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Introduction- This lab experiment serves as a model for community succession using bacterial colonies as the model. A bacterial colony grows from a single bacterium and is composed of millions of cells. Each colony has distinctive colony morphology: size, shape, color, consistency, and color. Community succession is a phenomenon observed in the organizational hierarchy of all living organisms. Community succession is not limited to bacterial colonies, but spans the entire community of life.
As the community grows, it changes the environment it inhabits, and the resulting community is different than at the start. As community succession occurs in bacterial colonies pH, odor, color, and consistency changes take place. In this experiment, four different milk age samples, fresh, 24 hour old, 4 day old, and 8 day old milk, will be prepared on different sets of agar plates that will be diluted to different levels for optimum bacterial growth and measurement. To identify the resulting colonies a procedure called gram stain is used in the identification of bacteria.
Bacteria can be gram-positive, gram-negative, or gram-variable. Most bacteria however, are usually gram-positive or gram-negative. Gram-positive and gram-negative bacteria react differently to staining because of the differences in their cell walls. Bacterial cell walls contain a polymer called peptidoglycan. Gram-positive bacteria have more of this polymer than gram-negative bacteria. They are also less complex than gram-negative cell walls, missing different polysaccharides, lipids, and proteins that are found in gram-negative bacteria.
Three different stain types are used in the identification process: safranin (pink/red), crystal violet (purple), and Gram iodine. Along with being either gram-positive or gram-negative, most bacterial cells can be classified according to three shapes: bacilli (rods), cocci (spheres), and spirilla (spirals, or corkscrews). The hypothesis for this experiment is that the bacterial colonies in each milk age sample and dilution will be unique as a result of community succession acting differently upon the various bacterial colonies.
Methods- We began the experiment by noting the characteristics of the different milk ages by smell, color, and pH was measured using pH paper. Fresh milk smelled like milk, had a white color, and a pH of 7. 24 hour old milk had a very slight sour smell to it, was still white colored with a pH of 7. Four day old milk had a sour smell, was white with visible chunks, and had a pH of 6. Lastly, four day old milk smelled like sour cream, was yellow colored with white chunks, and had a pH of 5.
Once the characteristics of the milk were noted we began to prepare the agar plates for the different milk samples. For cold milk, one agar plate was labeled undiluted, and one plate was labeled “10-1”. 0. 5 ml of milk was pipette onto the plate labeled undiluted. 0. 1 ml of milk was then pipette onto the plate labeled “10-1”, and the pipette was discarded. Using an alcohol and flame sterilized bent rod, the milk was distributed across the agar plates. The lid was never removed completely, just lifted up enough to allow the rod to thoroughly spread the milk.
For 24 hour milk, we prepared four agar plates by labeling them undiluted, “10-1”, “10-2”, and “10-3”. 1 ml of milk was pipette onto the plate labeled undiluted. Then 0. 1 ml milk was pipette onto the plate labeled “10-1”. We proceeded to pipette 0. 1 ml of milk into the 9 ml water blank labeled “10-2”. The pipette was then discarded and we repeated the spreading technique as done for the fresh milk samples. The lid of the 10-2 dilution was tightened and shaken 25 times. After obtaining a clean pipette, 1 ml of the 10-2 dilution was pipette onto the plate labeled “10-2”.
Then 0. 1 ml of the 10-2 dilution was pipette onto the plate labeled “10-3” and the pipette was discarded. Use the spread technique to distribute the bacteria. With the 4 day milk, we prepared 4 agar by labeling them “10-2”, “10-3”, “10-4”, and “10-5”. Then two 9 ml water blanks were prepared and labeled “10-2” and “10-4”. We then pipette 0. 1 ml of milk into the 9 ml water blank labeled “10-2” and discarded the pipette. The lid of the 10-2 dilution tube was tightened and shaken 25 times.