Introduction

Earth's biological world includes an enormous range of microscopic life -- bacteria, fungi, protozoa, certain algae, viruses, viroids and prions -- and these microbes make up a major share of everything living on the planet. While some microbes cause disease, a great many are genuinely beneficial, and human welfare depends on them in ways that go far beyond medicine: microbes exist virtually everywhere around us, in soil, water, air, and even within the bodies of animals and plants. Groups such as bacteria and fungi can be cultured on nutrient-rich media, where they multiply into colonies large enough to see with the naked eye, which is how scientists study and identify them. This chapter looks at a selection of the microbes that serve human welfare directly -- in the foods we eat, the products we use, and the environmental services they quietly perform.