QUESTIONS ON BIOCHEMISTRY
Questions on Biochemistry
Describe the difference between an obligate and a facultative aerobe.
Obligate and facultative aerobes are the same in terms of their ability to use oxygen in their respiration process. However, obligate aerobes requires oxygen to oxidize substrates such as sugar and fat to produce ATP (energy). Obligate aerobes are more efficient in converting substrates to ATP. The key process in aerobic respiration is the conversion of sugar to ATP shown in reaction 1 below:
In the reaction 36 molecules of ATP is produced per molecule of glucose. It is actually a slow process which comprises 3 steps happening in the mitochondria: (1) glycolysis, (2) Krebs cycle and (3) electron-transport process (Rosein, n.d.). Examples of obligate aerobes are fungi and some bacteria such as Pseudomonas aeruginosa and Bacillus (The Great Soviet Encyclopedia, n.d.).
On the other hand, facultative aerobes do not require oxygen in order to survive. Facultative aerobes grow best in oxygen-rich environments, but they can still survive without oxygen. Without oxygen, the facultative aerobes undergo anaerobic respiration in the cell cytoplasm. Anaerobic respiration is faster than aerobic respiration, but it only produces 2 molecules of ATP per molecule of glucose (Rosein, n.d.). This means that anaerobic respiration is less efficient in producing energy. This also implies that facultative aerobes grow less in no-oxygen environment. Examples of facultative aerobes include Salmonella and Escherichia coli.
Discuss how specified factors affect the growth of a given microorganism.
Some of the physical factors that affect the growth of a given microorganism are temperature, pH, osmotic pressure, hydrostatic pressure and radiation (Halligan, A., & Leatherhead Food International, 2007). First, temperature plays an important role in the enzymatic reactions inside the microorganisms. Different microorganisms have different optimum temperature for their survival, growth and multiplication. For instance there are four general types of microorganisms according to their optimum temperature ranges:
Thermophiles: 30 - 75 °C (optimal 55 °C)
Mesophiles: 10 - 45 °C (optimal 35 °C)
Psychrotrophs: 0 - 40 °C (optimal 20 - 30 °C)
Psychrophiles: -5 - 20 °C (optimal 15 °C)
Another factor is pH. pH is the negative logarithm of the hydrogen ion concentration [H+]. It refers to the acidity of the growth medium, with lower values being more acidic. pH variations can mean inhibition of enzyme activity, disruption of plasma membrane processes or deactivation of membrane transport proteins. Similar to temperature, different microorganisms have different optimum pH. Microorganisms can be classified according to their optimum pH:
Acidophiles: grow best in acidic environments (0-5.5)
Alkalophiles: grow best in alkaline environments (17.5-14.0)
Neutrophiles: grow best in neutral environments (5.5 to 8.0)
Next is osmotic pressure. It is the required pressure applied to a solution to prevent the inward flow of water across a semi-permeable membrane. There are microorganism that thrive in solutions with high salt concentrations. They are referred to as osmotolerant or halophiles. Another factor is hydrostatic pressure. It is the force per unit area exerted by the growing medium. Microorganisms that can survive high hydrostatic pressures are called barotolerant. Radiation is another key factor. Sunlight is beneficial to some microorganisms. However, too much radiation in the form of ionizing rays can cause genetic mutation resulting in death of microorganisms.
Aside from these physical factors, some chemical factors such as water activity and redox potential can also affect the growth of microorganisms. Water activity is the ratio of vapour pressure of water in the solution to that of pure water. It directly affects microorganisms since water is required for growth. Usually, bacteria do not grow in environments with water activity less than 0.85. Water activity is commonly controlled in food-manufacturing facilities to prevent spoilage of food and prolong shelf life. Another chemical factor is redox potential. It refers to the reduction-oxidation state of the growth medium. A positive redox potential (oxidized) is favored by aerobes while negative redox potential (reduced) is favored by anaerobes. Example of food with positive redox potential (+300-400 mV) is food of plant origin. This environment favors aerobic bacteria and molds. Meat has a negative redox potential (-200 mV) which favors anaerobic microorganisms.
Employ a given aseptic technique in a laboratory setting.
