Viruses and Bacteria

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Viruses• Infectious agents

• No cell structure

• Contain DNA or RNA

• Non-living

• Obligate parasites

Properties of Viruses• Can infect all forms of life

• Many infect only one species

Virus Size• Extremely small

• Comparison– Human red blood cell = 10,000 nm– Plasma membrane width 30 nm

Virus Structure• Nucleic acid core

– DNA (double or single stranded)– RNA

Virus Structure• Capsid = protein covering

• Some have envelope over capsid

Tobacco Mosaic Virus

Herpes Virus

Replication of Viruses• Cannot multiply outside cells

• Uses cell organelles to multiply

• Process is called lytic cycle

• Lysogenic cycle– Long term relationship of cell & virus– Viral nucleic acid replicates as cell multiplies

Ways Viruses Cause Disease• Can initiate cancer

• Can take over cell function

Cancer & Viruses• Some viruses initiate cancer

• Virus causes cell changes

• Virus does not “carry” cancer

Viruses as Pathogens

• Disrupt cell functions

• Use cells to make more virus

Viral Infections

• Acute infection– Rapid onset– Run a course– Subside– Examples

• Common cold• Influenza• Measles • Mumps

• Latent Infection– Infective agent present– Not manifesting disease– Causes latent infections– Examples

• Herpes • HIV

Genital Herpes• Common STD

– More than 1 in 5 age 12 & over in U.S.– Caused by herpes simplex virus– Develops a few days after sexual contact– Spreads when virus is active – Lesions develop– Latent infections

Prions & Viroids• Infectious agents

• Simpler than viruses

Prions

• Proteins that cause disease

• Only infectious agent with no nucleic acid

• Examples: – Bovine spongiform encephalopathy

• mad cow disease

– Creutzfeldt-Jacob disease

Healthy Brain Tissue Prion-infected Brain Tissue

Viroids• Small strands of RNA

• No protein coats

• Replicated in host cells

• Known to cause plant diseases

Bacteria• Prokaryotic cells

• Some harmful, some helpful

• Present virtually everywhere

Characters of Bacteria• No membrane-bound nucleus

• No membrane-bound organelles

Modes of Nutrition in Bacteria

• Photoautotrophic

• Chemoautotrophic

• Hetrotrophic

Photoautotrophs

• Produce food via photosynthesis

• Green & purple bacteria, cyanobacteria

• Cyanobacteria probably oxygenated Earth’s atmosphere

Chemautotrophs

• Energy from inorganic molecules

• Manufacture growth factors– Carbohydrates – Fats – Proteins – Nucleic acids

• Important in nitrogen cycling

Heterotrophs

• Feed on other organisms

• Play key role in carbon cycling

• Many bacteria are decomposers

• Release CO2 used by photosynthesizers

Archaea

• Prokaryotic cells

• Not bacteria

• None cause disease

• Often in harsh habitats

Reproduction in Bacteria• Asexual binary fission

Bacterial Disease

• Attach to cells

• Cause tissue damage

• Some plant diseases

• Many human diseases

Syphilis

• Caused by spiral bacterium

• Sexually transmitted

• Curable with penicillin

• Can cause death if untreated

Stages of Syphilis

• Primary – Hard chancre sore– Becomes painful

• Secondary – 6 weeks – months– Dispersed thru body– Lesions at other sites

Stages of Syphilis• Latent stage

– No symptoms– May last a lifetime

• Tertiary stage– New lesions – tumor-like– Paralysis and death

Gonorrhea

• Sexually transmitted

• May exhibit few symptoms

• Can cause sterility

Clamydia• Most prevalent STI

• Can lead to sterility

• Grow only within other cells

• Bacteria is “energy parasite”