Exam #1 Flashcards
Species
A group of organisms capable of interbreeding and producing fertile offspring.
Species: a group of closely related isolates or strains (microorganisms)
Strain/isolate: a subgroup within a species: operational taxonomic unit (OTU)
members of the microbial world
All living things can be classified into one of three groups, or domains
Bacteria
Archaea
Eukarya
Organisms in each domain share certain important properties
Two basic cell structures
Prokaryotes do not have a membrane-bound nucleus
Eukaryotes have a membrane-bound nucleus and organelles
domain Bacteria
Single-celled prokaryotes
= “prenucleus”
no membrane-bound nucleus
no other membrane-bound organelles
DNA in nucleoid
most have specific shapes (rod, spherical, spiral)
rigid cell wall contains peptidoglycan (unique to bacteria)
multiply via binary fission
many move using flagella
domain Archaea
like Bacteria, Archaea are prokaryotic
similar shapes, sizes, and appearances to Bacteria
multiply via binary fission
may move via flagella
rigid cell walls
However, major differences in chemical composition
cell walls lack peptidoglycan
ribosomal RNA sequences different
Many are extremophiles
high salt concentration, temperature
domain Eukarya
eukaryotes = “true nucleus”
membrane-bound nucleus and other organelles
more complex than prokaryotes
microbial members include fungi, algae, protozoa
algae and protozoa also termed protists
some multicellular parasites including helminths (roundworms, tapeworms)
Fungi
diverse group
single-celled (yeasts) or multicellular (molds, mushrooms)
energy from degradation of organic materials
primarily live on land
Algae
diverse group
single-celled or multicellular
photosynthetic
contain chloroplasts with
chlorophyll or other pigments
primarily live in water - rigid cell walls
many have flagella
cell walls, flagella distinct from those of prokaryotes
protozoa
diverse group
single-celled
complex, larger than prokaryotes
most ingest organic compounds as food sources
no rigid cell wall
most motile
Helminths
Parasitic helminths are worms that live at expense of a host
adult stage can be seen without magnification eggs and larvae - microscopic
helminths include roundworms, tapeworms, flukes.
acellular infectious agents
viruses, viroids, prions
not alive
not microorganisms, so general term microbe often used to include them
Viruses
nucleic acid packaged in protein coat
variety of shapes
infect living cells, termed hosts
multiply using host machinery, nutrients
inactive outside of hosts: obligate intracellular parasites
all forms of life can be infected by different types of viruses
Viroids
simpler than viruses
require host cell to replicate
single short piece of RNA
no protective protein coat
cause plant diseases
some scientists speculate they may cause diseases in humans
- no evidence yet
Prions
infectious proteins: misfolded versions of normal cellular proteins found in brain
misfolded version forces normal version to misfold
abnormal proteins bind to form fibrils
cells unable to function
cause several neurodegenerative
diseases in humans, animals
resistant to standard sterilization
procedures
Prion protein
PRNP gene encodes a protein called prion protein (PrP), which is active in the brain and several other tissues.
precise function of protein unknown
proposed roles in several important processes:
-transport of copper into cells
-protects brain cells (neurons) from injury (neuroprotection)
Theory of spontaneous generation
Theory of Spontaneous Generation
“organisms can arise from non-living matter”
Theory had its detractors
Francesco Redi
Louis Pasteur
John Tyndall
… each contributed to disproving the theory…
Francesco Redi
Italian biologist and physician
~1668 – demonstrated that worms found on rotting meat came from eggs of flies landing on meat
-proved this by placing rotting meat in jars:
-left one jar open
-covered one jar with fine gauze
-and another with parchment
Flies could only enter the uncovered jar, and in this, maggots appeared.
In the jar that was covered with gauze, maggots appeared on the gauze but did not survive.
No flies or maggots in the jar covered with parchment.
French chemist Louis Pasteur
Considered father of modern microbiology
~1860’s – demonstrated that air is filled with microorganisms
Proved this by filtering air in cotton plug
->Identified organisms in cotton as same organisms contaminating broths
Pasteur developed swan-necked flask
boiled infusions remained sterile despite opening to air
ended arguments that unheated air or broths contained “vital force” necessary for spontaneous generation
John Tyndall 1850’s
Irish physicist
Tyndall concluded different infusions
required different boiling times
-Some infusions were sterile after boiling five minutes…others not sterile after five hours of boiling
Attributed contamination to a heat-resistant life-form called endospore
Endospores
Bacterial genera that form endospores include Bacillus and Clostridium.
