Characteristics of Microorganisms Lecture Notes

Characteristics of microorganisms By Dr. Thaera H. Fruka Microorganisms are essential for the maintenance of life on earth as part of the carbon and nitrogen cycles, for example; without them, dead animals and plants would not decompose and the fertility of soils would drop. Their passive ben...

Characteristics of microorganisms By Dr. Thaera H. Fruka Microorganisms are essential for the maintenance of life on earth as part of the carbon and nitrogen cycles, for example; without them, dead animals and plants would not decompose and the fertility of soils would drop. Their passive benefits include the protection afforded by probiotic (‘friendly’) bacteria, which compete with disease-causing species for nutrients and attachment sites on body tissue; they also limit opportunities for harmful bacteria to establish infections in the body by producing antimicrobial chemicals. Infectious diseases can be caused by agents which are not living microorganisms: prions are simply ‘rogue’ protein molecules, and viruses usually consist of nucleic acid and protein but have no cellular structure. Bacteria represent the simplest living cells. Most of those of pharmaceutical interest can be grown easily in the laboratory. Fungi and protozoa are more complex than bacteria and most of them can show sexual reproduction. Relatively few fungi are pathogenic; most are important as contaminants and spoilage organisms in manufactured medicines. Protozoa are only of pharmaceutical interest as pathogens; they are not spoilage organisms.  Viruses and prions Viruses are parasites that infect all types of organisms: animals, plants, protozoa and bacteria too. They vary a lot in size and structure, but all contain both nucleic acid and protein; the protein surrounds and protects the nucleic acid core, which may be single-stranded or double-stranded DNA or RNA (Figure 1) Figure 1: presents herpes simplex virus viewed under the electron microscope. The largest common viruses are about 300 nm in diameter (e.g. chicken pox virus) and the smallest about 20 nm (e.g. common cold virus), although some which are elongated (e.g. Ebola) figure 2, may be up to 1400 nm long but very narrow, so none of them can be seen with an ordinary laboratory microscope but only with an electron microscope. Figure 2: shows Ebola Virus All viruses can only grow inside a host cell and usually the range of hosts is very narrow – often just a single species; rabies is a notable exception. Because they cannot be grown on Petri dishes in the same way as bacteria, they are difficult, time consuming and expensive to cultivate in the laboratory using fertile chickens’ eggs or artificially cultured mammalian cells as hosts. Many viruses only survive for a few hours outside their normal host cell, but a few survive much longer and so may, in theory, be present as contaminants of pharmaceutical raw materials of animal origin. However, viruses are relatively susceptible to heat and organic solvents so they are unlikely to arise in materials like gelatin, for example, because of the Some of the larger viruses have been studied as vectors (carriers) to deliver genes to cells, as in gene therapy for cystic fibrosis for example, but, in general, viruses are, like protozoa, important primarily as pathogens. Although viruses possess genes coding for enzymes to be made by the host cell, the majority contain few, if any, enzymes as part of their structure. One consequence of this is that they are unaffected by the antibiotics used to treat bacterial and fungal infections. This does not mean, though, that it is not possible to create antiviral drugs. One consequence of the HIV/AIDS pandemic is that the number of synthetic antiviral drugs on the UK market increased from fewer than 10 in the mid-1980s to more than 40 by 2010. Prions represent the simplest infectious agents, which, despite the fact that they are definitely nonliving, are nevertheless usually considered with microorganisms because of their capacity to transmit disease from one person to another. They are similar to viruses in that they have no cellular structure, but differ in that they do not even possess nucleic acids. They are merely atypical mammalian proteins that have the capacity to interact with normal proteins and induce structural changes so that the normal molecules are, in turn, changed into prions that are incapable of fulfilling their normal function. Prions are responsible for fatal, nerve-degenerative diseases termed transmissible spongiform encephalopathies, such as bovine spongiform encephalopathy (BSE; ‘mad cow disease’) in cattle, and Creutzfeldt–Jakob disease (CJD) in humans. They are particularly stable and difficult to inactivate by disinfectants, gamma-radiation and even by steam-sterilization conditions that far exceed those required to kill the most heat resistant spore forming bacteria.  