Viruses and Bacteria

The Difference in Definition• BacteriaBacteria: Prokaryotic Organisms

– Pro: Primitive or “prior to”– Karyon: Nucleus or “kernel”– Single-celled organisms – Has circular DNA; often has “plasmid” DNA that

helps codes for genes to increase fitness (ex. Antibiotic resistance)

• VirusesViruses: Submicroscopic, parasitic, acellular entity composed of a nucleic acid core surrounded by a protein coat.– Below the resolution of a microscope– Relies on a host– Does not have the properties of cellular life

Prokaryo

Submicroscopic parasitic acellular

The Difference in Size

• Bacteria can be measured in micrometers– 0.000001m or 10-6

• Viruses are measured in nanometers– 0.000000001m or 10-9

Comparing the size of a virus, a bacterium, and an animal cell

0.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus

Bacteria

Two main “domains” or groups1. Bacteria

Cell walls with peptidoglycan

2. ArchaebacteriaCell walls lack peptidoglycanAdapted to extreme environments:

- Extremely hot and cold, salty, without oxygen, etc.

peptidoglycan

Made up of types of peptide and sugar bonds

Bacteria: Shapes

• Three basic shapes:– Rod-shaped (Bacilli)

Bacillus anthracis (Anthrax), Yersinia pestis (Bubonic plague)

- Comma-shaped (Vibrios)

Vibrio cholerae– Spherical (Cocci)

Streptococcus, Staphylococcus– Spiral (Spirilla)

Treponema pallidum (Syphillis)

Bacterial Staining• Gram-positive: Retains the crystals of violet dye

in the peptidoglycan layer• Infection by this type can be treated by

antibiotics such as penicillin

• Gram-negative: Will not pick up the violet dye• Infection by this type must be treated by a broad-spectrum antibiotic

such as ciprofloxacin

Bacterial Staining

peptidoglycan

Bacterial Growth and Reproduction• Binary Fission: (video)

Asexual divisionDNA replicates and cytoplasm divides

• Conjugation (video) “Sexual” reproduction

Sex Pilus extends between bacteria plasmid DNA is transferred from one bacterium to another

• Spore Formation:occurs when growth conditions are unfavorableAn endospore is a “spore” with a thick internal wall of membrane that encloses and protects its DNA

Viral Shapes and structure

18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm

20 nm 50 nm 50 nm 50 nm

(a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

RNA

RNACapsomereof capsid

DNACapsomere

Glycoprotein Glycoprotein

Membranousenvelope

CapsidDNA

Head

Tail fiber

Tail sheath

Viruses Reproduction

Viruses reproduce by infecting other cells.

Two types of viral infections:

1. Lytic Infection

2. Lysogenic Infection

Figure 18.1 T4 bacteriophage infecting an E. coli cell

0.5 m

The Lytic Infection

Step 1: Attachment of virus to host cell

Step 2: Injection of viral DNA into cell

Step 3: Replication of viral DNA and Synthesis of Protein Capsule using cellular “machinery” –DNA and RNA polymerases, ribosomes, etc.

Step 4: Assembly of new viruses inside host cell

Step 5: New viruses “lyse” the host cell and are released for further infection

Characteristics of Lytic Infections

1. Fast acting

2. Symptoms emerge within 24 – 48 hours

3. Examples – influenza, west-nile

The Lysogenic InfectionStep 1: Virus attaches and inserts its DNA inside host

Step 2: Viral DNA attaches to the host DNA (prophage)

Step 3: The viral DNA lies “dormant” and only replicates each time the cell replicates

Step 4: Stress or other “factors” causes the infection to progress to the “lytic” phase

Characteristics of Lysogenic Infections

1. Slow Acting - Viral DNA can lie “dormant” for many years as proghage

2. The host are “symptom-free” during dormancy

3. Infection is fast acting when the infection progresses to the lytic phase

4. Example: HIV