Robert Koch
Robert Koch - supporting the GERM THEORY OF DISEASE
Experimental support for the concept of infectious disease – in 1876:
Koch discovered formation of endospores in Bacillus anthracis
Koch’s work with anthrax notable for
being first to link a specific microorganism with a specific disease
rejecting idea of SPONTANEOUS GENERATION and the MIASMA THEORY
miasma - a noxious form of “bad air” also known as night air
Koch showed that Bacillus anthracis caused anthrax
The bacterium could be observed in the tissue of anthrax victims
He extracted bacterium from sheep which had died of anthrax, grew it , injected a mouse with it …..
The mouse developed the disease as well.
Koch repeated this process in over 20 generations of mice, then he announced in 1876 that he had proved this bacterium caused anthrax.
Anthrax is caused by Bacillus anthracis
Koch’s postulates
The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
The microorganism must be isolated from a diseased organism and grown in pure culture.
The cultured organism should cause disease when introduced into a healthy organism.
The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
golden age of microbiology
As spontaneous generation was disproved,
Golden Age of Microbiology was born
The principle that microorganisms cause diseases is known as Germ Theory of Disease.
most pathogenic bacteria identified (1875–1918)
work on viruses began
understanding that microscopic agents could cause disease led to control efforts
huge improvements in past century in human health
antibiotics to treat infectious diseases
vaccines to prevent diseases
applications of microbiology
biodegradation: degrade PCBs, DDT, trichloroethylene and others
help clean up oil spills
bioremediation: using microorganisms to hasten decay of pollutants
Cellulose
Plant cellulose, which makes up the cell walls of most plants, is tough and mesh-like; cellulose fibrils are primary architectural elements.
Bacterial cellulose has the same molecular formula as plant cellulose, but it has significantly different macromolecular properties and characteristics:
it is more chemically pure
higher water holding capacity
greater tensile strength resulting from more polymerization
ultrafine network architecture.
Bacteria synthesize valuable products
cellulose
hydroxybutyric acid (manufacture of disposable diapers and plastics)
ethanol (biofuel)
hydrogen gas (possible biofuel)
oil (possible biofuel)
insect toxins (insecticides)
antibiotics (treatment of disease)
amino acids (dietary supplements)
Insect toxins
Bacillus thuringiensis is closely related to B. cereus, a soil bacterium, and B. anthracis, cause of anthrax; the three organisms differ mainly in their plasmids. There are several dozen recognized subspecies of Bacillus thuringiensis.
Upon sporulation, B. thuringiensis forms crystals of δ-endotoxins (called crystal proteins or Cry proteins), which are encoded by cry genes. The cry genes are located on a plasmid.
B. thuringiensis serves as an important reservoir of Cry toxins for production of biological insecticides and insect-resistant genetically modified crops.
When insects ingest toxin crystals, their alkaline digestive tracts denature the insoluble crystals, making them soluble, now able to be cut with proteases found in the insect gut, which frees the toxin. The Cry toxin then inserts into the insect gut cell membrane, forming a pore.
microorganisms as model organisms
excellent model organisms
metabolism, genetics similar to higher life-forms
all cells composed of same elements
synthesize structures in similar ways
replicate DNA
degrade foods via metabolic pathways
present and future challenges
Emerging diseases
pathogens can become resistant to antimicrobial medications (tuberculosis, malaria)
increased travel and immigration
many diseases eliminated from developed countries still exist in many parts of world (malaria, cholera, plague, yellow fever)
changes in population
weakened immune systems (elderly, HIV/AIDS)
chronic diseases may be caused by bacteria
-peptic ulcers caused by Helicobacter pylori
Compound light microscope
light is bent through lenses and enables a magnified view
view size, shape and motility of prokaryotes and unicellular eukaryotes
Iris diaphragm
controls the diameter of the light beam
condenser
focuses the light on the sample on your slide
refraction
Refraction is the bending of light as it passes from one medium to another.
air <-> glass: different refractive index
oil<-> glass: same refractive index
More light passing through your sample gives you better resolution.
Resolution
resolution: ability to clearly distinguish two objects that are close together together
light microscope resolution:~ 0.3µm
electron microscope resolution: ~0.3nm
resolution is affected by:
-quality and type of lens
-wavelength of light
-preparation of sample
Calculate magnifying power
I = 340/d
where I is the magnifying power
d is the diameter of the sphere expressed in mm.
