Sample Formal Lab Report

The following is report that repeats the two classic experiments that were used to disprove Spontaneous Generation--that life originates from non-living substances. This report does NOT contain the required bibliography or internal references.

Disproving Abiogenesis:
A Repeat of the Experiments of Spallanzani & Pasteur

ABSTRACT: Until the mid-1800's it was thought that life originated from non-living forms of matter. This idea of Spontaneous Generation was disproved by allowing boiled, nutrient broth cultures to come in contact with the air; or to contain narrow, S-shaped openings to come in contact with the air or to come in direct contact with the air. Since the only cultures that allowed the growth of microorganisms were the cultures that were in direct contact with the air, the idea that life could originate from non-living matter was disproved.

INTRODUCTION: Early man believed that life originated from non-living matter. Men observed that if food products were left at room temperature for long periods of time they developed "fuzzy" and "slimy" areas. The food decayed and developed an odor. Flies were also attracted to the food and maggots soon appeared in the food. If they ventured to a pond in spring they observed the emergence of tadpoles from the mud. All of these life forms (the fuzz, the slime, maggots and tadpoles) had no visible parent, thus it was believed that the life forms spontaneously generated from the non-living matter (the food or mud).

In the early seventeenth century scientists began to question the true origin of life. These men knew that seeds needed to be saved for next year's crop and that animals needed to be mated to create a new generation. It therefore seemed that all life forms should have their origins in previous life. Fancesco Redi performed the first controlled experiment to prove that life comes from life. Redi placed meat into two containers. The first container was felt open to the air and the second was covered with a mesh cloth. Both containers attracted flies but only the uncovered container had maggots. If Spontaneous Generation was true then both containers should have maggots. (We know now that maggots are the larval form of flies and the fly needed to lay its eggs on the meat so that the maggots would hatch.) These experiments proved that large life forms must come from existing life forms, but what of the slime and fuzz that are found on decaying food?

When Anton vonLeeuwenhoek invented the microscope in the early 1600's it became apparent that a microscopic world existed and that the slime and fuzz on the foods microscopically looked very similiar to microorganisms that were seen in the air and water. Thus, the origin of all life forms must be from pre-existing life: the slime and the fuzz came from microscopic life forms in the air or are present on the food. Lazzaro Spallanzani conducted experiments in the mid-1700's to prove that microscopic life forms also come from pre-existing life.

Spallanzani made a beef broth from bones. He placed this broth into three flasks. One flask was sealed to prevent the entrance of air borne microorganisms. Two other flasks were heated, but one of them was sealed to prevent contamination by air borne organisms. Growth developed in all the flasks but the heated-sealed one. Spallanzani said that if Spontaneous Generation was true then all the flasks should have supported growth. However, proponents of the theory said that the air contains a vital, activating force (miasma) which was required to start the process; thus an experimental design needed to be developed that would allow an unobstructed flow of air (the vital force) to enter the flask but also restrict the entrance of microbes.

In 1861, Louis Pasteur developed a flask with an S-shaped curve for its opening (swan necked flask; Figure 1). Pasteur was able to heat the broth that was in the flask and the curve in the opening acted as a trap that would catch any microbes that would fall into the flask. Pasteur's experiments showed that microbial growth would only become visible if microbes were already in a broth or if they came from the air.

Figure 1: Pasteur's Swan (S) Neck Flask

These experiments will repeat the experiments of Spallanzani and Pasteur. The theory of spontaneous generation will be disproven by showing that non-living matter cannot generate life and thus that life must come from pre-existing life.

MATERIALS AND METHODS: Four grams of nutrient broth powder were dissolved in 500 ml of tap water. Seventy ml of the broth solution was placed into seven, clean flasks. The openings on each flask were sealed as outlined below.

  1. Corked and not heated
  2. Open and boiled for 15 minutes
  3. Corked and boiled for 15 minutes
  4. Open and autoclaved (15 psi, 20 minutes)
  5. A straight, narrow tube is placed in opening (opening is "up") and autoclaved
  6. An inverted U-shaped tube (opening is "down") is placed in the opening and autoclaved
  7. An S-shaped tube (opening is "up") is placed in the opening and autoclaved

The flasks were incubated at room temperature and were observed daily for a week and then weekly for two additional weeks.

RESULTS:

Figure 2: Various Treated Flasks Incubated at Room Temperature

Time Cork, No Heat Open, Boil 15 min. Cork, Boil 15 min. Open, Autoclave Narrow, Autoclave C-open, Autoclave S-open, Autoclave
Day 1
g
ng
ng
ng
ng
ng
ng
Day 2
g
ng
ng
ng
ng
ng
ng
Day 3
g
g
ng
ng
ng
ng
ng
Day 4
g
g
ng
ng
ng
ng
ng
Day 5
g
g
ng
ng
ng
ng
ng
Day 6
g
g
ng
g
ng
ng
ng
Week 1
g
g
ng
g
ng
ng
ng
Week 2
g
g
ng
g
ng
ng
ng
Week 3
g
g
ng
g
g
ng
ng

Key
ng = no growth
g = growth

Each flask started as a clear, golden color--thus indicating visible growth in the broth. Growth was determined by turbidity of the broth or mold on the top of the broth.

DISCUSSION: If Sponteneous Generation is to be believed, all the flasks should have signs of growth. Heating of the the flasks killed any pre-existing microorganisms, therefore Flask 1 (unheated but corked) showed growth because the pre-existing life forms were not killed. Flask 3 (heated and corked) had all life destroyed and air borne organisms were not allowed to enter. Flasks that were heated but had straight openings (2, 4 and 5) all showed growth, although not at the same time. The size of the opening (for entrance of air borne microbes) and how they were heated (to kill pre-existing microbes in the broth) determined how quickly the growth became noticeable. Autoclaving was shown to be more effective than boiling and smaller openings also restricted the entrance of the air borne contaminants. The C- (6) and S- (7) shaped flasks were heated to kill the pre-existing bacteria but the design of the openings allowed for an unobstructed path for air to come in contact with the broth and at the same time trap the bacteria. In the C-flask the bacteria cannot travel up to come in contact with the broth while the S-flask allows the bacteria to fall but they, too, cannot travel up, thus they were trapped in the bend.

Since not all flasks showed signs of growth, Spontaneous Generations has been disproved. Support is also given to the theory of biogenesis that all life must come from pre-existing life forms, whether they are already present on a substance or come from the air to contaminate the substance.