Microbial Growth Cycle
Bacterial requirements for growth include sources of energy such as organic carbon molecules and metal ions . Additionally, a specific temperature, pH, and the need (or lack of need for oxygen) are important for optimal growth. All microbial metabolisms can be arranged according to three principles: 1) How the organism obtains carbon for synthesizing cell mass. 2) How the organism obtains reducing equivalents used either in energy conservation or in biosynthetic reactions. 3) How the organism obtains energy for living and growing (for more detail on this topic see atom on Growth Terminology). Unlike in multicellular organisms, increases in cell size (cell growth and reproduction by cell division) are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission which is a form of asexual reproduction. Under optimal conditions, bacteria can grow and divide extremely rapidly. These optimal conditions are discussed below.
Different kinds of bacteria need different amounts of oxygen to survive, which determines which bacteria can infect which parts of the body. Obligate aerobes must grow in the presence of oxygen. They cannot infect the skin because oxygen is present. Conversely, obligate anaerobes are killed by oxygen and carry out fermentation. Tetanus is an obligate anaerobe so it will infect areas where oxygen in limited. Aerotolerant anaerobes breath anaerobically (without oxygen), but can survive in the presence of oxygen. Facultative anaerobes can perform both fermentation and aerobic respiration.
For microbial growth to process, microorganisms require certain nutrients including carbon, nitrogen, phosphorus, sulfur, and metal ions.
Various types of bacteria thrive at different temperatures. Microorganisms that grow best at moderate temperatures are called mesophiles. Those surviving at high temperatures are thermophiles and microorganisms surviving at very low temperatures are called psychrophiles.
Many bacteria grow best at neutral pH. However, certain bacteria can survive and even grow in quite acid or alkaline conditions.