ABSTRACT
A microorganism or microbe can be dened as a living organism so small that it can only be observed through a microscope. Microorganisms consist of single cells or clusters of cells. A microbial cell is the fundamental unit of life; it maintains the structure of the microbe by taking up chemicals and energy from the environment and by responding to stimuli from its surroundings. Microbes reproduce and pass on their genetic makeup to their offspring, and evolve and adapt to the environment. Single-celled microorganisms were the rst life form to appear on earth approximately 3.5 billion years ago [1] and over the past 1.5 billion years there has been a tremendous diversication of life, culminating in complex multicellular organisms such as plants and animals. In many cases, microorganisms have evolved to live in intimate symbiotic relationships with other organisms, including plants and animals. Human beings, for example, have mutualistic relationships with many bacteria that live in the human gut. In other extreme cases, through evolution, microbes have been assimilated by and become part of other cells; many recent studies have provided hard evidence that chloroplasts and mitochondria were once free-living bacteria that were incorporated into eukaryotic cells to become energy-producing organelles [1].
