Viruses and bacteria are simple yet complex infectious agents. Viruses can only replicate inside living cells and are obligate intracellular parasites that contain either DNA or RNA. Bacteria were the first life forms on Earth and are prokaryotic cells that can metabolize and reproduce independently. Both viruses and bacteria can cause infections in humans and other organisms.

1. Viruses and Bacteria

2. Simple Yet Complex
• Virus – collection of genetic material and
proteins
– Causes infections in ALL life forms
• Bacteria – prokaryotic cell
– First “life” on earth

3. What is a Virus?
• Infectious agents, NOT CELLS
• No nucleus, organelles, cytoplasm
• Can replicate ONLY inside a living cell
– Obligate Intracellular Parasites
• Contain only 1 type of genetic molecule,
either DNA OR RNA
• Do not grow
• NOT ALIVE

4. Other Virus Characteristics
• Some viruses have:
– Envelop – lipid rich layer outside protein coat
• Envelop may have proteins that help infect cells
• Ex. HIV, Flu
• Can use presence or absence of envelop to classify
– Tails
– Spikes
– Legs

5. HIV
Chicken Pox
Common
Cold
Influenza

6. Other Virus Characteristics
• Host Range – kind of cells or organisms it can
infect
– Can ONLY infect cell that has target protein on the
outside
– Ex. HIV only infects human helper T cells
– Ex. Rabies can infect any mammal
• Reservoir – Place where viruses exist in
nature, acts as source of virus for host
– Ex. Birds – Bird flu, Mosquitos – yellow fever

7. Are Viruses Alive
• NO
– Does not metabolize
– Does not respond to stimuli
– Does not reproduce on its own
• How is it like things that are alive?
– Has genetic material (DNA or RNA)
– Viruses can evolve
– Classified similarly (Family, Genus, Species)

8. How Do Viruses Replicate?
• Uses host cell to make new viral particles
1. Attachment – attaches to host cell by sticking to
receptor on cell’s surface
2. Penetration – Viral genetic material enters host cell
(engulfing, injecting)
3. Synthesis – Virus produces lots of copies of genetic
material in host cell. Host cell provides all needed
materials
4. Assembly – subunits of capsid join, genetic material
packed
5. Release – Exocytosis, breakdown cell wall, bud from
host particle

9. 1
2
3
4
5
Attachment:
Virus binds cell surface receptor.
Penetration:
Viral nucleic acid is released inside
host cell.
Synthesis:
Host cell manufactures viral nucleic
acids and proteins.
Assembly:
New viruses are assembled from
newly synthesized coat proteins,
enzymes, and nucleic acids.
Release:
New viruses leave the host cell.

10. Does the Host Cell Die?
• Immediate Death
– Lytic Infection – virus enters cell, immediately replicates,
causes host cell to burst
– Newly released viruses infect other cells
• Delayed Death
– Lysogenic Infection – genetic material enters host cell,
replicates WITH host cells chromosomes
– Eventually switch to lytic infection
– Prophage – bacteriophage’s DNA inserted into host cells
DNA
– Only few copies are made that will determine if infection
becomes lytic

12. Severity of Infection
• Cell death causes symptoms
• Amount of cell death determines severity of
symptoms
• Symptoms not around host cells from immune
reaction
• Some viruses stick around for A LONG TIME
– Virus lays dormant as cell divides
– Latent infection – does not produce symptoms
• Herpes type I, HIV, HPV, Epstein-Barr

13. Fighting Off Infections
• Antiviral Drugs
– Interfere with enzymes, genetic information
– HOWEVER – body rarely ever is cured from a virus
b/c drugs kill host cell as well
• Vaccinations – teach body how to fight
infection when they see it again
– Some confer immunity for life (ex. Measles)
– Some have to get every year (ex. Flu)

14. Plant Viruses
• Must penetrate outer waxy leaf layers & thick
cell walls
– Usually spread by leaf eating insects
– Infection starts at site of infection spread by
plasmodesmata, vascular tissue
– Ex. Tobacco mosaic virus

