For each microorganism, there is a set of conditions (both physical and chemical) under which it can survive. Microbes have a variety of physical requirements for growth, including temperature, pH, and water stress.
Temperature
Microbes can be separated into groups based on the range of temperatures at which they can survive. At the edges of each range, microbes can usually survive but not thrive, whereas the perfect conditions for growth are usually somewhere in the middle.
There is also a lot of gray area between these groups because not all microbes in each group are the same. For instance, some psychrophiles can survive at 0 degrees C but prefer 15 degrees C, and others prefer 30 degrees C, bringing them almost into the mesophile group.
Here are the various groups of microbes based on their physical requirements:
Psychrophiles are microbes that can grow at 0 degrees C. Some are inhibited at higher temperatures, preferring to live in cold climates, whereas others can survive in conditions above 20 degrees C. This latter group are called psychrotrophs because they prefer colder temperatures but can live just fine in higher ones.
The range for psychropiles is –10 degrees C to 20 degrees C, with an optimum at about 10 degrees C. The range for psychrotrophs is from 0 degrees C to 40 degrees C with an optimum of about 20 degrees C.
Mesophiles like it best between 25 degrees C and 40 degrees C but can survive between 10 degrees C and 50 degrees C. Microbes that live within animals grow optimally at a temperature that matches that of their host. For instance, microbes that live in the human body grow between 34 and 37 degrees C, which is body temperature.
Thermophiles can tolerate temperatures up to 70 degrees C and like it best between 50 degrees C and 60 degrees C. This group contains a subset considered hyperthermophilic, or extreme heat loving. All the known microorganisms in this category are archaea and some can even grow in temperatures above 120 degrees C, deep in the sea where the pressure stops water from boiling at that temperature.
pH
Another physical growth condition important to microorganisms is pH. The pH is the measure of how acidic or alkaline a solution is, with values from 0 to 14. Acidic environments include acid mine drainage, iron lakes, and the jar of pickles in your cupboard, with ranges between 1 and 6. Neutral pH is around 7. Alkaline, or basic pH, is 8 to 14.
Most bacteria prefer a pH range of 6.5 to 7.5, whereas fungi can grow in more acidic conditions, preferring pH 5 to 6. Some bacteria and archaea are acididophilic (acid loving); they grow in conditions far too acidic for other species.
Water stress
The last physical condition to consider is water stress, either from the concentration of solutes in the microbe’s surroundings or from drying. As more solutes such as salts or sugar are dissolved in water, the concentration of water to solutes goes down.
A microbial cell is permeable to water, so if the concentration of water is lower outside the cell than inside the cell, water will move out in order to balance the inside and outside solutions.
Too much water leaving the cell will kill it. This fact has been used to preserve meats and other foods, by either drying or curing them with salt or adding a lot of sugar (for example, honey and jam). On the other hand, environmental microbes have adapted to salty conditions; some grow quite well in low-salt environments, like seawater, or high-salt environments, like brine ponds.
One way bacteria have developed to deal with bad conditions is to transform themselves into endospores. The endospore is a dormant form of the original bacterial cell surrounded by a tough coating that makes it resistant to drying, as well as toxic compounds in its environment.