Beta b -hemolysis: complete lysis of RBC - A clear zone with a clean edge around the colony. MSA contains a high concentration of salt NaCl , and therefore, selects for the growth of microbes that can tolerate high salt concentrations. Staphylococcus species are halotolerant, whereas Streptococcus species and many other organisms are inhibited by high concentrations of NaCl. MSA also differentiates on the basis of mannitol fermentation. Microorganisms that can ferment mannitol create acidic byproducts, which decrease the pH of the medium and cause the pH indicator, phenol red, to turn yellow.
Gram-positive cocci are often isolated from clinical samples. Catalase is an enzyme that converts hydrogen peroxide to water and oxygen gas. The test is easy to perform; bacteria are simply mixed with H 2O 2. If bubbles appear due to the production of oxygen gas the bacteria are catalase positive. If no bubbles appear, the bacteria are catalase negative.
Staphylococcus and Micrococcus spp. Since dead tissue is no longer supplied with oxygen by the circulatory system, the dead tissue provides pockets of ideal environment for the growth of C. The wound will have to be surgically debrided debridement refers to the removal of dead and infected tissue and a sample sent for microbiological lab analysis, but Charles will not have to have his foot amputated.
Many diabetic patients are not so lucky. In , nearly 70, diabetic patients in the United States lost a foot or limb to amputation, according to statistics from the Centers for Disease Control and Prevention.
Aerobic respiration constantly generates reactive oxygen species ROS , byproducts that must be detoxified. Even organisms that do not use aerobic respiration need some way to break down some of the ROS that may form from atmospheric oxygen.
Three main enzymes break down those toxic byproducts: superoxide dismutase, peroxidase, and catalase. Each one catalyzes a different reaction. Reactions of type seen in Reaction 1 are catalyzed by peroxidases. In these reactions, an electron donor reduced compound; e. The enzymes play an important role by limiting the damage caused by peroxidation of membrane lipids. Reaction 2 is mediated by the enzyme superoxide dismutase SOD and breaks down the powerful superoxide anions generated by aerobic metabolism:.
Figure 5. The catalase test detects the presence of the enzyme catalase by noting whether bubbles are released when hydrogen peroxide is added to a culture sample. Compare the positive result right with the negative result left. Obligate anaerobes usually lack all three enzymes. Aerotolerant anaerobes do have SOD but no catalase. Reaction 3, shown occurring in Figure 5, is the basis of a useful and rapid test to distinguish streptococci, which are aerotolerant and do not possess catalase, from staphylococci, which are facultative anaerobes.
Bacteria that grow best in a higher concentration of CO 2 and a lower concentration of oxygen than present in the atmosphere are called capnophiles. One common approach to grow capnophiles is to use a candle jar. A candle jar consists of a jar with a tight-fitting lid that can accommodate the cultures and a candle. After the cultures are added to the jar, the candle is lit and the lid closed. As the candle burns, it consumes most of the oxygen present and releases CO 2. The health-care provider who saw Nataliya was concerned primarily because of her pregnancy.
Her condition enhances the risk for infections and makes her more vulnerable to those infections. The immune system is downregulated during pregnancy, and pathogens that cross the placenta can be very dangerous for the fetus. Blood is considered sterile; therefore, competing microorganisms are not expected in the medium.
Small grayish colonies surrounded by a clear zone emerge. Such colonies are typical of Listeria and other pathogens such as streptococci; the clear zone surrounding the colonies indicates complete lysis of blood in the medium, referred to as beta-hemolysis Figure 6. When tested for the presence of catalase, the colonies give a positive response, eliminating Streptococcus as a possible cause.
Furthermore, a Gram stain shows short gram-positive bacilli. Cells from a broth culture grown at room temperature displayed the tumbling motility characteristic of Listeria Figure 6.
Figure 6. An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube.
What is your conclusion? An inoculated thioglycolate medium culture tube is clear throughout the tube except for dense growth at the bottom of the tube.
Pseudomonas aeruginosa is a common pathogen that infects the airways of patients with cystic fibrosis. It does not grow in the absence of oxygen. The bacterium is probably which of the following? Streptococcus mutans is a major cause of cavities. It resides in the gum pockets, does not have catalase activity, and can be grown outside of an anaerobic chamber.
Why do the instructions for the growth of Neisseria gonorrheae recommend a CO 2 -enriched atmosphere? Four tubes are illustrated with cultures grown in a medium that slows oxygen diffusion. Match the culture tube with the correct type of bacteria from the following list: facultative anaerobe, obligate anaerobe, microaerophile, aerotolerant anaerobe, obligate aerobe.
Skip to main content. Microbial Growth. Search for:. Oxygen Requirements for Microbial Growth Learning Objectives Interpret visual data demonstrating minimum, optimum, and maximum oxygen or carbon dioxide requirements for growth Identify and describe different categories of microbes with requirements for growth with or without oxygen: obligate aerobe, obligate anaerobe, facultative anaerobe, aerotolerant anaerobe, microaerophile, and capnophile Give examples of microorganisms for each category of growth requirements.
Figure 2. Diagram of bacterial cell distribution in thioglycolate tubes. Think about It Would you expect the oldest bacterial lineages to be aerobic or anaerobic? Which bacteria grow at the top of a thioglycolate tube, and which grow at the bottom of the tube? An Unwelcome Anaerobe Figure 4. Think about It What substance is added to a sample to detect catalase? What is the function of the candle in a candle jar? Key Concepts and Summary Aerobic and anaerobic environments can be found in diverse niches throughout nature, including different sites within and on the human body.
Microorganisms vary in their requirements for molecular oxygen. Obligate aerobes depend on aerobic respiration and use oxygen as a terminal electron acceptor. They cannot grow without oxygen.
Obligate anaerobes cannot grow in the presence of oxygen. They depend on fermentation and anaerobic respiration using a final electron acceptor other than oxygen. Facultative anaerobes show better growth in the presence of oxygen but will also grow without it.
Although aerotolerant anaerobes do not perform aerobic respiration, they can grow in the presence of oxygen. Most aerotolerant anaerobes test negative for the enzyme catalase. Optimum oxygen concentration for an organism is the oxygen level that promotes the fastest growth rate. The minimum permissive oxygen concentration and the maximum permissive oxygen concentration are, respectively, the lowest and the highest oxygen levels that the organism will tolerate.
Peroxidase , superoxide dismutase , and catalase are the main enzymes involved in the detoxification of the reactive oxygen species. Superoxide dismutase is usually present in a cell that can tolerate oxygen. All three enzymes are usually detectable in cells that perform aerobic respiration and produce more ROS. A capnophile is an organism that requires a higher than atmospheric concentration of CO 2 to grow. Multiple Choice An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube.
The organisms die in the presence of oxygen The organisms are facultative anaerobes. The organisms should be grown in an anaerobic chamber. The organisms are obligate aerobes. Show Answer Answer b. The organisms are facultative anaerobes. Show Answer Answer a. The organisms are obligate anaerobes. The bacterium is probably an obligate aerobe.
Show Answer Answer d. The bacterium is probably an aerotolerant anaerobe. Show Answer Answer c. It is a capnophile. Matching Four tubes are illustrated with cultures grown in a medium that slows oxygen diffusion. Show Answer Tube a is an obligate anaerobe. Tube b is an obligate aerobe.
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