15. Pepper Mild Mottle Virus
Plant Viruses
Tobacco Mosaic Virus

16. Viroids
• Highly wound circle of RNA without protein
coat
– Naked RNA that can infect cell
• Does not encode for protein
• Interferes with plant’s ability to produce
needed proteins
• Transmitted by seeds or pollen

17. Reported Viral Diseases

18. Bacteria: The First LIVE Organisms

19. What is a Prokaryotic Cell?
• Lack true nucleus or other membrane bound
organelles
• Belong to Domains:
– Archaea - Archaebacteria
– Bacteria – Eubacteria
• Parts of Prokaryotic Cell
– Cell membrane
– Cell wall
– Cytoplasm
– Ribosomes
– Granules, vessicles
– External structures – capsule, flagella, pili

20. Bacterial Cell Parts

21. Bacterial Classification
• Size – 0.5 – 2.0µm, surface/volume 3:1
• Shape – can sometimes change during lifetime
– Coccus – spherical
– Bacillus – rod shaped
– Spiral
• Comma – vibrio
• Rigid, wavy – spirilum
• Corkscrew – spirochaete
– Irregular

22. Common Shapes of Bacteria

23. Bacterial Classification
• Arrangement – distinctive arrangements/groups of
cells
– Form when cells divide without separating
• Division in one plane:
– Diplo – pairs of cells
– Strepto – chains of cells
• Division in two planes:
– Tetrads
• Division in three planes
– Sarcinae
• Random Division
– Staphylo

24. Common Arrangements of Bacteria

25. Prokaryotic Cell Wall
• Outside cell membrane
• Rigid but porous
• Functions:
– Maintains shape of cell
– Prevents cell from bursting
• Components
– Peptidoglycan – VERY large molecule, most important
component
– Teichoic Acid – gram positive, extend beyond cell wall
– Outer Membrane – gram negative, bilayer at outermost layer of
cell wall
– Lipopolysaccharide (LPS) – endotoxin, part of outer membrane
released when cell dies
– Periplasmic Space – gap between cell membrane and cell wall,
active area of cell metabolism

26. Gram Negative vs. Gram Positive
Anthracis bacillus
Bacillus subtilis

27. Typical Bacterial Parts
• Cell Membrane – similar to eukaryotic, fluid mosaic
model, semi-permeable
• Cytoplasm – mainly water (80%), other 20% = enzymes,
proteins, carbohydrates, lipids, ions
– Does NOT carryout streaming
• Ribosomes – protein synthesis, can be found in chains
(polyribosomes)
• Nuclear Region – centrally located, NOT bounded by
membrane
– Consists mainly of DNA, little RNA
– Can have 1 or 2 circular chromosomes, some with small
plasmids (small circular DNA molecules)

28. Typical Bacterial Parts
• Internal Membrane System
– Chromatophores – photosynthetic, has pigments,
used to capture light
– Nitrifying bacteria – enzymes used to derive
energy from nitrogen compounds
• Inclusions
– Granules – not bound by membrane, glycogen,
polyphosphate
– Vesicles – vacuoles – bound by membrane, air or
lipid deposits for energy

29. Typical Bacterial Parts
• Endospores – resting stage bacteria, NOT
reproductive, just used to stay alive
– Contain VERY little water
– Resistant to heat, desiccation, acids, bases,
disinfectants, radiation
– Bacteria induced to form endospore when some
nutrient missing

30. Typical Bacterial Parts
• Flagella – thin, helical EXTERNAL structure for
movement
– Chemotaxis – using flagella to move toward or away
from substances
– Phototaxis – using flagella to move toward or away
from light
• Axial Filaments – endoflagella
– Flagella that does not extend beyond cell wall
• Pili – small, hollow projections used to attach
bacteria to surfaces, transfer genetic material

31. Typical Bacterial Parts
• Glycocalyx – any polysaccharide containing
substance external to cell wall
– Capsule – protective structure outside wall, thick,
tightly bound, makes it difficult to destroy bacterial
cell
• Ex. Anthrax – naturally occurring in cattle, only has capsule
when infecting organism
– Slime Layer – outside wall, thin, but less tightly bound
to wall, protects cell from drying, helps bind cells
together and to their environment
• Ex. Slime layers, biofilms, tooth plaques

32. Common Bacterial Diseases