Bacteria Bacteria are responsible for a wider range of diseases than protozoa or fungi, and they were discovered at least 200 years before viruses. For those reasons, and because most of them can easily be grown in the laboratory, bacteria were the most widely studied group of microorganisms throughout much of the nineteenth and twentieth Typically, they are spherical or rod-shaped cells about 1–10 mm in their longest dimension, so when suitably stained they can easily b e seen with an ordinary light microscope (Figure 1). Compared to human cells, bacteria are quite robust: they have a cell wall which protects them against rapid changes in osmotic pressure. Many bacteria will easily survive transfer into water from the relatively high osmotic pressure at an infection site in the body, whereas human cells, without a wall to protect them, would rapidly take in water by osmosis, burst and die. Bacteria are also more tolerant than human cells of wide variations in temperature and pH, and will withstand exposure to higher intensities of ultraviolet light, ionizing radiation and toxic chemicals. Figure 1: presents the common bacterium Escherichia coli (E. coli) Compared to human cells, bacteria are quite robust: they have a cell wall which protects them against rapid changes in osmotic pressure. Many bacteria will easily survive transfer into water from the relatively high osmotic pressure at an infection site in the body, whereas human cells, without a wall to protect them, would rapidly take in water by osmosis, burst and die. Bacteria are also more tolerant than human cells of wide variations in temperature and pH, and will withstand exposure to higher intensities of ultraviolet light, ionizing radiation and toxic chemicals The recap box above highlights the major differences between bacteria (prokaryotes) and eukaryotes, but from a pharmaceutical perspective it is particularly relevant to contrast bacteria with mammals and consider the implications of their differences for the avoidance of microbial contamination and the treatment of infectious diseases. Two of the most fundamental distinctions are that bacteria reproduce asexually whereas mammals exhibit sexual reproduction, and that bacteria may reproduce in as little as 20 minutes but mammalian cells take many hours or days to divide. The cell-division process in bacteria is termed binary separation; this simply involves the chromosome being copied, and the cell enlarging. One copy of the chromosome, together with half the cell contents, becomes separated from the other by the formation of a cross-wall in the cell; a constriction may form which eventually causes the two so-called ‘daughter cells’ to separate. One bacterial cell doubling every 20 minutes can become over 16 million within 8 hours, and such a large number of cells located together on a Petri dish may become visible to the naked eye as a bacterial colony (Figure 2). Figure 2: seen a large number of cells located together on a Petri dish The consequence of bacteria reproducing asexually is that they are much more reliant on mutations as a means of producing genetic variation, and the fact that they grow so rapidly means that a mutant can quickly be selected and become the dominant cell type in the population. However, bacteria grow more slowly at an infection site in the human body than on a Petri dish (because they are attacked by the immune system and have to compete for food and oxygen with the body cells) but nevertheless, it is quite possible for antibiotic-resistant mutants to be selected during a course of antibiotic treatment. The cell structures that are unique to bacteria may be both a benefit and a disadvantage. The cell wall, for example, protects not only against osmotic pressure changes but against drying; consequently, many bacteria survive for long periods in dust. However, the bacterial enzymes that make the cell wall polymers are the targets for a number of important antibiotics, such as penicillins, which achieve their selective toxicity (killing bacteria without harming the patient) simply because human cells do not make cell walls and do not have the enzymes.  The same situation applies with respect to ribosomes possessed by bacteria; these are structurally different from those of eukaryotic cells, so antibiotics like tetracyclines and erythromycin interfere with protein synthesis in bacteria but not in humans. Despite the fact that all bacteria conform to the general description of prokaryotes, they still differ significantly in terms of shape, size and complexity, and these variations have in the past caused problems with classification.  Chlamydia and rickettsia are both groups of small, pathogenic bacteria that are obligate, intracellular parasites (meaning that they can only grow within a host cell in a similar way to viruses), whilst mycoplasmas differ from most bacteria in that they do not have a cell wall so they are unaffected by penicillins and other antibiotics that interfere with cell wall synthesis.