Topic 6: Bacteria & Viruses

What are the basic aseptic techniques?

• set up a bunsen flame nearby — creates convection current

• sterilize all equipment, disinfecting work area — kill any bacteria

• flaming necks of bottles — prevent transferring microorganisms into culture

Why is counting colonies an inaccurate method of determining the original number of bacteria?

colonies may clump together, often there will be lots of colonies so counting may be difficult

describe each stage of bacterial growth

lag — adjusting to the environment

log — rapid binary fission

stationary — nutrients deplete, waste builds

death — microorganisms die due to buildup of toxic waste products and lack of nutrients

name the 3 types of viruses & give an example for each

• DNA virus ex. lambda phage

• RNA virus ex. tobacco mosaic

• RNA retrovirus ex. HIV

describe lambda phage

DNA virus.
bacteriophage that infects E. coil , found in human intestines, can cause food poisoning.
Structure consists of a head, tail and tail fibres, the bacteriophage shape differs from the shapes of viruses that infect humans and mammals.

describe Tobacco Mosaic Virus (TMV)

RNA virus. plant virus that infects the chloroplasts, discoloring the leaves forming a mosaic pattern. it’s spread between plants naturally or through contact from farmers. it also causes the leaves to curl up, reducing their surface area and the leave’s ability to photosynthesis thus reducing the plant’s growth.

describe ebola

RNA virus
causes sever and often fatal fever, gets transferred via bodily fluids

Describe the lysogenic pathway

• Viral DNA inserted into host genome.

• remains Dormant/latent – copied with host DNA during replication.

• May later enter lytic cycle when triggered

Describe the lytic cycle

• Virus injects DNA/RNA into host.

• Host makes viral components.

• Viruses assembled.

• Host bursts (lysis), releasing new viruses.

describe HIV

Structure: RNA + enzymes (RT & integrase) inside a capsid, surrounded by a lipid envelope with attachment protein

Function: recognizes CD4 receptors on T-helper cells and uses its attachment proteins to bind to them. The capsid is released into the cell, where it breaks apart to release the RNA and enzymes. Reverse transcriptase converts the RNA into DNA. Integrase inserts it into the DNA of the T cell.

describe Tuberculosis (TB) bacterial pathogen

TB is caused by a bacteria which infects phagocytes in the lungs. Bacteria lie dormant inside the tubercles. When the immune system becomes weakened, the bacteria becomes active again and slowly destroy the lung tissue.

How to measure bacterial growth by cell count

Microscopic count from a known volume

How to measure bacterial growth by dilution plating

Serial dilutions grown on agar → count colonies → calculate original count.

How to measure bacterial growth by mass

Centrifuge sample, dry + weigh pellet → scale to full volume.

How to measure bacterial growth by turbidity

Spectrophotometer measures cloudiness → indicates cell density