Econstudentlog

Human Microbiology

By Simon P. Hardy. This is another one of those books from the Spring Sale at Stakbogladen (one of the university bookstores) that I got to one-fifth the normal price or so.

Some excerpts from the book:

i) “The variation in size of micro-organisms is not unlimited. The efficiency with which the organism can accumulate nutrients and dispose of waste material through the cytoplasmic membrane will restrict expansion. Many key metabolites (e.g. oxygen) pass passively through the cell wall and cell membrane into the cytosol. The surface area to volume ratio (SA/V) is the limiting factor for the extent to which passively diffusing molecules penetrate the cytosol. […] The physical packing of the nucleic acid […] and cytoplasmic components such as polysomes into a bacterial cell will also limit the minimum size achievable.”

ii) “Organisms growing within a fixed volume of culture media with no additional media added to the culture, is called a batch culture and is a closed system. Growing bacteria is a standard procedure in microbiology laboratories, hence the growth curve obtained [described in the previous section] is described in many microbiology texts. It is, however, artificial. Bacteria do not grow in such a closed system in vivo. There will be tremendous variation in the conditions that organisms find themselves in… […] When nutrients become scarce, bacteria not only reduce total metabolic activity but also synthesise proteins specifically designed to help the cell cope with starvation. In addition, bacteria alter cell wall structure so that the organism is more resistant to damaging chemicals. […] The significance of the stationary phase has been largely overlooked in preference to studies of growth rate, but there are implications for the transmission of bacteria to new hosts. Many bacteria are spread via contaminated water supplies, where nutrients are low and the antibacterial agents (chlorine) are employed to reduce bacterial numbers. These conditions are exactly opposite to those used to test the antibacterial activity of chlorine in the laboratory (actively growing bacteria cultured in nutrient-rich media).”

iii) “The bacteria that colonise or infect man are mesophiles; that is, have optimal growth temperatures between 20 and 40°C […]. Thermophiles are those organisms that grow at elevated temperatures (a good example being those found in thermal lakes) and are not known to infect man. Psychrophiles are those that grow at reduced temperatures below 20°C. These labels are not mutually exclusive. Listeria monocytogenes, for example, is a mesophilic organism that can cause infections in man, but can grow at 4°C…

iv) [Aerobes] are organisms that grow in the presense of atmospheric concentrations of oxygen. Strict aerobes will not grow in the absence of oxygen. […] Microaerophiles need reduced concentrations of oxygen (reduced oxygen tension) in order to grow and will not grow in air nor in the complete absense of oxygen. […] Obligate or strict anaerobes will not grow in the presence of very low concentrations of oxygen and many will also be killed. Different genera of anaerobic bacteria show a range of oxygen sensitivity. Anaerobes are unable to utilise oxygen for respiration and therefore will not grow in the presence of oxygen. Certain anaerobes may tolerate exposure to oxygen for a period, so a distinction has to be made between oxygen killing organisms and oxygen just inhibiting their growth.”
Facultative anaerobes are organisms that will grow in air but can also grow in anaerobic conditions. […] Facultative anaerobes will be the best equipped to deal with varying oxygen tensions, whereas the strict aerobes and strict anaerobes can be considered specialists that have adapted to particular gaseous environments.”

v) “Bacteria that live on and infect man grow best at pH 7 (neutral pH) and may be described as neutrophiles. Acidophiles prefer low pH, less than pH 6, whereas alkaliphiles grow at alkaline pH values over 8. Most organisms will tolerate a range of pH values that extend either side of their pH optimum as a bell-shaped curve (although the shape of the plot need not necessarily be symmetrical). The organisms will possess adaptive mechanisms with which to deal with the limits of tolerance, not least because the organisms themselves will force pH changes through their production of acids or bases as metabolic waste products.”

vi) “In contrast to the degree of detail that has been worked out concerning bacterial metabolic pathways, the routine culture of bacteria in laboratories is often little short of mysticism. The use of culture media with defined ingredients is rarely necessary when cheaper, undefined or semi-defined media will suffice. The media used to culture organisms must be considered highly artificial in comparison to the nutritional conditions encountered by organisms when growing in the natural environment, be that in the environment or on man.”

vii) “One difficulty that undermines our confidence in controlling bacteria and viruses is the definition of death in microbes. Death in bacteria and viruses is a retrospective diagnosis. A bacterium is defined as dead when it cannot be grown. If an organism fails to grow when cultured in a broth or on a plate, having previously been successfully cultivated, then we can say it is dead. Or we can say that we failed to grow the organism in the correct conditions. Because it is impossible to prove a negative event (absence of growth) there is always the worry that we have not killed the organism but simply failed to grow it. The reasons for organisms not growing in laboratory culture media are considerable… […]

A large number of compounds including chemical disinfectants and antibiotics have been identified that can be used in controlling the multiplication of bacteria. Although the mode of action will differ between different compounds, it is helpful to distinguish between whether they act by killing bacteria, in which case they are termed bactericidal or only inhibit bacterial proliferation rather than kill the organism, when they are described as bacteriostatic. […] When exposed to a lethal agent not all of the bacteria in the culture die immediately but, instead, a proportion of the total will be killed per unit time. […] In other words, the more organisms there are, the more organisms are killed.”

viii) “Most non-sporing bacteria (i.e. the vegetative form) are killed when heated to 60°C. Yeasts and fungi need temperatures over 80°C. Bacterial endospores, however, are only killed to any significant extent when held at temperatures above 100°C for over 5 minutes. Bacterial endospores, therefore, pose the greatest problems in obtaining sterility. Because water acts as a better conductor of heat than air, the transfer of energy into the microbe is achieved more efficiently when the organisms are heated in moist or wet conditions…”

And so on. Lots of good stuff here. Some of it is quite hard (probably in part because I don’t remember HS chemistry all that well – but also because a lot of background medical knowledge is assumed throughout the book, some of which I have obtained elsewhere – and some of which I have not..)

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August 6, 2011 - Posted by | Biology, Books, Infectious disease, Medicine, Microbiology

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