An aseptic technique is a set of complimentary procedures in microbiology laboratories and hospitals that is performed in sterile conditions (Garcia, 2010). Sterile means without living microorganisms. An example of this technique is the preparation of a sterile media for growing bacteria. The rationale of this activity is to prepare an uncontaminated and pure medium for a specific bacteria to grow. First, the powder (growth media) is dissolved in distilled water. It is then poured into sterile containers. These steps are done in a sterile environment, usually in a laboratory hood. The aseptic technique here is the use of the autoclave to sterilize the bottles with the growth media. At this point, the growth media is now sterile and ready to be used. The use of Bunsen burner is also widely employed to heat surface and bottle necks to prevent contamination especially during pouring media to petri dishes, etc.
Recognize common methods used to control microbial growth.
Some common methods include: (1) autoclaving, (2) pasteurization, (3) direct flaming, (4) incineration, (5) hot-air oven, and (6) membrane filtration (The Control of Microbial Growth, n.d.). First is autoclaving which is the use of high-pressure steam. It is an effective sterilization techniques using moist heat since the steam is in direct contact with the material to be sterilized. Next is pasteurization which employs high temperature at a short period to destroy microorganisms without affecting the food taste. This is a standard practice usually in food and beverage industries.
Next is direct flaming using Bunsen burner. This practice is employed in microbiology laboratories to transfer microbial strains. Another microbial control method is incineration. This involves burning the microorganisms through physical destruction. This is usually employed in some slaughter houses to prevent contamination. Hot-air ovens are also used in sterilizing equipment using heat. Last in the list is membrane filtration. In this process, liquid or gas passess through a filter with specified pore size. Usually, the targeted microorganism is retained in the filter such as in the case of HEPA filter. Other less-common techniques include radiation, chemical methods (use of germicide, antibiotics and disinfectants), and desiccation (drying).
Recognize appropriate uses for a specified sub type of culture media.
In terms of consistency, culture media can be classified as solid or liquid. In terms of use, they can be classified as routinary laboratory media and synthetic media for research purposes. Routinary laboratory media is classified as (1) basal media, (2) enriched media, (3) selective media, (4) indicator media, (5) transport media, and (6) storage media (Cartwright and Shah, 1994). Basal media are used as starter media for growth of bacteria. One example is the nutrient broth which consists of meat, peptone, sodium chloride and water. This nutrient broth is then added with agar to become nutrient agar. Next is enriched media. Enriched media consist of basal medium plus blood, serum, egg, etc. This provides additional substrates for a specific bacteria such as the case of blood agar media for Streptococcus.
Another is the selective media. Inhibitors are added to grow a specific type of microorganism. One example is the MacConkey agar for Enterobacteriaceae. It contains bile salt which does not inhibit growth of Enterobacteriaceae but inhibits other types of bacteria. Indicator media contain an indicator which changes color when a specific type of bacteria is grown. Lactose fermenters such as E. coil turn the medium from pink to red due to increased acid content. Non-lactose fermenters such as Salmonella and Shigella results in colorless colony. Transport media are used for transporting samples such as Stuart’s medium. Storage media are used for storing microorganisms for a long period of times. Examples of storage media include egg saline medium and chalk-cooked meat broth.
References:
Aerobes. (n.d.) The Great Soviet Encyclopedia, 3rd Edition. (1970-1979). Retrieved April 9 2016 from http://encyclopedia2.thefreedictionary.com/aerobes
Cartwright, T., & Shah, G. P. (1994). Culture media. Basic Cell Culture: A Practical Approach, (146), 57.
Garcia, L. S. (Ed.). (2010). Clinical microbiology procedures handbook. American Society for Microbiology Press.
Halligan, A., & Leatherhead Food International. (2007). Micro-facts:[the working companion for food microbiologists]. P. Wareing (Ed.). Leatherhead Publ.
Rosein, Patrick. (n.d.) Aerobic Respiration - Biology Video by Brightstorm. Retrieved April 9 2016 from https://www.brightstorm.com/science/biology/cell-functions-and-processes/aerobic-respiration/
Rosein, Patrick. (n.d.) Anaerobic Respiration - Biology Video by Brightstorm. Retrieved April 9 2016 from https://www.brightstorm.com/science/biology/cell-functions-and-processes/anaerobic-respiration/
The Control of Microbial Growth. (n.d.) Lecture Notes from Midlands Technical College. Retrieved April 9 2016 from http://classes.midlandstech.edu/carterp/Courses/bio225/chap07/lecture4.htm
