria (prochloron)
• More recent endosymbiosis (an evolutionary snap shot)
o Eucaryotes phagocyte prokaryotic cells.
o Need time to co-evolve and create a stable new organism.
• Aphids and related insects: a recent endosybiotic relationship (200 million years)
o The endosymbiont still has its gram-negative double membrane and cell envelope, phylogenetically closely related to E.coli
o Stable endosymbiotic event
The endosymbiont can no longer grow outside its host
Aphids are also dependant of their endosymbiont, they die if the endosymbiont is killed (antibiotic)
Building a phylogenetic tree
• Get the sequence of an ubiquitous gene (like the 16S ribosomal RNA gene) for all the organisms to be included in the phylogenetic tree.
• Align the gene sequences using a sequence alignment program.
• Feed the aligned sequences to a phylogenetic algorithm.
o Find which sequences share the highest level of homology and link closest neighbors.
o Create phylogenetic tree, branching the organisms related to the similarity of their gene sequence.
o The length depends on the evolutionary distance.
Bacterial taxonomy
• Science of taxonomy or systematic includes
o Classification (biological class.)
o Nomenclature (how to n ame organisms)
o Identification (visible characteristics)
• Goals, stability, predictability, build larger groups, study one member and learn about group
The hierarchical scheme of classification
• 18th century, Carolus Linnaeus, Linnaeus scheme of classification.
• Based on the comparison of visible characteristics.
• Darwin…
o Kingdom or domain
o Phylum or division
o Class
o Order
o Family
o Genus
o Species (basic taxon)
How bacteria are named.
Basic taxon in bacterial taxonomy is the species. The names are binomial and are a binary combination of Latin words. They consist of a genus name and a single epithet. The genus name has an initial capital letter. Species epithet has initial lower case. Species name is always italicized. They tell us
about the organism (appearance, habitat, characteristic property, scientist name).
Concept of a species in bacteriology.
• A collection of strains that share many stable properties and differ significantly from other groups of strains.
• An assemblage of microorganisms so alike with no useful distinction drawn between them.
• Divided into strains (clones) (genetically different).
• Biovars (biochemical or physiological differences)
• Morphovars (different morphology)
• Serovars (different antigenic properties)
Numerical taxonomy (bacteria)
• Equal weight to all compared characters (different from eukaryotes)
• Sj = similarity coefficient (Jacquard) Number of similarities shared / Number of similarities compared.
• Sj are used to create dendograms.
Traditional characters
• Morphology
o Shape (round, elongated, single, chains, clusters)
o Flagella (presence, location, number)
o Gram-stain (+/-)
• Biochemistry and physiology
o What supports growth
Aerobic or not
Optimal temperature
Optimal pH
Salt concentration
Carbon source
Fermentation properties
o Serology
Antibodies against specific bacteria (Antisera)
o Phage typing
What bacteriophages can infect
Bacteriophages have host range (infect only a limited number of bacterial cells)
• Genomic characteristics
o Indirect methods
G + C content (% G + C) GC are nucleotides
• Melting point (more GC, temp. higher)
• Density (proportional to GC content)
DNA hybridization
• Total genome hybridization
• Probe hybridization (target a specific gene)
o Direct methods
Sequencing genes (rRNA, protein coding), give phylogenetic relationship, real evolutionary links)
o
•
Chapter 3 : Procaryotic cell structure
Major differences between prokaryotic and eukaryotic cells
• Eukaryotic cells are more complex. They contain numerous organelles (only chloroplasts and mitochondria have their own DNA).
• In eukaryotes, the genome (DNA) is contained in a membrane-bound structure called the nucleus (nuclear membrane).
• Prokaryotic cells are surrounded by a cell wall of peptidoglycan.
• Eukaryotic cells are approximately 10x larger.
