Toxic Derivatives of Oxygen O2

Although not toxic itself, O2 can be converted into a number of compounds that are highly toxic. Some of these, such as superoxide (O2:), are produced both as a part of normal metabolic processes and as chemical reactions involving oxygen and light. Others, such as hydrogen peroxide (H2O2), result from metabolic processes involving oxygen. To survive in an environment containing O2, cells must have enzymes that can convert these toxic compounds to non-toxic forms. The enzyme superoxide dismu-tase degrades superoxide to produce hydrogen peroxide. Catalase breaks down hydrogen peroxide to H2O and O2. Together, these two enzymes detoxify these reactive products of O2.

Although most strict anaerobes do not have superoxide dismutase, some do, while a few aerobes lack it. Therefore, other factors must also be playing a role in protecting organisms from the toxic forms of oxygen.

Each bacterial species can survive within a range of pH values; within this range, it has a pH optimum. Despite the pH of the external environment, cells maintain a constant internal pH, typically near neutral. ■ pH, p. 23

Most bacteria can live and multiply within the range of pH 5 (acidic) to pH 8 (basic) and have a pH optimum near neutral

4.3 Environmental Factors that Influence Microbial Growth 89

(pH 7). These bacteria are called neutrophiles. Preservation methods that acidify foods, such as pickling, are intended to inhibit these organisms. Surprisingly, some neutrophiles have adapted special mechanisms that enable them to grow at a very low pH. For example, Helicobacter pylori grows in the stomach, where it can cause ulcers. To maintain the pH close to neutral in its immediate surroundings, H. pylori produces the enzyme urease, which splits urea in the stomach into carbon dioxide and ammonia. The ammonia neutralizes the stomach acid in the bacterium's immediate surroundings. ■ pickling, p. 806

Acidophiles grow optimally at a pH below 5.5. For example, Thiobacillus ferroxidans, a member of the Bacteria, grows best at a pH ofapproximately 2. This bacterium obtains its energy by oxidizing sulfur compounds, producing sulfuric acid in the process. It maintains its internal pH near neutral by pumping out protons (H+) as quickly as they enter the cell. Picrophilus oshimae, a member of the Archaea, has an optimum pH of less than 1! This prokaryote, which was isolated from the dry, acid soils of a gas-emitting volcanic fissure in Japan, has an unusual cytoplasmic membrane that is unstable at a pH above 4.0.

Alkalophiles grow optimally at a pH above 8.5. For example, Bacillus alcalophilus grows best at pH 10.5. It appears alka-lophiles maintain a relatively neutral internal pH by exchanging internal sodium ions for external protons. Alkalophiles often live in alkaline lakes and soils.

Water Availability

All microorganisms require water for growth. Even if water is present, however, it may not be available in certain environments. For example, dissolved substances such as salt (NaCl) and sugars interact with water molecules and make the water unavailable to the cell. In any environment, particularly in certain natural habitats such as salt marshes, prokaryotes are faced with this situation. If the solute concentration is higher in the medium than in the cell, water diffuses out of the cell due to osmosis. This causes the cytoplasm to dehydrate and shrink from the cell wall, a phenomenon called plasmolysis (figure 4.5). ■ solute, p. 54 ■ osmosis, p. 54

Prokaryotes can maintain the availability of water in a high salt environment by increasing the solute concentration inside the cell. This can be done in two ways. The organism can pump

High concentration of dissolved components

Cytoplasmic membrane shrinks from the cell wall (plasmolysis)

Cytoplasmic membrane

Cell wall

High concentration of dissolved components

Cytoplasmic membrane shrinks from the cell wall (plasmolysis)

Cytoplasmic membrane

Cell wall

Cell Called Shrinkage
Figure 4.5 Effects of Solute Concentration on Cells The cytoplasmic membrane allows water molecules to pass through freely. If the solute concentration is higher outside of the cell, water moves out.The dehydrated cytoplasm shrinks from the cell wall, a process called plasmolysis.

90 Chapter 4 Dynamics of Prokaryotic Growth ions, most commonly potassium (K+), from the outside to the inside of the cell, or it can synthesize certain small organic compounds such as the amino acid proline and sugar alcohols, which have no effect on normal cellular activity. Bacteria that can tolerate high salt concentrations, up to approximately 10% NaCl, are called osmotolerant. Staphylococcus species, which reside on the dry salty environment of the skin, are osmotoler-ant. ■ proline, p. 26

Organisms that require high levels of sodium chloride to grow are called halophiles (halo means "salt" and phile means "loving"). Many marine bacteria are mildly halophilic, requiring concentrations of approximately 3% sodium chloride. Certain members of the Archaea are extreme halophiles, requiring at least 9% sodium chloride or more; some can even grow in saturated salt solutions. Extreme halophiles are found in environments such as the salt flats of Utah and the Dead Sea.

The growth-inhibiting effect of high concentrations of salt and sugars is used in food preservation. High levels of salt are added to preserve such foods as bacon, salt pork, and anchovies. High concentrations of sugars can also inhibit the growth of bacteria. Many foods with a high sugar content, such as jams, jellies, honey, preserves, and sweetened condensed milk, are naturally preserved. ■ drying of foods, p. 124

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  • merigo
    What are different toxic derivatives of oxygen microbiology?
    3 years ago
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    What are the various toxix derivatives of oxygen?
    2 years ago

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