Bacteria & Viruses

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!