Why do you suppose this habitat contains such a high microbial count?
Variable; possible answers include, temperature, moisture, amount of human traffic. Types of organisms we find on skin (gram+ cocci, gram- cocci), soil (gram+ bacilli endosporeformers, gram+ filamentous, molds), in water (gram- bacilli), in dust (gram+ cocci, gram+ bacilli endosporeformers, mold spores, variable)
a. Were any plates completely lacking in colonies?
b. Do you think that the habitat sampled was really sterile?
c. If your answer to b is no, then how can you account for the lack of growth on the plate?
d. If your answer to b is yes, defend it.
(a) Variable answers (b) no/yes (c) Organisms sampled were not able to grow on the kind of nutrient agar used, or organisms sampled require longer time to grow. (d) Students may feel an area sampled is sterile because it was recently cleaned.
In what ways do the macroscopic features of bacterial colonies differ from those of molds?
Bacterial colonies are generally smooth and small as compared to fungal colonies, which are large and “fuzzy.”
Why is the level of contamination measured as number of colonies rather than size of colonies?
Since each colony is produced from a single cell, the number of colonies indicates the number of cells originally present or level of contamination. Colony size reflects growth rate. See Ex 38
Should one be concerned to find bacteria on the skin? How about molds? Explains.
Bacteria, such as the staphylococci and the diphtheroids, are part of the normal skin flora. Molds, however, are likely transient contaminants picked up from the environment.
How can microbial levels be controlled on the skin? On surfaces in the environment? in the air?
Microbial levels on skin are best controlled by hand washing, on surfaces in the environment with use of disinfectants like bleach, and in the air by HEPA filtration systems.
Compare the following features of bacteria to those of eukaryotic microorganisms:
- cell wall
- respiration and photosynthesis
- motility mechanisms
- Bacteria are smaller, about 0.5-10 µm in diameter.
- Bacterial DNA is not enclosed in a nucleus but rather is organized in the cytoplasm.
- Bacteria have 70S ribosomes.
- Bacteria have a cell wall composed of peptidoglycan.
- Bacteria lack mitochondria and chloroplasts but can carry out respiration and photosynthesis.
- Bacteria may have flagella that are simpler in structure but may be more numerous.