For example with a sphere of 1,7 mm of diameter you will obtain about a magnification of 200 X.
Maintaining cell shape
Bacterial cell walls maintain cell shape and rigidity and protect the cells from bursting due to osmotic pressure (turgor pressure).
Bacterial cell walls are composed of peptidoglycan (but Archeal cell walls are not!).
Spheroplasts
Disruption of the cell wall of rod-shaped Bacillis species or Escherichia coli with lysozyme or penicillin results in formation of round, osmotically sensitive cells (SPHEROPLASTS)
after microbe’s cell wall digested, membrane tension causes cell to acquire spherical shape
spheroplasts - osmotically fragile, lyse in hypotonic solution
cytoskeletal elements and cell shape
FtsZ –forms part of ring in the middle of dividing cell required for constriction of cell membrane and cell envelope to yield two daughter cells. FtsZ can polymerise; bacterial homolog of Eukaryotic cytoskeletal component tubulin
MreB - bacterial homolog of the Eukaryotic cytoskeletal component actin
Crescentin is an intermediate-filament-like protein with an essential role in the curved-rod shape of Caulobacter crescentus
dyes
basic dyes (positive charge)
attracted to negatively charged cellular components
acidic dyes (negative charge)
negative staining:
cells repel,
so dye colors background
bacterial stains
simple staining - involves one dye
differential staining - used to distinguish between different groups of bacteria
Gram stain is a differential stain that distinguishes between
Gram-positive bacteria and Gram-negative bacteria
special stains (capsule, endospore, flagella) also useful
Gram stains
Gram stain reflects fundamental difference in cell wall structure
separates most bacteria into two groups:
Gram stain distinguishes between Gram-positive bacteria (purple)
& Gram-negative bacteria (pink)
Steps in gram staining
- CRYSTAL VIOLET (PRIMARY STAIN)
Cells stain purple - IODINE (MORDANT)
Cells remain purple - ALCOHOL (DECOLORIZER)
Gram positive:purple
Gram negative: colorless - SAFRANIN (COUNTERSTAIN)
Gram positive: purple
Gram negative: pink
differential stain: acid fast stain
Mycobacterium genus: cell wall has mycolic acid – waxy fatty acid prevents uptake of dyes then resists decolorization.
tuberculosis
Mycobacterium tuberculosis:
leprosy - chronic infectious disease caused by Mycobacterium leprae, an acid-fast, rod-shaped bacillus. Leprosy curable – treatment in early stages averts disability.
multi-cellular arrangements
Plane refers to the orientation of the septum during division.
If division occurs on one plane a chain is formed (strepto- Greek: twisted chain)
Perpendicular division planes cause packets to form: tetrads and sarcinae
Random division planes causes clusters to form (staphylo- Greek: grapelike cluster)
Prokaryotic cells divide by binary fission
Origin of replication (OriC) is replicated
oriC and newly synthesized DNA move to opposite ends of cell
Plasma membrane and cell wall form septum
(a partition separating two chambers)
2 identical daughter cells are formed
Surface-volume ratio of a cell
Surface area is the area around the cell membrane
Volume is the space inside the cell
The ratio is the surface area divided by the volume
As radius of cell increases 1x to 3x,
surface area increases from 1x to 9x
volume increases from 1x to 27x
As radius of cell increases 1x to 3x, surface area increases from 1x to 9x, volume increases from 1x to 27x
The smaller the cell, the greater the surface-to-volume ratio.
As the cell gets bigger, you have less surface area per unit of volume.
.
typical structures of a prokaryotic cell
cytoplasmic membrane
phospholipid bilayer: essential part of cytoplasmic membrane - separates internal contents of cell from outside environment
embedded integral membrane proteins - communication, transport
cytoplasmic membrane lipids
—>Bacterial, Eukaryotic cell membrane: fatty acids linked to glycerol by ester linkage
Bacteria (and Eukaryotes):
glycerol moiety is ester-linked to glycerol-3-phosphate backbone
Archeal membrane
Archeal cytoplasmic membrane:
Hydrocarbons linked to glycerol by ether linkage
Archaeal cell membranes are chemically different from all other living things, including a “backwards” glycerol molecule and isoprene derivatives in place of fatty acids.
