How do microbes survive?

Microbes may be invisible to the unaided eye, but they form an integral part of every ecosystem on Earth: grasslands, forests, tundra, deserts, marine and freshwater habitats.

In Step 1.11, you learned that microbes need to get specific compounds from their environment to survive, grow and reproduce. Different species of microbe use different combinations of organic and inorganic compounds, but these compounds are unevenly distributed across the planet. Nutrient availability is therefore one of the key factors that determines where a microbe can live.

In addition to nutrient availability, there are a range of other factors that affect where a microbe can live. Every species of microbe has evolved adaptations that enable them to live under specific environmental conditions, which we refer to as their niche. These include oxygen (O₂) concentration, temperature, pH, light intensity, radiation, pressure, solute concentration (osmolarity) and water activity. In this Step, we consider why oxygen and temperature affect where microbes live.

O₂ concentration varies between 20% in air (fully aerobic conditions) to 0% in fully anoxic conditions. Aerobes use O₂ as the final electron acceptor in the electron transport chain during aerobic respiration, which converts O₂ to H₂O. Some aerobes die when the O₂ supply gets too low, but others survive by generating ATP via fermentation (this does not use an electron transport chain and is less efficient than aerobic respiration).

Other microbes use alternative electron acceptors such as sulphate or nitrate in the electron transport chain (anaerobic respiration) and do not need oxygen at all. In fact, some anaerobes are killed by oxygen because they cannot detoxify reactive oxygen species (ROS). These highly reactive chemicals, including superoxide (O₂-), hydrogen peroxide (H₂O₂) and hydroxyl radicals (O₂•), are generated as a by-product of metabolism and cause damage to cellular components, including DNA and membranes.

The bacterium Escherichia coli (Figure 1) is a facultative anaerobe. In the laboratory it grows best under fully aerobic conditions at 37°C, but it is more at home in the nutrient-rich intestines of warm-blooded animals. This is an anoxic environment because facultative anaerobes rapidly use up any available oxygen.

Figure 1: Scanning electron microscope (SEM) image of Escherichia coli © CDC/ National Escherichia, Shigella, Vibrio Reference Unit at CDC

Microbes are only able to grow within a certain temperature range and grow best at an optimum temperature. This is because temperature affects the activity of enzymes and the function of cell membranes. If the temperature is too low, enzymes are inactive, membranes solidify (this prevents the exchange of nutrients) and metabolism shuts down. As the temperature increases enzymes become more active: metabolic reactions speed up and cells replicate faster. Above a certain temperature, proteins (including enzymes) are damaged (denatured) and no longer function and membranes fall apart. Organisms that thrive in extreme temperatures (either very hot or very cold) have evolved special adaptations to tackle these problems. We will meet some extremophiles, microbes that live in extreme conditions later in the course.

Pseudomonas fluorescens is a strict aerobe that lives in soil and on the surfaces of plant roots and leaves, where temperatures can range from sub-zero in winter to over 30°C in the height of summer. It grows fastest at the optimum temperature of 27°C, but it can grow slowly at much lower temperatures. It uses a tail-like structure called the flagellum to swim (Figure 2) towards oxygen and other nutrients (chemotaxis). This provides it with an advantage as soil can quickly become nutrient limited due to competition with other microbes.

Figure 2: Left: Pseudomonas fluorescens is able to swim over the surface of a motility agar plate. Right: A mutant strain of Pseudomonas fluorescens that is unable to swim (due to deletion of the flagellar master regulator FleQ) © University of Reading

You can download a summary of the main environmental conditions that determine where a species of microbe can live in this additional resource PDF.