Airborne transmission influenza
Influenza is a disease of the respiratory tract caused by members of the Orthomyx-oviridae. Transmission occurs as a result of inhaling airborne respiratory droplets from an infected individual. Infection by the influenza virus results in the destruction of epithelial cells of the respiratory tract, leaving the host open to secondary infections from bacteria such as Haemophilus influenzae and Staphylococcus aureus. It is these secondary infections that are responsible for the great majority of...
Microorganisms and food
To the general public, the association of microorganisms and food conjures up negative images of rotten fruit or food poisoning. On reflection, many people may remember that yeast is involved in bread and beer production, but how many realise that microorganisms play a part in the manufacture of soy sauce, pepperoni and even chocolate In the following pages, we shall look at the contribution of microorganisms to the contents of our shopping baskets before considering one of the negative...
The zooflagellates Mastigophora
Members of this, the biggest and most primitive group of protozoans, are characterised by the long flagellum (mastigos 'a whip'), by which they propel themselves around. Although typical zooflagellates have a single flagellum, some types possess several. The prefix 'zoo-' distinguishes them from plant-like flagellates such as Euglena, but as we have already mentioned, such a distinction is not necessarily warranted on molecular and structural grounds (see the end of this chapter). Figure 9.11...
Antifungal and antiviral agents
Lies the problem anything that damages fungal cells is likely to damage human cells too. Polyene antibiotics such as amphotericin and nystatin (both produced by species of Streptomyces) act on the sterol components of membranes their use is limited, because human cells can also be affected by their action (to use a term we learnt earlier in this chapter, they have a low therapeutic index). Nystatin is used topically against Candida infections, while amphotericin B is generally used against...
Beyond the cell wall
A number of structural features are to be found on the outer surface of the cell wall these are mainly involved either with locomotion of the cell or its attachment to a suitable surface. Perhaps the most obvious extracellular structures are flagella (sing flagellum), thin hair-like structures often much longer than the cell itself, and used for locomotion in many bacteria. There may be a single flagellum, one at each end, or many, depending on the bacteria concerned (Figure 3.10). Each...
Transmission by water or food viral gastroenteritis
Everybody must surely be familiar with the symptoms of gastroenteritis - sickness, diarrhoea, headaches and fever. The cause of this gastroenteritis may be bacterial (e.g. Salmonella) or viral. The major cause of the viral form is the human rotavirus , which, together with the Norwalk virus, is responsible for the majority of reported cases. The rotavirus has a segmented, dsRNA genome, and is a non-enveloped virus. The virus damages the villi in the upper part of the intestinal tract, affecting...
Fungi and disease
A limited number of fungi are pathogenic to humans (Table 8.1). Mycoses (sing mycosis) in humans may be cutaneous, or systemic in the latter, spores generally enter the body by inhalation, but subsequently spread to other organ systems via the blood, causing serious, even fatal disease. Cutaneous mycoses are the most common fungal infections found in humans, and are caused by fungi known as dermatophytes, which are able to utilise the keratin of skin, hair or nails by secreting the enzyme...
Products derived from genetically engineered microorganisms
In Chapter 12 we saw how recombinant DNA technology can be used to genetically modify microorganisms so that they produce commercially important proteins such as human insulin. This is done by incorporating the gene for the desired protein into an PRODUCTS DERIVED FROM GENETICALLY ENGINEERED MICROORGANISMS 419 PRODUCTS DERIVED FROM GENETICALLY ENGINEERED MICROORGANISMS 419 Figure 17.7 A continuous flow stirred tank reactor. Parameters such as pH and concentrations of specific metabolites are...
Microbial Nutrition and Cultivation
In Chapter 2 we introduced the major groups of macromolecules found in living cells the raw materials from which these are synthesised are ultimately derived from the organism's environment in the form of nutrients (Table 4.1). These can be conveniently divided into those required in large quantities* (macronutrients) and those which are needed only in trace amounts (micronutrients or trace elements). You will recall that carbon forms the central component of proteins, carbohydrates, nucleic...
Acids bases and pH
Only a minute proportion of water molecules, something like one in every 5 x 108, is present in its dissociated form, but as we have already seen, the H+ and OH- ions play an important part in cellular reactions. A solution becomes acid or alkaline if there is an imbalance in the amount of these ions present. If there is an excess of H+, the solution becomes acid, whilst if OH- predominates, it becomes alkaline. The pH of a solution is an expression of the molar concentration of hydrogen ions...
Basidiomycota
This large group of some 25 000 species contains the true mushrooms and toadstools as well as other familiar fungi such as puffballs and bracket fungi. In fact the great majority of the fungi that we see in fields and woodlands belong to the Basidiomycota. They are of great economic importance in the breakdown of wood and other plant material (Chapter 16). The group derives its common name of the club fungi from the way that the spore-bearing hyphae involved in reproduction are swollen at the...
Phylum Chlamydiae
Formerly grouped with the Rickettsia (see above), these non-motile obligate parasites of birds and mammals are now assigned a separate phylum comprising only five genera, of which Chlamydia is the most important. Like the Rickettsia, members of the Chlamydiae have extremely small cells, and very limited metabolic capacities, and depend on the host cell for energy generation. Unlike that group, however, they are not dependent on an arthropod vector for transmission from host to host. Chlamydia...
Phylum Firmicutes The low GC Grampositive bacteria
The low GC Gram-positive bacteria form volume 3 of the second edition of Bergey. The spore-forming Gram-positive bacteria include two large genera, Clostridium and Bacillus. Although not particularly close in phylogenetic terms, they are both capable of propagation by endospores. Clostridium species are obligate anaerobes, and common inhabitants of soil. Sugars are fermented to various end-products such as butyric acid, acetone or butanol. Lacking an electron transport system, they obtain all...
Airborne transmission strep throat
Streptococcal pharyngitis, commonly known as strep throat, is one of the commonest bacterial diseases of humans, being particularly common in children of school age. The primary means of transmission is by the inhalation from coughs and sneezes of respiratory droplets containing Streptococcus pyogenes (p-haemolytic type A streptococci), although other routes (kissing, infected handkerchiefs) are possible. The primary symptoms are a red and raw throat (and or tonsils), accompanied by headaches...
Chemical bonds
The force that causes two or more atoms to join together is known as a chemical bond, and several types are found in biological systems. The interaction between sodium and chloride ions shown in Figure 2.4 is an example of ionic bonding, where the transfer of an electron from one party to another means that both achieve a complete outer electron shell. There is an attractive force between positively and negatively charged ions, called an ionic bond. Certain elements form ions with more than a...
Carcinogenicity testing the Ames test
The great majority of carcinogenic substances, that is, substances that cause cancer in humans and animals, are also mutagenic in bacteria. This fact has been used to develop an initial screening procedure for carcinogens instead of the expensive and time-consuming process of exposing laboratory animals not to mention the moral issues involved , a substance can be tested on bacteria to see if it induces mutations. The Ames Test assesses the ability of a substance to cause reverse mutations in...
Estimation of microbial numbers
Several methods exist for the measurement of bacterial numbers, most of which are also applicable to the enumeration of other unicellular forms such as yeasts. Such methods fall into two main categories those that count total cell numbers, and those that count viable cells only. Total cell counts are generally done by direct microscopic examination. A specialised glass slide is employed, which carries an etched grid of known area Figure 5.1 . The depth of the liquid sample is also known, so by...
