Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
Prokaryotes represent the oldest kind of organisms on earth; their fossil record extends back over 3.5 billion years; all are members of the Domains Archaea and Bacteria. The term prokaryote means pre nucleus; they lack the membrane bound nucleus found in eukaryotes (members of the Domain Eukarya). In fact prokaryotes have no membranous cellular compartments called organelles in eukaryotes. There are about 2,700 known living species. In numbers prokaryotes are the most abundant kind of organisms. The examples you will see are all members of the Domain Bacteria. Most members of this group are heterotrophs and are important as decomposers. They play an extremely important role in nitrogen fixation, making atmospheric nitrogen (N2), available to eukaryotic organisms in the form of ammonia (NH3). Some bacteria are photosynthetic and have light absorbing pigments embedded in their plasma membranes. These bacteria appear green and are called Cyanobacteria.
Most bacteria are surrounded by a cell wall that is chemically different from those in fungi and plants. H. C. Gram discovered staining differences that are related to the wall structure. Gram-negative bacteria have a complex wall and are resistant to many antibiotics. Gram-positive bacteria, e.g., Staphylococcus, have a simpler wall and are more susceptible to antibiotics and to lysozyme, an enzyme in such secretions as tears. Some bacteria form a polysaccharide capsule around themselves and are thus very resistant to chemical attack, e.g., a certain form of Streptococcus pneumoniae.
Bacteria have three common shapes:
coccus (pl. cocci) = sphere
Bacteria mixed shows bacteria of the three different shapes; some slides are Gram stained and bacteria may be dark purple (Gram positive) or light pink (Gram negative).
Examination of dental bacteria
Bacteria are present everywhere on the surface of your body and in your digestive tract. Your mouth has an especially rich flora. To examine your dental bacteria proceed as follows:
Isolation of Bacteria from the Environment
Bacteria are everywhere. In this experiment you will collect bacteria from the environment and transfer them to a nutrient agar in a petri dish for incubation. Don't open the petri dish until step 2 below; bacteria from the air may settle on the agar and contaminate your experiment. Do note that the smaller half of the dish contains the gelatinous agar.
Living Cyanobacteria are available for study. Prepare slides of Oscillatoria and Anabaena. Both are filamentous--chains of elongated cells. Occasional larger cells in the chain are heterocysts which have dark ends; these cells fix nitrogen. The green color in the cells is chlorophyll; note that the color is everywhere and is not localized in chloroplasts as you saw in the algae Elodea and Spirogyra, which are eukaryotes. If you look at free filaments at the edge of the Oscillatoria clump, you will observe their characteristic slow and smooth undulating (or oscillating) movement.
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