A few of the microorganisms most harmful to humankind are Staphylococcus aureus, Escheria coli, Enterococcus faecium and Pseudomonas aeruginosa. Microorganisms that cause disease are referred to as pathogens.
All bacteria, most protozoa and some fungi are microorganisms. The term "microorganisms" is used to indicate a life form that is not visible to the human eye without the aid of a microscope. These may be a single cell or made up of many cells. There are microorganisms in every type of environment on Earth, but there are more beneficial microorganisms than there are harmful ones. Genetic engineering involves manipulation of microorganisms to create specific characteristics in larger organisms.