9. Microbial Growth and Biosynthesis
9.7 Other Environmental Conditions that Affect Growth
Learning Objectives
- Identify and describe different categories of microbes with specific growth requirements other than oxygen, pH, and temperature, such as altered barometric pressure, osmotic pressure, light and nutrient levels
- Give at least one example microorganism for each category of growth requirement
Microorganisms interact with their environment along more dimensions than pH, temperature, and free oxygen levels, although these factors require significant adaptations. We also find microorganisms adapted to varying levels of salinity, barometric pressure, humidity, light and nutrient concentrations.
Osmolarity
All life depends on available water to grow. Available moisture is measured as water activity (aw), which is the ratio of the vapour pressure of the medium of interest to the vapour pressure of pure distilled water; therefore, the aw of water is equal to 1.0. Bacteria require high aw (0.97–0.99), whereas fungi can tolerate drier environments; for example, the range of aw for growth of Aspergillus spp. is 0.8–0.75.
Most natural environments tend to have lower solute concentrations than the cytoplasm of most microorganisms. Rigid cell walls protect the cells from bursting in a dilute environment. Not much protection is available against high osmotic pressure. In this case, water, following its concentration gradient, flows out of the cell. This results in plasmolysis (the shrinking of the protoplasm away from the intact cell wall) and cell death. This fact explains why brines and layering meat and fish in salt are time-honoured methods of preserving food. Microorganisms called halophiles (“salt loving”) actually require high salt concentrations for growth. These organisms are found in marine environments where salt concentrations hover at 3.5%. Extreme halophilic microorganisms, such as the red alga Dunaliella salina and the archaeal species Halobacterium, grow in hypersaline lakes such as the Great Salt Lake (3.5–8 times saltier than the ocean) in Utah (Figure 9.40) and the even saltier Dead Sea (10 times saltier than the ocean).
Dunaliella spp. counter the tremendous osmotic pressure of the environment with a high cytoplasmic concentration of glycerol and by actively pumping out salt ions. Halobacterium spp. accumulate large concentrations of K+ and other ions in their cytoplasm. Their proteins are designed for high salt concentrations and lose activity at salt concentrations below 1–2 M. Although most halotolerant organisms, for example Halomonas spp. that grow in salt marshes, do not need high concentrations of salt for growth, they will survive and divide in the presence of high salt. Not surprisingly, the staphylococci, micrococci, and corynebacteria that colonize our skin tolerate salt in their environment. Halotolerant pathogens are an important cause of food-borne illnesses because they survive and multiply in salty food. For example