Medical Microbiology

Over the centuries, thoughtful observers, such as Fracastoro and Bassi (see Table 1.2), noted a connection between microbes and disease. Ultimately, researchers developed the germ theory of disease, the theory that many diseases are caused by microbes. The first to establish a scientific basis for determining that a specific microbe causes a specific disease was If all life on Earth shares descent from a microbial ancestor, how did the first microbe arise? The earliest fossil evidence of cells in the geological record appears in sedimentary rock that formed as early as 3.8 billion years ago. Although the nature of the earliest reported fossils remains controversial, it is generally accepted that "microfossils" from over 2 billion years ago were formed by living cells. Moreover, the living cells that formed these microfossils looked remarkably similar to bacterial cells today, forming chains of simple rods or spheres (Fig. 1). The exact composition of the first environment for life is controversial. The components of the first living cells may have formed from spontaneous reactions sparked by ultraviolet absorption or electrical discharge. American chemists Stanley Miller (1930-2007) and Harold C. Urey (1893-1981) argued that the environment of early Earth contained mainly reduced compounds compounds that have a strong tendency to donate electrons, such as ferrous iron, methane, and ammonia. More recent evidence has modified this view, but it is agreed that the strong electron acceptor oxygen gas (02) was absent until the first photosynthetic microbes produced it. Today, all our cells are composed of highly reduced molecules that are readily oxidized (accept electrons from 02). This seem ingly hazardous composition may reflect our cellular origin in the chemically reduced environment of early Earth. In 1953, Miller attempted to simulate the highly reduced conditions of early Earth to test whether ultraviolet absorption or electrical discharge could cause reactions producing the fundamental components of life (Fig. 2A). Miller boiled a solution of water containing hydrogen gas, methane, and ammonia and applied an electrical discharge (comparable to a lightning strike). The electrical discharge excites electrons in the molecules and causes them to react. Astonishingly, the reaction produced a number of amino acids, including glycine, alanine, and aspartic acid. A similar experiment in 1961 by Spanish-American researcher Juan Or (1923-2004) (Fig. 2B) combined hydrogen cyanide and ammonia under electrical discharge to obtain adenine, a fundamental component of DNA and of the energy carrier adenosine triphosphate (ATP). How could early cells have survived the heat and chemically toxic environment of early Earth? Clues may be found in the survival of archaea that thrive under habitat conditions that we consider extreme, such as solutions of boiling sulfuric acid. The specially adapted structures of such microbes may resemble those of the earliest life-forms.