bacteria & viruses. by the end of this class you should understand: how life may have arisen...
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Bacteria & Viruses
By the end of this class you should understand:
• How life may have arisen from nonliving materials
• Modern classifications of bacteria and familiar species we encounter every day
• The life cycle of parasitic protists• How viruses work and whether they are alive• How to protect yourself from bacteria and
viruses
Early Life on Earth
• Life first appeared around 3.8 billion years ago, soon after liquid water formed– Not recognizable as modern
life• Fulfilled the basics of life– Reproduced– Took in energy– Probably didn’t maintain
homeostasis very well yet
Nonliving Synthesis• Life is formed from large assemblies
of macromolecules– How could these have been
assembled without life?• There are a number of hypotheses
on how these molecules could have been assembled– Clay deposits that preferentially
collect charged particles and then dried up
– Hydrothermal vents at the ocean floor naturally produce carbon molecules
Protocells• One way or another, these
molecules eventually formed into protocells– Protocells have been studied as a likely
mechanism for a precursor to life• Essentially, fatty acids collect into a
simple membrane, and inside, enzymes made of RNA called ribozymes convert other molecules into RNA– Cell grows as more RNA is absorbed
and splits naturally
Evidence for Protocells
• A lot of evidence exists that this mechanism is possible, but that’s not the same thing as evidence that it actually happened
• The key evidence is protein synthesis– Common to all life– Uses RNA-based process
(ribosomes and tRNA)
Prokaryotes Revisited• Modern bacteria are nothing
like early bacteria• Through natural selection,
only the hardiest and fastest-reproducing bacteria were represented in the next generation– Bacteria can ideally reproduce in
20 minutes– One bacterium in a soup of
nutrients can make a colony of 100 million bacteria in 12 hours
Three Modern Domains of Life
Archaea Bacteria that resemble early life Live in unusual environments
Eubacteria What we think of as bacteria All pathogens are eubacteria
Eukarya All eukaryotes including humans
Archaea
Oldest and least changed bacteria Many live in hostile environments
Extremely acidic Deep ocean vents Boiling hot springs (extreme thermophiles) Salt marshes (extreme halophiles) Oil wells
Others live in more “normal” environments
Eubacteria Most bacteria that we are familiar
with Many are harmless or even helpful
Bacteria that live on our skin and in our gut are called normal flora and they crowd out more dangerous bacteria
Some are dangerous to our health Usually eat our tissues for nutrients Some release dangerous toxins
instead
Classifications of Eubacteria Gram staining
Gram positive: turn purple in a Gram stain• Has a cell membrane and a crunchy cell wall
Gram negative: stay pink in a Gram stain• Has two cell membranes with a wall between
Another Classification Aerobic
– Aerobic bacteria can survive in air– Many bacteria on the skin– Mitochondria are related to early aerobic
bacteria Anaerobic bacteria
– Die when exposed to air– Lack catalase and peroxidase enzymes– Many bacteria in your gut and that spoil food
How do anaerobes get around? Solution: spores! Some bacteria create
spores in harsh conditions
These spores can survive almost anything
Turn back into bacteria when the conditions have improved
Bacteria shapes Coccus – sphere Bacillus – rod Spirillum – spiral Many other
weirder shapes also exist
Often in the bacteria name
Staphylococcus
Gram positive Common on human skin,
usually harmless Some are harmful
Infections Food poisoning
Responsible for “Staph infection”
Streptococcus Gram positive Many also found on human
skin Dangerous strains responsible
for “strep throat” Even more dangerous types
cause Scarlet fever Most dangerous is “flesh-
eating” bacteria Pictures not provided for the faint
of stomach
Escherichia coli “E. coli” Gram negative Common in human digestive tract Helpful for metabolism Some virulent strains exist
“E. coli” outbreaks are really “Rare and dangerous strain of mutated E. coli outbreak”
Often not the bacteria that is harmful but the toxins it produces
Salmonella Gram negative Commonly found in humans
and farm animals Can cause food poisoning in
undercooked poultry products Other species cause typhoid
fever Infect human cells using a
capsule Potentially lethal
Antibiotics Antibiotics are chemicals that
selectively kill bacteria and not eukaryotic cells Ineffective against human, yeast, and
protozoa Effective against certain types of
bacteria only Penicillin was the first antibiotic that
killed all Gram-positive bacteria Ampicillin and Streptomycin kill all
bacteria
Antibiotic resistance Bacteria constantly mutate
and develop new genes Bacteria also pass genes to each
other on plasmids, small rings of DNA
As a result, bacteria may develop a resistance to an antibiotic, so taking antibiotics regularly is generally inadvisable Additionally, antibiotics kill the
normal flora of your intestines and possibly your skin
Better the devil you know... MRSA (methicillin-
resistant Staph aureus) is very dangerous
Can have many resistance plasmids Becoming increasingly
common in hospitals Best defense is a healthy
immune system and normal flora
This is a real problem…
Protists
• Protists are eukaryotic single-celled organisms– Amoebas, paramecium, etc.– Many probably resemble early eukaryotes
• I will not be covering most of the protists since there are approximately a hojillion of them, but I will cover Plasmodium, the protist that causes malaria– Because I’m friendly like that!– Most protists are NOT infectious
Malaria Life Cycle
Virus
A virus is a nonliving infectious particle that uses genetic information to reproduce
All viruses reproduce by injecting their genetic molecules into a host cell The genetic molecules (DNA or RNA) force the cell
to make more viruses Viruses have no metabolism of their own and do
not maintain homeostasis or reproduce independently
Parts of a Virus All viruses have:
Protein coat Spikes that
target host cell Genetic material
Some viruses have a membrane called an envelope
What about these weirdos? This is a bacteriophage Virus that infects bacteria Useful in scientific research Keeps its DNA in the capsule,
injects it through the tube into the bacteria Bacteria have enzymes that try
to chop up DNA that doesn’t belong, so it’s an ongoing arms race!
What does the genetic info do? Sample: HIV in
human cell Essentially the
DNA or RNA causes the cell to make more viral proteins
The viruses are exocytosed or bud off with envelope until the cell dies
Host specificity The virus spikes can only latch onto certain
receptors This keeps viruses from freely moving
between different species of animals Crossover occurs due to mutation, e.g. “bird flu”
Viruses can only infect certain cells in the host HIV only infects Helper T cells Rhinovirus infects epithelial cells Some viruses (e.g. herpes) infect nerve cells
• These are generally incurable
Antibiotics NOT for Viruses
• Do NOT take antibiotics for viruses– Not only will it do nothing, it exposes you to potential
bacterial infections to go with your viral infection!• Vaccines are available for some viruses– A vaccine is a piece of a bacterium or virus that your
immune system “learns” to kill– Pre-training your immune system so you don’t get the
same disease again• Many viruses and some bacteria mutate too
quickly to have effective vaccines made!
Protip:
• VACCINES DO NOT CAUSE AUTISM• There was a time when vaccines were packaged with
an antiseptic called thimerosal, which is a mercury-containing compound– They are no longer packaged as such
• The original study linking vaccines and autism was discredited, and many scientists have repeated the experiment but found no statistically significant difference
• Autism is a developmental disorder and more prevalent in males, it cannot be caused by an injection at six months of age
See you in lab!
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