A. Describe the difference between an obligate and facultative aerobe. An obligate aerobe is a microorganism that must have oxygen for growth and will grow at the top of the test tube. A facultative aerobe prefers oxygen but it is not necessary for growth. Many of these microorganisms will grow at the top of the tube, but will also be dispersed throughout as well. B. Discuss the temperature requirements of the organisms (you grew) in this lab. Both Staphylococcus epidermis and Lactobacillus acidophilus are mesophiles. Mesophiles grow best between 25- 45 degrees Celsius (Alonzo, n.d.). In this experiment both bacteria have grown easily at a maintained room temperature of 26 Celsius.
C. Discuss under what conditions you would want to control the growth of microbes.
To minimize risk of contamination with unwanted microorganisms, aseptic technique was employed during collection. This includes hand washing with bactericidal soap, cleaning area with an “all-purpose” germicide, and prior sterilization of media and refrigeration upon receipt. My eight-year-old son, fresh from baseball practice, unwashed and teeming with microbes, was the source of Staphylococcus epidermis culture.
I disinfected our working area, washed my hands, readied our working material, with aseptic technique swabbed his antecubital area with a sterile swab, placed directly in the nutrient broth test tube without (further) contamination, loosely recapped and racked above the refrigerator for incubation. The Lactobacillus acidophilus culture was similarly done. After cleaning my area and preparing the necessary material, I washed my hands, opened the L. acidophilus capsule and transferred the contents into the MRS media test tube. The culture was recapped without contamination, inverted to mix, cap loosened, and racked above the refrigerator.
D. Discuss the basic forms of culture media.
All media are prepared in a sterile fashion.
Nutrient broth is liquid, containing various material as nutrients that ensure microbial growth (Betsy & Keogh, 2005). Liquid media is used for plentiful growth. An agar slant is a nutrient medium in a test tube, containing a solidify agent, positioned to become solid at a slant to increase the area available for inoculation. Solid media is used to study the characteristics of colonies. Solid media also enables the isolation of a pure culture. An agar stab is a nutrient medium that contains agar as a solidifying agent. It is prepared in a test tube and positioned upright when becoming solid and the inoculum stabbed deep into the medium (Alonzo, n.d.). Agar dishes are made in petri dishes. Agar medium is allowed to become solid in dishes of various sizes, providing more area than a test tube would allow.
When culturing, separating, and counting, dishes are an easier choice over stab and slant agar (Alonzo, n.d.). E. Discuss whether you saw growth in the tubes after 24 hours. After 24 hours, S. epidermis showed cloudiness concentrated low in the tube around the swab. The top of the nutrient broth within the test tube was clear. L. acidophilus developed a pellicle. Sediment appears to be the white powder solute and is likely due to the incomplete mixing in the MRS media solution. No increase of this is noted since initial amount marking. F. Discuss whether you saw growth in the tubes after 48 hours. After 48 hours, S. epidermis turbidity has increased noticeably, with pellicle formation evident. L. acidophilus pellicle formation has increased. Turbidity is now noted. Slight increase of sediment level is also noted.
G. Explain why you did or did not see growth.
Test tubes containing bacteria cultures were kept at room temperature with loose coverings. These conditions allowed for an appropriate temperature for growth of these specific microorganisms and access to sufficient oxygen to meet requirements for growth. The nutrient media provided nutrients for the microorganisms to use for growth.
Alonzo, Cynthia. (n.d.). Aseptic Technique & Culture Microbes. In Microbiology – MLT1 (pp. 84-99) Englewood, CO: Hands-On Labs, Inc.
Betsy, T., & Keogh, J. (2005). Microbiology Demystified. Blacklick, OH, USA: McGraw-Hill Professional Publishing. Retrieved from http://www.ebrary.com