9.2 Oxygen requirements for microbial growth  (Page 4/10)

Detoxification of reactive oxygen species

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 peroxidase s.

In these reactions, an electron donor (reduced compound; e.g., reduced nicotinamide adenine dinucleotide [NADH]) oxidizes hydrogen peroxide , or other peroxides, to water. 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:

The enzyme catalase converts hydrogen peroxide to water and oxygen as shown in Reaction 3.

Obligate anaerobes usually lack all three enzymes. Aerotolerant anaerobes do have SOD but no catalase. Reaction 3, shown occurring in [link] , 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. A sample of culture rapidly mixed in a drop of 3% hydrogen peroxide will release bubbles if the culture is catalase positive.

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 .

Part 2

The health-care provider who saw Jeni 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. A note on the provider’s order to the microbiology lab mentions a suspicion of infection by Listeria monocytogenes , based on the signs and symptoms exhibited by the patient.

Jeni’s blood samples are streaked directly on sheep blood agar , a medium containing tryptic soy agar enriched with 5% sheep blood. (Blood is considered sterile; therefore, competing microorganisms are not expected in the medium.) The inoculated plates are incubated at 37 °C for 24 to 48 hours. 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 ( [link] ). 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 ( [link] ). All of these clues lead the lab to positively confirm the presence of Listeria in Jeni’s blood samples.

• How serious is Jeni’s condition and what is the appropriate treatment?

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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 .
• Microaerophiles need oxygen to grow, albeit at a lower concentration than 21% oxygen in air.
• 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.

Matching

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.

(a) obligate anaerobe, (b) obligate aerobe, (c) microaerophile, (d) facultative anaerobe

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