Shape and arrangements of cells
• Spherical or ellipsoidal cells (cocci/coccus)
• Cylindrical or rodlike (bacilli/bacillus)
• Helical or spiral cells (spirilla/spirillum)
• Others… ( square, star, etc.)
Schematic on powerpoint Different types of coccus, bacillus, etc. Diplobacillus are side by side (not one on top of the other).
• Even if quite smaller than eukaryotes, prokaryotic cells have a higher surface/volume ratio, allowing them to grow much faster.
Some exceptions :
• Giant bacteria
o Epulopiscium fishelsoni
Name means “guest at a fish’s banquet”
Found in the gut of the brown surgeon fish
First identified as a protozoon because of its size
1 000 000 times bigger than E.coli
o Thiomargarita namibiensis
Name means “sulfur pearl of Namibia”
Found in ocean sediments (coast of Namibia)
100 times larger than E. fishelsoni
Size of a period at the end of a sentence
• Tiny eukaryote
o Nanochlorum eukaryotum
10 times smaller than a human cell
In the size range of a typical bacterial cell
True eukaryote
• Nucleus
• Mitochondria, chloroplast
Prokaryotic cell organization
Powerpoint shows the morphology of a prokaryotic cell and the functions of prokaryotic structures.
Bacterial envelopes
1. Gram negative (3 layers)
a. Outer membrane
b. Cell wall
c. Cytoplasmic membrane (inner or plasma)
2. Gram positive ( 2layers)
a. Cell wall
b. Cytoplasmic membrane (inner or plasma)
3. Mycoplasma (1 layer)
a. Cytoplasmic membrane (inner or plasma)
The cytoplasmic (plasma) membrane
• Functions
o Contain the cytoplasm
o Regulate what comes in and goes out the cell using specific transporters
• Composed of a fluid phospholipid bilayer
• Contains proteins (50% dry weight)
o Transmembrane proteins (transporters)
o Peripheral membrane proteins, stuck on either side of the membrane not spanning it.
Hydrophilic polar head (because it is charged) and hydrophobic part because they are saturated??
The peptidoglycan cell wall
• Very thick around G-. Very thin around G+.
• Functions (coat of mail around the cell)
o Gives the cell its shape and rigidity
o Withstand osmotic pressure
Hypotonic (concentration en eau inférieure à l’intérieure)
Hypertonic (concentration en eau supérieure à l’intérieure)
Isotonic (no change in water movement)
Cells in media are always in hypotonic situation (due to the high concentration of ions, chemicals, proteins, nucleic acids)
• Water tends to diffuse inward. The peptidoglycan protects the cell. (refer to Powerpoint)
• Composed of peptidoglycan (murein – autre nom).
o Peptido Peptides / Gylcan polysaccharide
o Glycan : long chains of alternating sugars.
o Peptides : cross links between adjacent chains.
• In gram-positive the cell wall also contains teichoic acid that runs perpendicular to the peptidoglycan chains which solidifies its structure.
• In gram-positive (no outer membrane) the cell wall is anchored to the cytoplasmic membrane by lipoteichoic acid.
• In gram-negative the cell wall is anchored to the outer membrane by lipoproteins.
The outer membrane (gram negative)
• Functions
o Protective barrier (toxic substances, antibiotics)
o Only water and few gases can go through
o Other molecules need to travel through pores (porins)
• Composed of a fluid phospholipid bilayer
o Inner layer is made of phospholipids
o Outer layer is made of lipopolysaccharides (LPS, endotoxins) and some phospholipids.
Lipid A (hydrophobic), polysaccharides (hydrophilic)
• Anchored to the cell wall by lipoproteins
o Protein part linked to the cell wall
o …
Capsule or slime layer (S-layer)
• Secreted outside the envelope (polysaccharides)
• Protection from phagocytes and protozoa
• Protection from desiccation, toxic compounds, ions, pH fluctuations and destructive enzymes.
