N. Shetty, E. Aarons, J. Andrews
Prokaryotic and eukaryotic cells Bacteria Sizes, shapes and arrangement of bacteria Phases of bacterial growth Viruses General properties Viral transmission Fungi Fungi of medical importance Protozoa Classification Helminths
Microbes and their habitats have held a peculiar fascination for mankind ever since Antony van Leeuwenhoek (1632-1723) recorded some of the most important discoveries in the history of biology. Once Leeuwenhoek succeeded in creating the simple microscope, he described bacteria, free-living and parasitic creatures, sperm cells, blood cells, microscopic nematodes and much more. His publications opened up an entire world of microscopic life for scientific study. Microbes continue to ecite intense research because of their virulence; their ability to cause tissue damage and death. They have been responsible for the great plagues and epidemics and have often changed the course of human history. The HIV pandemic has emerged as the single most defining occurrence in the history of infectious diseases of the late 20th and early 21st centuries. Microbes continue to baffle human ingenuity; they defy attempts at control by chemotherapeutic agents, vaccines, and the human immune system. The threat of a future pestilence is never far away.
In order to study the microbial etiology of infectious disease, an understanding of the basic principles of microbiology and their interaction with the human host are essential. There are four basic groups of microbes:
Bacteria
Viruses
Fungi: yeasts and molds
Protozoa.
Multicellular organisms such as helminths also need to be included in the broad description of infectious disease agents.
Prokaryotic and eukaryotic cells
The cell is the basic unit of life, whether it is of human or bacterial origin. Differences in bacterial (prokaryotic cells) and human (eukaryotic cells) have been eploited for diagnostic and treatment purposes. It is important to understand what these differences are and how they contribute to disease pathogenesis (Table 1.1).
Bacteria
Sizes, shapes and arrangement of bacteria
Bacteria are unicellular organisms, ranging from 0.4 µm to 2.0 µm in size. They eist broadly in one of three morphological forms, spheres (cocci), rods (bacilli), or spirals. All of these forms are subject to variation depending on eisting growth conditions. The morphology of a bacterium is maintained by a unique cell wall structure and it is the chemical nature of this cell wall that is eploited by the Gram staining technique (see Chapter 4). The Gram stain remains the single most important diagnostic test in the study of infection - dividing bacteria into two basic groups: Gram positive bacteria and Gram negative bacteria - thereby influencing the all too important decision: which antibiotic does the clinician use immediately and empirically before full microbiological results are available.
With the help of the Gram stain and a microscope it is possible to visualize the size (relative to a human red or white cell), the shape, the arrangement (if distinctive), and the Gram reaction of the bacterial cell. All the above features are important clues that help identify the infectious agent from a patient's clinical specimen.
Cocci are spherical or oval bacteria having one of several distinct arrangements based on their planes of division:
1 Division in one plane produces either a diplococcus (paired; Figure 1.1) or streptococcus (chain) arrangement.
If you were to Gram stain a smear of the specimen containing a putative diplococcus you would be able to see a Gram positive (purple) or -negative (pink) coccus in pairs; note the size relative to a polymorphonuclear leukocyte (Figure 1.2a and b). Streptococci, including medically important ones such as Streptococcus pyogenes, are Gram positive (Figure 1.3). The streptococci can be arranged in pairs (e.g. Streptococcus pneumoniae, Figure 1.2a) or in chains (e.g. S. pyogenes, Figure 1.3).
2 Division in random planes produces a staphylococcus arrangement. Note the Gram stained smear of a pus sample showing numerous polymorphs 'pus cells' and staphylococci: cocci in irregular, grape-like clusters (Figure 1.4). Ordered division in two or three planes can result in sarcinial arrangements (tetrads) respectively.
Bacilli are rod-shaped bacteria (Figure 1.5). Bacilli divide in one plane and are arranged singly or in chains as in Bacillus anthracis. For many clinically important bacilli the arrangement is not distinctive; some bacilli may be rounded off looking more coccoid; they are often called cocco-bacillary forms. Bacilli, like cocci, can be Gram positive or -negative (Figure 1.6)
Other common shapes of bacteria are: curved bacteria as in Campylobacter (Figure 1.7) and Vibrio species; Spirillum species have thick rigid spirals; and spirochaete forms such as Leptospira species have fleible spirals (Figure 1.8). Spirals range in size from 1 µm to over 100 µm in length.