Archea: isoprenoid side chains are ether-linked to an glycerol-1-phosphate moiety
b. monolayer-forming tetra-ether lipids:
glycophospholipid from thermoacidophilic Thermoplasma acidophilum - heat resistant
c. bilayer formed of archaeal diether lipids, found in order Halobacteriales
Membrane proteins
Within phospholipid bilayer
some membrane proteins function as selective gates and/or sensors of environmental conditions
plasma membrane: a selectively permeable barrier
cytoplasmic membrane and energy transformation
electron transport chain
uses energy from electrons to move protons out of cell
creates electrochemical gradient across membrane
energy called proton motive force
harvested to drive cellular processes including
ATP synthesis, some forms of transport, and motility
protons outside
hydroxide ions inside
Proton motive force
ejection of protons creates electro-chemical gradient
used to synthesize ATP, power transporters and flagella
Plasma membrane
a selectively permeable barrier – directed movement through selective gates
Highly specific transport system - carriers transport a certain molecule type.
directed movement of molecules across cytoplasmic membrane
facilitated diffusion: form of passive transport
movement down gradient; no energy required
not typically useful in low-nutrient environments
active transport: requires energy*
movement against gradient
two main mechanisms:
transporters use proton motive force
transporters use ATP (ABC transporter) ATP Binding Cassette
group translocation chemically alter compound phosphorylation common glucose
symport
Green circles moving against their concentration gradient through a transport protein (requires energy)
Yellow circles move down their concentration gradient (releases energy). The movement is coupled.
antiport
Blue circles moving against their concentration gradient through a transport protein (requires energy)
Yellow circles move down their concentration gradient (releases energy).
The movement is coupled.
Antiporters
Many different antiporters support bacterial pH homeostasis, nutrient uptake, motility and ATP synthesis.
Both Escherichia coli and Bacillus subtilis establish a proton motive force, (PMF).
The PMF is used to energize solute transport, motility and ATP synthesis.
ABC transporters
Use the energy of ATP binding and hydrolysis to transport substances across cell membranes.
ABC transporters consist of trans-membrane domains which determine specificity of the transporter, and cytoplasmic ATP-binding domains.
Can export and import substances across the cytoplasmic membrane.
Importers also have a high-affinity binding protein that recognizes the substrate in the periplasm and delivers it to the transporter.
Prokaryotic cell structure
Cell membranes
Cell wall
peptidoglycan
Outer structures:
Capsule
Flagella and bacterial motility
Pili
Internal structures
Nucleoid
Plasmids
Ribosomes
Storage granules
Endospores
Cell wall-peptidoglycan
Only bacteria have a peptidoglycan cell wall
peptidoglycan structure
Lysozyme hydrolyses the glycosidic bonds that link NAM and NAG
peptidoglycan structure
inhibition of cell wall biosynthesis by antibiotics
Penicillin inhibits formation of crosslinks in peptidogycan wall
binds enzyme: transpeptidase (penicillin binding protein)
transpeptidase forms tetra-peptide crosslinks between adjacent glycan chains
Penicillin becomes covalently linked to the enzyme’s active site - inhibits it, irreversibly
Why does penicillin kill only actively multiplying cells, while lysozyme kills cells in any stage of growth?
Penicillin will kill only cells that are actively synthesizing peptidoglycan (cells that are growing) because it interferes with peptidoglycan synthesis.
Lysozyme breaks the bonds that join the subunits, thereby weakening the existing structure.
Gram-positive cell wall
Gram-positive cells
thick (20-80 nm) cell wall peptidoglycan layer outside plasma membrane
Gram-positive cell walls contain
teichoic acids
negatively charged; give Gram-positive bacteria a negative exterior charge
PAMP – recognition by immune system cells
lipopolysaccharide (LPS)
structure and function
LPS consists of:
O side chain - vary sugar composition in response to antibodies
lipid A portion (part of outer membrane lipid bilayer)
LPS slows entry of antibiotics and other toxins
Lipid A
toxic (endotoxin) to humans - part of LPS molecule recognized by our host defenses.
When large amounts accumulate (such as in a bloodstream infection), response by defense system itself can be deadly.
Cell wall in archaea
Archaea have several different types of cell wall. Some contain a structure reminiscent of
peptidoglycan called pseudomurein. Other microbes will have a surface layer (S-layer) composed of repeating units
of one or a few proteins, glycoproteins or sugar. These crystal lattices serve to protect the cell.
S-layers
S-layers in Archaea: glycoprotein lattices : wall component composed of subunits with pillar-like, hydrophobic trans-membrane domains, or lipid-modified glycoprotein subunits.