• Reservoir of stored food (polysaccharides)
• Site for waste disposal
• Helps maintain shape and rigidity
• Prevent infection by bacteriophages or attacks by predacious bacteria
• Aid in cell adhesion and motility (gliding bacteria)
Surface appendages of bacteria
• Flagella (singular – flagellum)
o Movement of bacterial cells (rotate)
o Vary in number and arrangements
o Made of a protein polymer (flagellin) (helical)
• Pili (singular – pilus) or fimbriae (singular – fimbria)
o Hairlike, thinner than flagella (1 000 / cell)
o Adherence to solid surfaces
• Sex pili for mating (DNA transfer)
o Larger than fimbriae (1-10/cell)
o Genetically determined by sex factors or conjugative plasmids
Typical arrangements of bacterial flagella
• Monotrichous : single flagellum at one pole
o Move counterclockwise (cell goes forward), clockwise (cell tumbles)
• Amphitricous : single flagellum at each pole
• Lophotriccus : two or more flagella at one or both poles
o …
• Peritrichous : flagella all over the surface
Bacterial movement (taxis)
• Chemotaxis : towards or away specific chemical agents
o Movement towards or away from a specific chemical.
Towards nutrient source or pheromone, away from detergent
o Movement is directional
o Runs are longer than tumbles, the movement is not completely random
• Aerotaxis : towards regions rich in oxygen
• Phototaxis : towards light
• Magnetotaxis : follow magnetic lines of force
The cytoplasm
• Site of numerous chemical reactions (metabolisms)t
• Composed of water at 90%
• Can be seen by a microscope
o Nucleoid, irregular mass of DNA, circular, no nucleus.
o Ribosomes (produce proteins, large and small subunits made of RNA and proteins)
o Inclusion bodies (storage granues, carbon sulfur, nitrogen, phosphate
o Gaz vacuoles (help the bacteria to float)
Bacterial endospores
• Most resistant biological structures known
o Can resist to very harsh conditions
• Nongrowing, resting structures, no metabolism, survival state
o Survive hundreds, thousands of years (heat dehydration radiation)
o Their formation is induced when growth conditions are unfavorable.
o In favorable conditions they start growing again.
• The formation of endospores is called sporulation
o Reduce water content from 90% to 15%
o Increases concentrations of calcium and dipicolinic acid
Sporulation and germination
• Sporulation (from a vegetative cell to an endospoer)
o Cell divides in 2 unequal parts
o The larger part engulfes the smaller part (forespore)
o Forespore matures to become an endospore
Synthesis of a protective thick wall
• Inner cortex made of peptidoglycan
• Spore coat (keratin-like protein)
Dehydration of the endospore
o ……….
Microbial growth : READ CHAPTER 6 !!!!!!!!!!!!!!
• Cell growth and division
• Population growth
o Exponential growth
o Synchronous growth
o Asynchronous growth
o Generation time
• Mathematical expression of population growth
o The growth curve
Cell growth and division
• Cells get larger and divide cerating two identitcal daughter cells
o Binary fission : divide near the midpoint to form two daughter cells
o Budding: forming a bubble like structure that comes out and eventually ceparates from the parent cell
o Others : fragmentation (filamentous growth), exospores formation.
Population growth
• During active growth, populations of unicellular microorganisms increase exponentially by geometric progression (2^n)
• When timing of cell division is random, asynchronous cultures result and produce at a smooth exponential growth plot.
• When cells divide synchronously, a step wise growth plot can be seen.
Growth rate or doubling time
• Generation time : period required for a cell to enlarge, divide and produce two daughter cells.
o Quite variable between different bacteria
o All cells in a population have the same doubling time, same growth condition for all.
o Varies with the availability of nutrients.