It is worth remembering that not all bacteria stain with the Gram stain. In Chapter 2 we will discuss organisms that do not take up the Gram stain readily, those that do not have typical bacterial cell wall structures or arrangements, and those that are obligate intracellular microorganisms.
Phases of bacterial growth
When an organism is inoculated into suitable media such as a liquid culture medium in the laboratory or if it were to encounter a susceptible human/animal host it will ehibit a growth curve (Figure 1.9).
In the lag phase the microorganism adapts to a new and often more favourable environment. During this phase, there is a marked increase in enzymes and intermediates, in preparation for active growth. The lag phase is a period of adjustment necessary for the accumulation of metabolites until they are present in concentrations that permit cell division to resume.
In the eponential or logarithmic phase, cells are in a state of balanced growth. The cells increase in number and there is a logarithmic epansion of mass and volume. In other words imagine a single cell dividing into two, each further divides in a binary manner and two becomes four, four to eight, eight to siteen, and so on. A steady state is reached where one of many factors come into play; either essential nutrients become ehausted, there is accumulation of waste products, change in pH, induction of host immune mechanisms and other obscure factors eert a deleterious effect on the culture, and growth is progressively slowed.
During the stationary phase, accumulation of toic products or ehaustion of nutrients causes net growth to cease. The viable cell count remains constant. The formation of new organisms equals the death of organisms in the system. The stationary phase is important to the clinical microbiologist as microbial toins, antimicrobial substances and other proteins such as bacteriocins and lysins accumulate to significant levels at the end of the stationary phase. Thus, they affect not only growth in the laboratory but also pathogenesis of disease in the host.
As factors detrimental to the bacteria accumulate, more bacteria are killed than are formed. During the phase of decline there is a negative eponential phase, which results in a decrease in the numbers of viable bacteria within the system.
Viruses
General properties
Of all the agents infectious to man, and indeed to other living things, viruses are the smallest. (See Bo 1.1 for a description of prions.) An individual infectious unit, comprising a nucleic acid genome, packaged inside a protein coat with or without a surrounding lipid-containing envelope membrane, is known as a viral particle or virion (Figure 1.10).
Only the very largest of these, the poviruses measuring up to 400 nm in their longest dimension and the even larger mimivirus, can be visualized with a light microscope. Cell-free, intact virions are entirely metabolically inert: they cannot be said to be "alive" at all. Yet, on entering an appropriate cell (which is anything but a matter of chance proimity), the cellular machinery is hijacked and diverted towards production of new viral particles. Normal cellular function may be disrupted to a greater or lesser etent and the consequences of this may be manifest as disease.
As viruses can replicate themselves only inside living prokaryotic or eukaryotic cells, they have evolved alongside cellular organisms and specific viral infections are known in mycoplasma and other bacteria, algae, fungi, plants, and animals. In that certain viruses depend on co-infection of their cellular targets with other viruses, viruses might be said to even parasitize each other (e.g. hepatitis D virus cannot replicate in the absence of hepatitis B virus (HBV) because it requires the HBV protein coat for the packaging of its own nucleic acid).
The host range for a given virus may be relatively broad (e.g. influenza A, which can infect ducks, chickens, pigs and horses as well as humans), or etremely narrow (e.g. measles, which infects only humans). This is known as host-specificity. Within a multicellular host, a particular virus may be able to infect many types of cell (e.g. Ebola virus) or be restricted to the cells of only certain tissues. This phenomenon is known as tissue tropism. Both host-specificity and tissue tropism are determined largely by the molecular properties of the viral surface (the viral envelope proteins or in the absence of an envelope, the viral coat proteins) precisely interacting with specific cell surface molecules. In the absence of the cell surface molecule(s) required, the virus cannot enter the cell. This will be discussed further later in this chapter.
The "purpose" of a virus, then, is to find a susceptible cell, enter it and replicate in such a way as to facilitate its progeny finding susceptible cells in new hosts, i.e. transmission. The strategies used to achieve this end are hugely diverse, and accomplished with the most etraordinary economy of material. A virion may carry an enzyme or two, necessary for the initiation of the replication cycle, but is essentially just a sophisticatedly addressed package bearing an auto-start program: a blueprint for making more of itself. Compared with the genomes of cellular organisms, viral genomes are minute. For comparison, the human genome comprises 3 x [10.sup.6] kilobases (kb), that of Haemophilus influenzae 1.8 x [10.sup.3] kb, that of a po virus around 200 kb, and that of hepatitis B virus (HBV) only 3.2 kb. The numbers of genes encoded by viral genomes are commensurately small. HBV encodes just four. Moreover, there is almost no redundancy in viral genomes and, not infrequently, genes overlap, with different proteins being transcribed from different open reading frames (see Chapter 19). The possession of relatively so few genes does not however imply that viruses are easily understood and hence eliminated by human intervention. It should be noted that while the 9.7-kb genome of the human immunodeficiency virus (HIV) was first fully sequenced in 1985 by Ratner and his co-workers, over 20 years later, we do not entirely understand its pathogenesis, have no cure, and no preventative vaccine.
