دانلود کتاب اصول میکروبیولوژی – ویرایش چهارم

Microbiology is a specialized area of biology that deals with living things ordinarily too small to be seen without magnification. Such microscopic organisms are collectively referred to as microorganisms (my″- kroh-or′-gun-izms), microbes, or several other terms depending on the kind of microbe or the purpose. There are several major groups of microorganisms that we’ll be studying. They can be either cellular or noncellular. The cellular microorganisms we will study are bacteria, archaea, fungi, and protozoa. Another cellular organism that causes human infections is not technically a microorganism. Helminths are multicellular animals whose mature form is visible to the naked eye. Acellular microorganisms causing human disease are the viruses and prions. Table 1.1 gives you a first glimpse at these microorganisms. There is another very important group of organisms called algae. They are critical to the health of the biosphere but do not directly infect humans, so we will not consider them in this book. Each of the other seven groups contains members that colonize humans, so we will focus on them. The nature of microorganisms makes them both very easy and very difficult to study—easy because they reproduce so rapidly and we can quickly grow large populations in the laboratory, and difficult because we usually can’t see them directly. We rely on a variety of indirect means of analyzing them in addition to using microscopes. For billions of years, microbes have extensively shaped the development of the earth’s habitats and the evolution of other life forms. It is understandable that scientists searching for life on other planets first look for signs of microorganisms. Single-celled organisms appeared on this planet about 3.8 billion years ago according to the fossil record. One of these organisms—referred to as LCA, or the Last Common Ancestor—eventually led to the appearance of two newer single cell types, called bacteria and archaea. A little bit later this singlecelled ancestor gave rise to eukaryotic (yoo-kar′-ee-ot-ic) cells. The type of cell known as LCA no longer exists. Only its “offspring”—bacteria, archaea, and eukaryotes—remain. Eu-kary means “true nucleus,” and these were the only cells containing a nucleus. Bacteria and archaea have no true nucleus. For that reason, they have traditionally been called prokaryotes (pro-kar′-ee-otes), meaning “prenucleus.” But researchers are suggesting we no longer use the term prokaryote to lump them together because archaea and bacteria are so distinct genetically. Some scientists have started calling them akaryotes, meaning “no nucleus.” If you consider the seven types of microorganisms we will be dealing with in this book, you will recognize bacteria and archaea as each having their own domain. The protozoa, fungi, and helminths are all in the domain Eukarya. Viruses and prions do not appear on the tree of life because they are not cells, and not considered living. That sounds strange, but we will delve into that in the virus chapter, which comes later.