Effects of the Environment on Microbial Growth

EFFECTS OF THE ENVIRONMENT ON MICROBIAL GROWTH

Organisms survive and grow under optimal conditions but exactly what constitutes optimal conditions can vary with the type or organism. Some anaerobic organisms are not equipped to survive the reactive oxygen species that come out of living in a high oxygen environment. Organisms that survive in extreme locations are actually anaerobic and do not live well when oxygen is present. This can include places like sewers, the intestines, deep in the earth’s crust, bogs, and marshes do not survive well with oxygen present.

Bacteria can be grown in thioglycolate tube cultures that is low in oxygen. The organism is stabbed into the tube and can become motile within it. It gradually picks up oxygen from the top of the tube on down. The maximum growth occurs at the level where the oxygen concentration is optimal. Organisms at the top are obligate aerobes, while organisms at the bottom are obligate anaerobes. Organisms that grow throughout are called facultative anaerobes but they will concentrate at the top. Aerotolerant anaerobes will growth throughout. Microaerophiles grow somewhere in between, where oxygen content is lower than atmospheric oxygen levels.

Obligate anaerobes are difficult to grow. They need to be grown in an anaerobic jar, which chemically removes the oxygen, or in anaerobic chambers, which are also anaerobic. Mixed bacterial infections are common in humans, which usually involve anaerobes and aerobes. Some organisms will operate somewhere between a minimum permissive oxygen concentration and a maximum permissive oxygen concentration.

The process of aerobic respiration involves the making of reactive oxygen species as byproducts. These need to be detoxified from the cell. There are three important enzymes that do this: catalase, peroxidase, and superoxide dismutase. Each does a slightly different thing to detoxify reactive oxygen species. Obligate anaerobes will not have any of these enzymes, while other organisms will have just one or two of these.

Some organs are called capnophiles because they like high carbon dioxide levels and low oxygen levels. They can grow in candle jars in which a candle has been lit to get rid of oxygen and release carbon dioxide as a byproduct of the burning process.

Organisms vary also in their desire for growth in an acidic or alkaline environment. Levels of pH less than 7 are acidic environments, while pH levels higher than 7 are alkaline environments. Extremes of pH can denature certain macromolecules. The proton motive force so important in making ATP will fall apart in an alkaline environment. Proteins are especially sensitive to pH levels.

And yet, certain bacteria prefer more extremes in pH. These include the organisms that make pickles, yogurt, and other fermented food. Acidic foods can be healthy for digestion because they inhibit the growth of certain bacteria. There will be a minimum growth pH and a maximum growth pH with an optimum pH level for each organism. Most bacteria will be neutrophiles that like a pH near 7.

Acidophiles like low pH levels. There are those that live in hot springs or sulfur fields that are considered extreme acidophiles. Lactobacillus in the human vagina like the pH 4 levels of that area. Many of these bacteria will pump hydrogen ions out of the cell actively in order to adapt to low pH levels.

Alkaliphiles like higher pH levels. This is true of Vibrio cholerae, which is inactivated by stomach acidity. There are certain lakes that are very alkaline and contain these types of alkaliphile organisms. They have mechanisms to maintain the proton motive force despite the circumstances.

Organisms can also tolerate ranges in temperature. There are organisms that thrive in the Antarctic and organisms at the bottom of hot ocean vents. There are optimal temperature levels for most of the organisms. Most organisms are called mesophiles and adapt to moderate environments. Psychrotrophs like cooler environments but those above freezing. Psychrophiles like temperatures below freezing. Thermophiles prefer heat and are seen in hot springs and compost piles. Hyperthermophiles like extremely hot environments so they cannot be killed with an autoclave.

Many psychrophiles have characteristics, including antifreeze proteins, that protect it from extremes in temperatures. The lipids are hydrophobic and more flexible at low temperatures. Growth rates in these organisms are, however, quite slow. It should be noted that thermophiles and mesophiles can withstand freezing but of course do not