Viral transmission
Many viral diseases that occur in humans are zoonoses, i.e. they are communicable from animals to humans under natural conditions. Unlike most other human pathogens, viruses have no free-living form outside their host(s). In the environment, a virus particle can do no more than passively survive intact and any damage is liable to render it noninfectious. In this respect it is important to note that the lipid bilayer of enveloped viruses is very vulnerable to disruption by desiccation, detergents, and solvents. Because infectivity depends on the integrity of the envelope bearing its viral attachment (glyco)proteins, these viruses do not survive long in the environment and are readily susceptible to decontamination methods (e.g. even simple soap and water). Conversely, nonenveloped viruses tend to be considerably more durable. Caliciviruses are particularly resilient and the difficulty of adequately decontaminating fomites (contaminated inanimate objects) in the contet of norovirus outbreaks has no doubt contributed to outbreak persistence on many occasions.
Despite their passivity in the environment, viruses have evolved so as to maimize their chances of transmitting from host to host. Some viruses, arthropod borne or arboviruses, multiply to high viral loads in the bloodstream and are transferred to new hosts by arthropods that feed on human blood. Transmission is also assured if viruses shed in vast numbers into human body fluids respiratory secretions, saliva, genital secretions, urine, and stool. Human behavior takes care of the rest: face to face conversation, seual activity, use of the hands for eating as well as for toileting facilitates transmission and continued survival. Certain viruses, influenza and rotavirus for eample, go even further by causing the volume of the infectious fluid to be considerably increased and literally sprayed into the environment by sneezing or eplosive, watery diarrhea respectively. Mother-to-child transmission (also known as vertical transmission) is usually an incidental means of transmission rather than the predominant one. The transmission of blood-borne viruses through parenteral eposure (blood transfusion, contaminated surgical implements, tattooing, sharing of equipment for IV drug use, etc.) is of course an artifact of very recent (in evolutionary terms) human behavior, where the major means of transmission is mucosal or skin lesion contact with blood or genital secretions. In the absence of universal immunization, knowledge of the route by which a viral disease is transmitted is essential to the control of that infection in human populations.
Fungi
Fungi are an etremely diverse group of organisms, ubiquitous in the environment. They are found as two main forms, yeasts and molds. They are nonphotosynthetic organisms with the ability to absorb soluble nutrients by diffusion from living or dead organic matter. Molds consist of branching filaments (hyphae), which interlace to form a mycelium. The hyphae of the more primitive molds remain aseptate (without walls) whereas those of the more developed groups are septate with a central pore in each cross wall. Yeasts are unicellular organisms consisting of separate round or oval cells. They do not form a mycelium, although the intermediate yeast-like fungi form a pseudomycelium consisting of chains of elongated cells.
Many fungi, including some of clinical importance, can eist in both forms dependent on temperature and other environmental conditions. These are known as dimorphic fungi.
Like mammalian cells, fungi are eukaryotes (Table 1.1) with DNA organized into chromosomes within the cell nucleus. Fungi also have distinct cytoplasmic organelles including Golgi apparatus, mitochrondria, and storage vacuoles. Homology with mammalian cells also etends to biosynthesis, where fungi share similar pathways for both protein synthesis and DNA replication.
A formal classification scheme of fungi has little medical relevance so a simplified clinical classification for pathogenic fungi, based on initial site of infection, is more commonly used (Table 1.2).
Cutaneous superficial fungal infections are very common. The majority are caused by three groups of fungi: mold dermatophytes such as Microsporium spp. and Trichophyton spp., Candida albicans, and Malassezia spp. Keratin-containing structures such as hair shafts, nails, and skin are affected. Dermatophyte skin infection (sometimes called ringworm) is commonly named after the area affected, for eample tinea capitis (head) or tinea corporis (body).
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Excerpted from Infectious Disease by Nandini Shetty Julian W Tang Julie Andrews Copyright © 2009 by John Wiley & Sons, Ltd. Excerpted by permission.
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