Microbial physiology is the study of the function and biology of microbes. All physiology is the study of living organisms and their parts, so the microbial variation investigates the relationship of structure and function in microorganisms like bacteria. The metabolism of microbes and their growth and reproduction is of interest to the microbial physiologist as well. A particular area of interest is the way these organisms behave under stress and the mechanisms through which they adapt to change.
Microbe is a general term for organisms too small to see. Microbes include bacteria, fungi, archaea (organisms which superficially resemble bacteria), protista (including algae and slime molds), and others. The word microbe comes from the Greek mikros, meaning small, combined with bios, life.
In practice though, microbial physiologists frequently study bacteria, or bacterial relationships with other organisms. The structure and function of bacteria interact, of course. The construction of a bacterium directs its function a certain way, and a certain function, through time, affects structure.
Microbial metabolism is the way microorganisms use energy and nutrients. Microorganisms can be classed according to where they get the carbon they use to build their bodies and where they get the energy they need to power their functions. For example, all oxygen-using photosynthetic bacteria are cyanobacteria, while all anoxygenic photosynthesizing bacteria are purple bacteria.
Growth is another interest of this discipline. In culture, there are four stages in the growth of a colony of bacteria. The first stage is the lag stage. In this phase the bacteria are adapting to circumstances. They ingest nutrients and mature. In the second phase, the log phase, the size of the colony will double and redouble in whatever time period the bacteria need to mature, because bacteria reproduce by splitting in half, which doubles their numbers. This is called the log phase because it produces exponential growth, with each succeeding generation that is old enough to split. The next phase is the stationary phase. The food is depleted, and the colony is polluted with waste. Growth is impossible. In the death phase, the colony dies.
Cell division, reproduction, is another area of interest in microbial physiology. The process of binary fission in bacteria and the way the genome is maintained or altered through generations is studied. Because the daughter cell of an organism that has merely split in half ought to be identical to the parent, science is interested in the way the bacterial genome changes in changed circumstances.
Some bacteria have incorporated bits of genetic code from
bacteriophages, viruses that have invaded them, into their genome. For example deadly E. coli 0157:H7 is an evolution of harmless Escherichia coli that incorporated some of the DNA of a phage and acquired the ability to make a toxin when it did. Another way bacterial genomes change is through bacterial conjugation, in which genetic material is changed through direct contact between microorganisms. This is a form of horizontal gene transfer and may actually be fairly common in nature. It is a factor in antibiotic resistance, as resistant organisms that have survived the onslaught of a certain antibiotic may transfer their resistance to organisms which have never encountered the drug.
Microbial physiology has brought many practical insights to the field of food science, protecting and enhancing the food supply. It is useful in industry as well, where a surprising number of products are made with the help of microorganisms. It has also helped enormously in the still young field of biotechnology, where new cures are sought for diseases old and new.