A main difference between the multicellular organisms (such as humans, and other species) and unicellular organisms (like bacteria) is that growth and reproduction are very closely linked in unicellular organisms. Bacteria reproduce by asexually through binary fission, or by division of the cell. The result here is that the offspring is identical to the parent.
Growth of bacteria is usually interpreted as the increase in its population (although there is also some growth or increase in size prior to division, but being unicellular, the main 'growth' is interpreted as increase in population).
Basically, since bacteria are living organisms, one of the factors to be considered is the nutrient source. Initially, then, with a low population, growth will be fast due to abundance of food source. Upon reaching a point when the source becomes insufficient, however, the population decreases and the bacteria will decreases:
- Lag phase- phase of bacterial growth wherein the bacteria try to equilibrate with the environment. Cells start to increase in size as they intake nutrients and increase in size prior to division.
- Exponential phase - once equilibrated, the bacteria will experience a rapid increase in population due to rapid multiplication. The nutrient source is abundant and the intake is maximized.
- Stationary phase - the increase in population is accompanied by an increase in accumulated waste material, which changes the environment such as the pH, usually a change unfavorable to bacteria. The nutrients are also consumed. The rate of growth (increase) is equal to rate of death.
- Death phase- further accumulation of waste and depletion of nutrients will start the death of the colony.
Bacteria can change over time due to mutations. Please note that I'm talking about bacteria as a whole, not a single bacterium. Although the organisms are unicellular and use mitosis as a means of reproduction (identical copies), mutations can happen during DNA replication. Some forms of DNA mutations include deletion, insertion, etc. When mutations in the DNA do occur, new traits are introduced to the population.
An example of a mutation benefitting and changing the species are antibiotic-resistant bacteria. Although a population of bacteria may have experienced detrimental effects when introduced to a certain antibiotic, a simple mutation may cause one bacterium to be immune. As the other bacteria die out, this single bacterium begins to reproduce and create an antibiotic-resistant colony.