Madical brain Notes 10

  Why is microbiology important? To the lay person, microbiology means the study of sinister, invisible ‘bugs’ that cause disease. As a subject, it generally only impinges on the popular consciousness in newcoverage of the latest ‘health scare’. It may come as something of a surprise therefore to learn that the vast majority of microorganisms coexist alongside us without causing any harm. Indeed, many perform vital tasks such as the recycling of essential elements, without which life on our planet could not continue. Other microorganisms have been exploited by humans for our own benefit, for instance in the manufacture of antibiotics and foodstuffs. To get some idea of the importance of microbiology in the world today, just consider the following list of some of the general areas in which the expertise of a microbiologist might be used:   ·                Medicine ·                environmental science ·                food and drink production fundamental research ·                agr

  How do we know? Microbiology in perspective: to the ‘golden age’ and beyond We have learnt an astonishing amount about the invisible world of microorganisms, particularly over the last century and a half. How has this happened? The penetrating insights of brilliant individuals are rightly celebrated, but a great many ‘breakthroughs’ or ‘discoveries’ have only been made possible thanks to some (frequently unsung) development in microbiological methodology. For example, on the basis that ‘seeing is believing’, it was only when we had the means to  see  microorganisms under a micro-scope that we could prove their existence. Microorganisms had been on the Earth for some 4000 million years, when Antoni van Leeuwenhoek started out on his pioneering microscope work in 1673. Leeuwen-hoek was an amateur scientist who spent much of his spare time grinding glass lenses to produce simple microscopes (Figure 1.1). His detailed drawings make it clear that the ‘animalcules’ he observed from a varie

  Light microscopy Try this simple experiment. Fill a glass with water, then partly immerse a pencil and observe from one side; what do you see? The apparent ‘bending’ of the pencil is due to rays of light being slowed down as they enter the water, because air and water have different  refractive indices . Light rays are similarly retarded as they enter glass and all optical instruments are based on this phenomenon. The compound light microscope consists of three sets of lenses (Figure 1.3): ·                the  condenser  focuses light onto the specimen to give optimum illumination ·                the  objective  provides a magnified and inverted image of the specimen ·                the  eyepiece  adds further magnification Most microscopes have three or four different objectives, giving a range of magnifica-tions, typically from 10 ×  to 100 × . The total magnification is obtained by multiply-ing this by the eyepiece value (usually 10 × ), thus giving a maximum magnification of 1

  Electron microscopy From the equation shown , you can see that if it were possible to use a shorter wave-length of light, we could improve the resolving power of a microscope. However, because we are limited by the wavelength of light visible to the human eye, we are not able to do this with the light microscope. The electron microscope is able to achieve greater magnification and resolution because it uses a high voltage beam of electrons, whose wavelength is very much shorter than that of visible light. Consequently we are able to resolve points that are much closer together than is possible even with the very best light microscope. The resolving power of an electron microscope may be as low as 1–2 nm, enabling us to see viruses, for example, and the internal structure of cells. The greatly im-proved resolution means that specimens can be meaningfully magnified over 100 000 × . where  λ  is the wavelength of the light source,  n  is the refractive index of the air or liquid between

  Atomic structure All atoms have a central, positively charged  nucleus , which is very dense, and makes up most of the mass of the atom. The nucleus is made up of two types of particle,  protons  and  neutrons . Protons carry a positive charge, and neutrons are uncharged,hence the nucleus overall is positively charged. It is surrounded by much lighter, and rapidly orbiting,  electrons  (Figure 2.1). These are negatively charged, the charge being equal (but of course opposite) to that of the protons, but they have only 1 / 1840 of the mass of either protons or neutrons. The attractive force between the positively charged protons and the negatively charged electrons holds the atom together. The number of protons in the nucleus is called the  atomic number , and ranges from 1 to over 100. The combined total of protons and neutrons is known as the  mass number . All atoms have an equal number of protons and electrons, so regardless of the atomic number, the overall charge on the atom wil