Exercise 24: Effects of Oxygen on Growth
Name two enzymes that are present in obligate aerobes but lacking in obligate anaerobes. What is the function of each enzyme?
Obligate anaerobes lack superoxide dimutase, which converts toxic superoxide to a harmless compound, and catalase, which converts toxic peroxide to oxygen.
What is the role of oxygen in aerobic respiration?
Oxygen acts as a terminal electron acceptor during respiration.
Anaerobic respiration
Process in which inorganic compounds such as nitrates and sulfates replace oxygen as the terminal electron acceptor in the ETC.
Why is resazurin a useful media additive for the study of anaerobes?
Resazurin is an indicator for the presence of oxygen.
Where in the human body would you expect to find an obligate aerobe? Where would you expect to find an obligate anaerobe?
Aerobes can be found in the human skin, i.e. Staphylococcus. Anaerobes can be found in hair follicles, i.e. Propionibacterium acnes, and in the intestinal tract, i.e. Bacteroides fragilis.
What type(s) of metabolism can occur in the absence of oxygen?
Anaerobic respiration or fermentation occurs in the absence of oxygen.
Why is a GasPak anaerobic jar necessary for the culture of anaerobes on plates of Brewer's anaerobic agar but not in tubes of fluid thioglycollate medium (FTM)?
Brewer's anaerobic agar plates provide growth of organisms in an oxygen-rich environment on the surface of the medium; therefore the GasPak anaerobic jar is necessary to remove the oxygen for anaerobic growth. Anaerobes grow in FTM because in the depths of the tube, the conditions are anaerobic; therefore the GasPak is unnecessary.
Differentiate between a microaerophile and an aerotolerant organism.
Microaerophiles require low amounts of oxygen for metabolism, although higher concentrations are lethal. Aerotolerant organisms can grow in oxygen but do not use it for metabolism and are indifferent to its presence.
Aerotolerant anaerobes (obligate fermenters)
These anaerobes can tolerate oxygen and even grow in its presence, but they do not require oxygen for energy production. They produce their energy strictly by fermentation and not by respiratory means. Examples: streptococci, Enterococcus faecalis, and Streptococcus pyogenes
Obligate (strict) anerobes
These bacteria cannot tolerate oxygen and must be cultured under conditions in which oxygen is completely eliminated; otherwise, they are harmed or killed by its presence. Only found among the prokaryotes and in some protozoa. Examples: Clostridium, Methanococcus, and Bacteroides
Facultative anaerobes
These bacteria grow very well aerobically but also have the capacity to anaerobically if oxygen is not present. Can switch between respiration (with oxygen) and fermentation (without oxygen) to produce energy. Example: Escherichia coli
Obligate (strict) anaerobes
These bacteria must grow in oxygen because their metabolism requires oxygen. They carry out aerobic respiration in which oxygen is utilized as the last terminal electron acceptor in the ETC. Examples: Pseudomonas, Micrococcus, and many Bacillus
Microaerophiles
These bacteria prefer to grown in oxygen concentrations of 2-10% rather than the 20% found in the atmosphere. The lower concentration of oxygen is necessary for their respiratory metabolism. Example: Helicobacter pylori
Classify each organism based on its oxygen requirements: a. Escherichia coli b. Enterococcus faecalis c. Staphylococcus aureus d. Bacillus subtilis e. Clostridium sporogenes f. Clostridium beijerinckii
a. facultative anaerobe b. aerotolerant organism c. facultative anaerobe c. aerobe d. aerotolerant anaerobe e. obligate anaerobe