Transcript Why Don't Antibiotics Work on Viruses?

• Summer 2008 Workshop
• in Biology and Multimedia
• for High School Teachers
Bacterial vs. Viral Infections
Why don’t antibiotics work for viral infections?
MicrobeLibrary.org; © Jean-Yves Sgro, University of Wisconsin
Polio Virus
http://www.microbelibrary.org/microbelibrary/files/ccImages/Articleimages/simonson/Images/Streptococcus%20sobrinus%20fig1.jpg
Streptococcus
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
College.
Can you think of some illnesses caused by viruses or bacteria?
Bacterial Infections
Viral Infections
• strep throat
• the flu
• gastroenteritis
• colds
• cholera
• AIDS
• tuberculosis
• hepatitis
• food poisoning
• chicken pox
• botulism
• gastroenteritis
• gangrene
• measles
• necrotizing fasciitis
• mumps
• boils, abscesses
• E. Bola
• pneumonia
• pneumonia
• acne
• West Nile
• meningitis
• cervical cancer
• ulcers
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
College.
Sample Bacterial Illness: Strep Throat
en.wikipedia.org/wiki/Strep_throat
http://www.microbelibrary.org/microbelibrary/files/ccImages/Articleimages/simonson/Images/Streptococcus%20sobrinus%20fig1.jpg
• infectious agent: Streptococcus pyogenes
- spherical bacteria usually found in pairs or chains
• most sore throats are actually caused by viruses and are
NOT considered strep throat (~15-35% are strep throat)
• symptoms: sudden, severe sore throat, fever over 101°F,
swollen tonsils and lymph nodes, white or yellow spots on the
back of a bright red throat
• transmission usually by air-born bacteria
• incubation period prior to symptoms 2-5 days
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Sample Bacterial Illness: Strep Throat
en.wikipedia.org/wiki/Strep_throat
http://www.microbelibrary.org/microbelibrary/files/ccImages/Articleimages/simonson/Images/Streptococcus%20sobrinus%20fig1.jpg
• diagnosed with a throat culture and detection of strepspecific molecules
• treated with antibiotics, usually penicillin for 10 days
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Viral Illness: The Common Cold
http://en.wikipedia.org/wiki/Image:Rhinovirus.PNG
• symptoms: sore throat, runny nose, nasal congestion, cough
• prevalance: 3 colds/ person/ year
• incubation period 2-5 days
• no cure for the common cold (or the flu)
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Should you ask for an antibiotic when you have a cold or the flu?
To understand why antibiotics DO NOT work for viruses,
you should understand:
• how antibiotics work
• basic structural properties of bacteria and viruses
• basic biosynthetic pathways of bacteria and viruses
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Structure of a Bacterial Cell
Cell Wall
-maintains cell structure
-composed of peptidoglycan, a
polymer of sugars and amino acids
Flagella
-tail-like structure used for locomotion
http://en.wikipedia.org/wiki/Image:Average_prokaryote_cell-_en.svg
Plasma Membrane
-phospholipid bilayer surrounding cell
-contains proteins that play a role in
transport of ions, nutrients, and
wastes
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Structure of a Bacterial Cell
Nucleoid
-region DNA is found in prokaryotes
DNA
-single double-stranded circular
chromosome
-no histone proteins
Plasmid
-small circular chromosome
-may carry an antibiotic resistance
gene
http://en.wikipedia.org/wiki/Image:Average_prokaryote_cell-_en.svg
Ribosomes
-site of protein synthesis (translation)
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Some Features of an Infectious Bacteria
virulence factors: molecules produced by a
pathogen that aid in its survival in a host
• capsule: surrounds bacterial cell wall;
protects bacteria from phagocytosis
• pili: allows bacteria to attach to and
invade other cells despite mucous and cellturnover
http://en.wikipedia.org/wiki/Image:Average_prokaryote_cell-_en.svg
• enzymes: break down matrix between cells
allowing bacteria to spread throughout tissues
Toxins
• exotoxins: cause lysis of specific host cells
• enterotoxins: cause secretion of fluid into the small intestine leading to
vomiting and diarrhea
• endotoxins: cell-bound lipopolysaccharides; causes fever and
inflammation
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Bacterial Transcription and Translation
- occur independent of host cell
• transcription occurs using a single RNA polymerase
• transcription occurs in bacteria independent of a host
cell
• translation occurs using a bacterial ribosome consisting
of a large and small subunit
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Bacterial Reproduction
-can occur independent of host cell
-bacterial chromosome replicates
-cell growth occurs (includes cell wall synthesis)
-cell elongates (includes cell wall synthesis) and the two
chromosomes segregate
- ring forms of FtsZ fibers and closes dividing the
cytoplasm in two
http://en.wikipedia.org/wiki/Image:Binary_fission.png
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Properties of Viruses
Basic Structure
• single stranded OR double
stranded RNA or DNA
• protein shell capsid
• some have a lipid bilayer with
embedded proteins
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard College.
Nucleic acid
Protein Capsid
Lipid
Membrane
Adapted from: http://en.wikipedia.org/wiki/Image:Virus_Replication.svg#file
Influenza
http://microbewiki.kenyon.edu/index.php/Image:8430_lores.jpg
Host
Cell
Properties of Viruses
Virus Life Cycle
Virion
• virus attaches to host cell
• virus enters cell via
endocytosis
• capsid degraded
• host transcribes viral DNA
• host ribosomes translate viral
RNA
Host Cell
Nucleus
Host Cell
Ribosomes
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Host cell
Golgi
• new viruses assemble
Host
Cell
Adapted from: http://en.wikipedia.org/wiki/Image:Virus_Replication.svg#file
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
Antibiotic Treatment of Bacterial Infections
• antibiotics kill bacteria or prevent bacteria from dividing
• antibiotics are produced naturally by bacteria and fungi
• antibiotics are mass produced by growing huge cultures of the source
microbe
Why might microbes produce antibiotics in nature?
• to prevent the growth of microbe competitors
Staphyloccocus
aureus
antibiotic
http://en.wikipedia.org/wiki/Image:Staphylococcus_aureus_%28AB_Test%29.jpg
Life Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard College.
Antibiotics: Mechanisms of Action
Inhibition of Bacterial Protein Synthesis
- some antibiotics bind to the large or small subunit of the
bacterial ribosome
Examples: neomycin,
streptomycin,
azithromycin,
erythromycin,
tetracycline
http://en.wikipedia.org/wiki/Image:Average_prokaryote_cell-_en.svg
ife Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard
College.
Antibiotics: Mechanisms of Action
Inhibition of Cell Wall Synthesis
-some antibiotics prevent peptidoglycan formation
Examples: vancomycin,
amoxicillin, ampicillin,
penicillin
e Sciences-HHMI Outreach. Copyright 2008 President and Fellows of Harvard College.
http://en.wikipedia.org/wiki/Image:Average_prokaryote_cell-_en.svg