Viruses, Bacteria, and Epidemiology

Part 1: Viruses

Virus Characterization

Viruses lack a cell membrane and are obligate parasitic agents that lack the ability to replicate away from their host cell. A virus consists of either DNA and/or RNA encapsulated within a protective protein coat. Many animal viruses also contain an envelope surrounding the protein coat consisting of host membrane-derived lipids, etc. Viruses vary greatly in size, ranging from a few nanometers to roughly one micrometer. Most viruses appear either polyhedral or helical (rod-like in appearance).


Structure of a enveloped helical virus.


Icosahedral virus. There is a protein coat surrounding the DNA.


The structure of a typical bacteriophage


Antigen-Antibody complex.A parasite that causes disease is called a pathogen. Since viruses are parasites of cells, they have the potential to be pathogens. All living things on this planet are hosts to viral parasites. Throughout history, humans have been plagued with a multitude of diseases caused by viruses, including influenza, encephalitis, rabies, polio, mumps, measles, small pox, AIDS (caused by HIV) and hemorrhagic fever (caused by EBOLA and Marburg virus).

The Immune System

The body’s main defense against viral and bacterial pathogens is the immune system. Proteins and glycoproteins (sugar-protein surface markers) on the surface of pathogens stimulate the production of antibodies in the host. Any substance that stimulates the immune response is called an antigen. Each antigen the body is exposed to results in the production of a specific antibody that binds to only that antigen.

Antibodies in Medical Diagnosis

By developing specific antibodies to surface antigens found on a pathogen, a diagnostic procedure known as Enzyme-Linked Immunosorbent Assay (ELISA) can be used to detect the presence of the pathogen.

Part 2: Bacteria and Archaea

Both bacteria and archaea are prokaryotes. Though this makes them very similar to each other, they also have many unique characteristics.

Bacteria have the following characteristics:

  • No membrane-bound organelles
  • Single circular strand of DNA with no associated histone proteins
  • Most bacteria have a rigid cell wall composed of peptidoglycan (carbohydrate matrix linked with polypeptide units)

Archaeans have the following characteristics:

  • No membrane-bound organelles
  • Single circular strand of DNA with associated histone proteins
  • Cell membrane contains isoprene chains

The earliest fossils discovered on earth are archaeans dating to 3.8 billion years ago.

A diagram of a typical prokaryotic cell. This diagram, made in Adobe Illustrator, is an improved version of a similar diagram, Image:Prokaryote cell diagram.svg, which was also made by LadyofHats. Besides general appearance changes, this version adds plasmids and pili, and notes that DNA is circular. Latina: Diagramma cellulae naturalis prokaryoticae.


  1. Obtain a piece of water fern and place on a glass slide with a drop of water. Using a single-edged blade, dice the water fern into small pieces and place a cover slip on top.
  2. Search for a symbiotic cyanobacterium called Anabaena that lives within the tissue of the water fern under scanning (40X), low (100X), and high (400X) power.
  3. Carefully focus, using the fine focus knob only, until you can see the bacterial cells.

Lab Question

Draw several cells connected together in a chain. For comparison of size, draw a single water fern cell next to the Anabeana cells. Why are the Anabeana cells so small compared with the water fern cells?

Part 3: Epidemiology—How does an epidemic spread?

Some people are carriers of disease. A pathogen (disease agent) may not cause its host any immediate discomfort, with no or only mild symptoms. At some point the infected individual may become diseased. The danger to others is that a carrier may not be recognized as being infected and could inadvertently spread the pathogen to others that they come in contact with. Examples include carriers of viruses that cause hepatitis and AIDS. These pathogens have the capability of spreading very quickly due to the lack of symptoms in infected hosts. The rapid spread of a life threatening disease agent is known as an epidemic. Epidemics that spread from continent to continent are referred to as a pandemic, such as AIDS.

The study of disease at the population level is known as epidemiology. For some diseases, public health officials must identify the original carrier, called the index case. This person is sometimes referred to as the “Typhoid Mary.” By identifying such a person and finding out who he or she came in contact with, officials learn how many individuals have potentially picked up the pathogen and how the disease spreads.

In today’s lab you will learn how a disease agent can be spread through a group of people, leading to an epidemic. Each individual will obtain a solution. Noncarriers will receive an acid stock solution while one unknown individual will receive a base stock solution. Let’s review acids and bases briefly. Acidic solutions have an excess of H+ ions relative to OH ions, with a pH of less than 7. Basic solutions, on the other hand, have an excess of OH ions relative to H+ ions, with a pH in excess of 7. The acids and bases we will use in lab today are relatively mild and pose little harm. Nevertheless, be sure they do not get in your eyes or mouth.

Phenol red is a pH indicator. It changes color depending upon whether the solution it is added to is an acid or base. If phenol red is added to an acidic solution, the solution will be yellow. If it is added to a basic solution, the solution will be magenta/red.


  1. Each individual should obtain an unknown stock solution, a clean test tube with stopper, and a pipette.
  2. Using the pipette, transfer three pipettes full of your unknown solution to the clean test tube.
  3. Remove one pipette full of solution from your test tube and proceed as follows:
    1. Choose someone at random from your class.
    2. Empty your pipette into your contact’s test tube as he/she does the same to you.
    3. Place the stopper in your test tube and shake your test tube gently to mix solutions.
    4. In the first data table below (Round 1) write down the name of the person with whom you exchanged solutions.
  4. Repeat step 3 twice more (Rounds 2 and 3), each time with a different contact. At each addition, mix the combined solutions thoroughly.
  5. When you have exchanged solutions with three different contacts, add one drop of phenol red to your test tube to see if you are “infected.” Two test results are possible:
    1. Solution turns red/magenta—you are infected!
    2. Solution turns yellow—you are not infected (sure dodged a bullet)


Table 1. Personal data gathered while swapping fluids with 3 different people
Your Name Round 1 Round 2 Round 3

Enter the class data in Table #2 below for those that are infected, using information from others in your class.

Use an asterisk (*) to indicate which student contacts tested positive in columns labeled 1, 2, and 3.

Table 2. Exchange history for all persons in the class who are infected
Infected Person Round 1 Round 2 Round 3

Lab Questions

  1. Who was the index case (first person to have disease)?
  2. Assuming a lab of 20 students, what are the minimum number of rounds that it would it take before everyone is infected?