Some Archaea have a rigid wall layer (pseudomurein in methanogens) as intermediate layer between plasma membrane and S-layer.
In Gram-positive bacteria S-layer proteins are bound to rigid peptidoglycan-containing layer via secondary cell wall polymers.
In Gram-negative bacteria S-layer closely associated with lipopolysaccharide of outer membrane.
capsules and slime layers
Bacteria with capsules attaching to intestinal cells(TEM)
Bactria adhering to each other in a layer of slime (SEM)
glycocalyx: extracellular polymer of glycoprotein (polysaccharide)
protective outer layer
not all bacteria have one
if thick and sturdy, a capsule.
if thin and diffuse, a slime layer
Capsules
capsule considered virulence factor - enhance ability of pathogenic bacteria to
evade phagocytosis
attach to surfaces
be protected from toxins, detergents, bacteriophages
Example: Streptococcus pneumoniae capsule
assembly of capsule - steps in Streptococcus pneumoniae capsule biosynthesis
glycosyltransferases assemble oligosaccharide repeats on cytoplasmic face of membrane
Wzx flippase transports repeat units to external surface of membrane.
repeat units polymerized by Wzy capsular polymerase to form high-molecular-weight capsular polysaccharides, which are then ligated to cell wall
Flagella
MONOTRICHOUS: single flagellum at one end
Example: Caulobacter crescentus Vibrio cholerae, Pseudomonas aeruginosa, Isiomarina loihiensis
LOPHOTRICHOUS: flagella lined up at one end
Example: Vibrio fischeri, Helicobacter pylori
PERITRICHOUS: flagella are distributed all over the cell E. coli, Example: Salmonella typhimurium
SPIROCHETES: specialized flagella inside periplasm causes corkscrew motion
Example: Borerelia, Treponema and Leptospira
bacterial flagella are powered by the proton motive force, but Archea use ATP for energy
bacterial chemotaxis
A cell moves via a series of runs and tumbles.
The cell moves randomly when there is no concentration gradient of attractant or repellent.
When a cell senses it is moving toward an attractant, it tumbles (T) less frequently, resulting in longer runs (R).
What mechanism causes a cell to tumble?
flagellum bacterial propulsion-
Driven by a transmembrane proton gradient, rotates both CCW and CW
filament is helical and converts torque into thrust. The motor consists of stators or Mot complexes and a rotor or C ring, which also serves as the CCW⇄CW switch.
Archaea flagellin
Archaeal flagellins possess a highly conserved hydrophobic N-terminal sequence that is similar to that of type IV pilins and clearly unlike that of bacterial flagellins.
The Archaellum is a rotating Type IV pilus
After the pre-archaellin has been processed, the motor complex assembles the filament. The motor complex is formed by the ring-forming scaffold protein FlaX
The dimeric soluble domain of FlaF interacts with the S-layer.
protein appendages
pilli, fimbriae and adhesins
overview of type IV pili system
dynamic adhesive structures, major virulence determinants in several human pathogens
pilus fiber composed of pilin subunits made by prepilin peptidase
cleaved by prepilin peptidase PilD
for proper pilus assembly and function
pilin translocated across inner membrane where it forms dynamic multimeric filament
secreted via pore-forming secretin PilQ to bacterial surface
PilC - transmembrane protein on inner membrane
ATPases (PilB and PilT) mediate pilus extension and retraction
horizontal gene transfer by transformation of exogenous DNA.
DNA in the environment can be entangled by retracting type IV pili and introduced into the cell through the outer (OM) and inner membranes (IM). In the cytoplasm, the incoming DNA is integrated into the genome by homologous recombination
Pilus
structure used by bacteria during conjugation (direct contact)
transfer of genetic material between a donor and a recipient cell
Plasmid DNA forms a mating bridge
Pili works in attaching to naked DNA, other cells, and cellulose (to move around).
Pili also retract and extend
adhesins
cell-surface components on pili that facilitate adhesion to other surfaces
Nucleoid
The nucleoid is a chromatin-dense area within the cytoplasm and contains the bacterial DNA, associated proteins and RNA that are responsible for controlling the bacteria’s activity and reproduction
Binary fission
The bacterial chromosome is packed tightly
Although there are no histones in prokaryotes, other bacterial proteins condense the bacterial chromosome within the cell.
