Microbiology is the study of microorganisms. Microorganisms are all single-celled

microscopic organisms and include the viruses, which are microscopic but not cellular. Microbial cells differ in a fundamental way from the cells of plants and animals in that microorganisms are independent entities that carry out their life processes independently of other cells. By contrast, plant and animal cells are unable to live alone in nature and instead exist only as parts of multicellular structures, such as the organ systems of animals or the leaves of plants.

PENICILLIN :the first antibiotic discovered by accident. Alexander Fleming, a Scottish physician and bacteriologist, almost tossed out some culture plates that had been contaminated by mould. Fortunately, he took a secondLook at the curious pattern of growth on the contaminated plates.Around the mould was a clear area where bacterial growth hadbeen inhibited (Figure 1.5). Fleming was looking at a mold that could inhibit the growth of a bacterium.

The mold was later identifiedas Penicillium notatum (pen-i-sillē-um nō-tātum), laterrenamed Penicillium chrysogenum (krĪ-sojen-um), and in 1928Fleming named the mold’s active inhibitor penicillin. Thus, penicillinis an antibiotic produced by a fungus. The enormous usefulnessof penicillin was not apparent until the 1940s, when it wasfinally tested clinically and mass produced.Since these early discoveries, thousands of other antibioticshave been discovered. Unfortunately, antibiotics and otherchemotherapeutic drugs are not without problems. Many antimicrobialchemicals are too toxic to humans for practical use;they kill the pathogenic microbes, but they also damage the infectedhost. For reasons we will discuss later, toxicity to humansis a particular problem in the development of drugs for treatingviral diseases. Viral growth depends on life processes of normalhost cells. Thus, there are very few successful antiviral drugs,because a drug that would interfere with viral reproductionwould also likely affect uninfected cells of the body.Another major problem associated with antimicrobial drugsis the emergence and spread of new strains of microorganismsthat are resistant to antibiotics. Over the years, more and moremicrobes have developed resistance to antibiotics that at onetime were very effective against them. Drug resistance resultsfrom genetic changes in microbes that enables them to toleratea certain amount of an antibiotic that would normally inhibitthem (see the box in Chapter 26, page 757). For example a microbemight produce chemicals (enzymes) that inactivate antibiotics,or a microbe might undergo changes to its surface thatprevent an antibiotic from attaching to it or entering it.The recent appearance of vancomycin-resistant Staphylococcusaureus and Enterococcus faecalis (en-te-rō-kokkus fe-kālis)has alarmed health care professionals because it indicates thatsome previously treatable bacterial infections may soon be impossibleto treat with antibiotics.

In 1929 Alexander Fleming, a British biologist, inadvertently discovered penicillin. He had observed bacterial Staphylococci colonies disappearing in cultures that were contaminated with mold.Fleming eventually extracted the compound from the mold that had been responsible for destruction of the bacterial colonies. The product was named penicillin, after the Penicillium mold from which it was derived.