Less nutrients doubling time increases
Accumulation of toxic metabolic compounds doubling time increases
o Generation time determination is easy (look at the slope…)
• Growth curve is easy to look at. Lag phase (adaptation to a new media, synthesis of proteins necessary to survive in this media), exponential phase, stationary phase (nutrients are being absorbed and consumed) and death phase (lack of nutrients).
o Lag phase : time for cells to adjust to fresh or new medium.
o Exponential phase: period where nutrients are not in limited amounts.
o Stationary phase: the culture gradually switches from the log to the stationary phase when nutrient concentration is limiting and toxic waste accumulates, cells divide slower and eventually stop.
o Death phase : nutrients are completely consumed. Cells stop growing and die However, not all cells die.
• Reasons for a longer Lag phase
o Inoculum taken from an old culture.
o Inoculum taken from a rich medium and added to a poorer one
No lag occurs when growing cells are transferred from a poor to a richer medium
o Inoculum taken from a chemically different medium
o Inoculum taken from refrigerated cells
o Cells inoculated into a cold medium
Continuous culture in a chemostat constant flow of nutrient. (old media comes out)If you increase the nutrient concentration, the generation time will decrease, but you’re flushing bacteria. Therefore, the cell density will decrease too even if the generation time goes down.
Conditions used in a chemostat
• Concentration of one of the nutrient should be provided in limiting amounts to reduce growth rate.
• All nutrients should be provided continuously at the same rate to the growing culture.
• Some of the growing culture (cells and waste products) should be removed continuously at the same rate.
Growth rate is then constant and equals loss rate, same amounts of cells actively growing.
Uses of continuous cultures
• Provide a constant supply of cells in balanced growth (exponential phase) and growing at a known growth rate.
• Study microbial growth at variable and very low nutrient concentrations, resemble the environment bacteria encounter in nature.
• Facilitate research in microbial interaction and ecological situations.
• Allow industrial production of food and metabolites continuously.
Parameters affecting microbial growth
• Temperature (OPTIMAL growth temperature)
o Hyperthermophiles (95)
o Thermophiles (65)
o Mesophiles (37)
o Psychotrophs (25)
o Psychrophiles (10)
• Hydrogen ion concentration (pH)
o Neutrophiles (7)
o Acidophiles (below 7)
o Alkalophiles (above 7)
• Gaseous environment
o Aerobes (need O2)
o Anaerobes (absence of O2)
o Facultative (with/without O2)
o Capnophiles (need oxygen and high CO2 concentration)
•
Lecture 10
Temperature, pH, … influence bacterial growth. See lecture content for more information. Some bacteria grow only at a certain pH (acidic environment, basic environment, etc).
Oxygen may produce toxic molecules for microorganisms.
Growth on solid vs liquid media :
• In liquid cultures, all cells are in the same condition, surrounded by the liquid medium.
• On a petri dish, cells in a colony face different conditions.
o Cells in the center are in the stationary phase or death phase.
o Cells on the edge are actively growing (exponential phase).
o Cells at the surface are in aerobic conditions.
o Cells under the surface of the colony are in anaerobic conditions.
Spectrophotometers allow us to know approximately how many cells we have in a liquid culture, by measuring the degree of absorption of light.
Another cell count technique consists in dilutions. See lecture.
Mathematics of cell growth lectuer
Chapter 11 : READ. GENE STRUCTURE REPLICATION AND EXPRESSION
Some basic definitions
• Gene: DNA segments that codes for a protein, ribosomal RNAs or transfer RNAs.
• Genome: all the genes present in a cell or virus.
• Genotype: specific set of genes of an organism.
• Phenotype: observable characteristics.
• Procaryote :bacteria (haploid, one set of genes, 1N)
• Eukaryotic cells : diploid, 2N.
Lecture 11
Nucleic acids : definitions
• Two types of nucleic acids; DNA and RNA
o DeoxyriboNucleic Acid
o RiboNucleic Acid
• Nucleoside polymers (strings of beads)
• Nucleosides are joined by phosphate groups.
• Nucleosides are made of a sugar ring and a base.
• Nucleotides : nucleosides with one or more …
•
Related Documents