Ribosomes
Site of protein synthesis
Prokaryotic ribosome: 70S ribosome composed of 30S and 50S subunits
Eukaryotic ribosome: 80S - composed of 40S and 60S subunits
Ribosomes in translation
consist of RNA and protein
differences between bacterial and eukaryotic ribosomes exploited to create antibiotics that specifically target bacterial ribosomes
similarities and differences between DNA sequences that encode small subunit of ribosomal RNA are used to identify organisms and to create phylogenies
16S rRNA genes are sequenced and compared in Bacteria and Archea
18S rRNA genes are sequenced and compared in Eukaryotes.
Storage granules
accumulations of polymers synthesized from a nutrient a cell has in excess.
Endospores
dormant cell type formed by species of Bacillus and Clostridium
(anthrax, botulism, food poisoning)
resist: high temperatures (including boiling), most disinfectants, low energy radiation, desication, UV light
can survive many years until an environmental stimulus triggers germination
germinating endospores exit dormant stage to become typical multiplying cell (vegetative cell).
Antibiotics
attack essential molecular
machines in bacteria, stopping
or slowing their action, ultimately
slowing growth or killing the cell.
IN CELL WALL
BETA-LACTAM ANTIBIOTICS
such as penicillin and
methicillin, contain an extremely
reactive beta-lactam ring that
attacks BPs (penicillin-binding
proteins) that build the cell wall.
VANCOMYCIN
sequesters the building
blocks of the cell wall so
that they can no longer
be crosslinked to form a
tough protective layer.
IN CYTOPLASM
MACROLIDES and
AMINOGLYCOSIDES
attack ribosomes, blocking
manufacture of new proteins.
FUSIDIC ACID glues
elongation factor G
(EF-G) to ribosomes,
stalling protein synthesis.
RIFAMPICIN, QUINOLINES
and ANTIFOLATES attack
essential enzymes
in bacteria.
endocytosis in Eukaryotic cells
Phagocytosis: pseudopodium is made when cell membrane pinches to trap a solid particle and a phagosome is made (food vacuole)
Pinocytosis: membrane pinches to make a vesicle that traps extracellular fluid
Receptor mediated endocytosis: receptors trap particles and a coated vesicle is made
Endosymbiont theory
Endosymbiont theory
states that the ancestors of mitochondria as well as chloroplasts were bacteria residing within other cells in a mutually beneficial partnership.
The intracellular bacterium in such a partnership is called an endosymbiont. As time went on, the endosymbiont lost key features sucha s a cell wall amd the ability to replicate independently.
Mitochondria multiply by elongating and then dividing (binary fission).
Plastids (bacteria) also became plants (eukaryotes)
Phagocytosis
the process by which a cell engulfs a solid particle to form an internal vesicle (phagosome)
phagocytosis - specific form of endocytosis involving vesicular internalization of bacteria
In immune system, a major mechanism used to remove pathogens.
prokaryotic cells divide by binary fission
oriC is the region where replication is initiated
septum is the division site - division is initiated by assembly of the tubulin homologue FtsZ into a membrane‐tethered ring‐like structure
- Origin of replication (OriC) is replicated
- oriC and newly synthesized DNA move to opposite ends of cell
- Plasma membrane and cell wall form septum
(a partition separating two chambers) - 2 identical daughter cells are formed
Bacillus subtilis has two alternative life cycles
a | The vegetative life cycle. In favorable conditions Bacillus subtilis elongates, replicates its chromosome and divides by binary fission.
b | Bacillus subtilis can develop a highly resistant, dormant cell to survive harsh environmental conditions.
When conditions improve the endospore germinates and B. subtilis re-enters vegetative life cycle.
Growth
exponential growth: population doubles each division
generation time - time it takes for population to double
varies among species
environmental conditions
growth can be calculated:
Nt = N0 x 2n
Nt = number of cells in population at time t
N0 = initial number of cells
n = number of generations at that point
example: pathogen in potato salad at picnic in sun
assume 10 cells with 20 minute generation time
N0 = 10 cells in original population
n = 12 (3 divisions per hour for 4 hours)
Nt = N0 x 2n = 10 x 212
Nt = 10 x 4,096
Nt = 40,960 cells of pathogen in 4 hours!
Power of exponential growth
rapid generation time with optimal conditions can yield huge populations quickly
remember that generation time depends on species and growth conditions
Pure culture
pure culture is defined as a population of cells derived from a single cell
pure culture obtained using aseptic technique
minimizes potential contamination
cells grown on culture medium
contains nutrients dissolved in water
broth (liquid) or solid gel
