2 Types and Properties of Bioaerosols
Bioaerosols differ from other air pollutants because they have complex and varied organic structures and may be capable of reproducing, thereby causing infections. This section presents an overview of the different types and properties of agents associated with bioaerosols. For those who desire more details and further explanation, there are numerous excellent texts on microbiology, aerobiology, and the toxicology of airborne contaminants (11-14). 2.1 Bacteria
Bacteria are free-living, unicellular organisms that can self-perpetuate without the aid of a host cell. Their chemical composition is not greatly different from that of other living materials. They are composed of water, proteins, fats, carbohydrates, and various inorganic compounds such as sulfur, phosphorus, and salts. Nearly all bacteria are encased in porous but rigid cell walls that protect them from osmotic rupture and give different types of bacteria characteristic shapes. They occur in three general shapes: spherical (coccus), rod-shaped (bacillus), and spiral (spirochete, spirillum, and vibrio). They also differ in size. Cocci range from 0.15 to 2.0 mm in diameter. The smallest bacillus is about 0.5 mm long and 0.2 mm in diameter, and the largest pathogenic bacillus may approach 1 mm in diameter and 3 mm in length. The spirilla are usually 1 to 14 mm long. An important fact associated with bacterial cells is that the ratio of surface area to volume is extremely high. This high surface area provides a good opportunity for direct contact with the surface tissue of susceptible host cells. Typically pulmonary diseases resulting from bacterial infections include pneumonia, brucellosis, Legionnaires' disease, tuberculosis, meningitis, and anthrax.
A mucilaginous capsule surrounds many pathogenic bacteria. The presence of a capsule may increase the virulence of an organism by protecting it against certain host defenses. Most unencapsulated microorganisms are readily destroyed after being engulfed by local phagocytes. As an example, unencapsulated Streptococcus pneumoniae are generally considered avirulent, whereas encapsulated strains can produce pneumonia. Some airborne bacilli form resistant structures, known as spores. In a spore state, the organism has a relatively high degree of resistance to all sorts of injurious environmental influences such as high temperatures, germicidal chemicals, dryness, etc. When spores are brought under conditions favorable for growth, they germinate; the spore becomes a vegetative form of the bacillus, which then multiplies and behaves in the usual manner.
The large and heterogeneous group of microorganisms called viruses are alike in that they are all obligate intracellular parasites that live in cells of their selected hosts. Viruses, the smallest parasites (0.02 to 0.3 mm), are intracellular molecular particles, in some instances crystallizable. The virus particle consists of a central core of a nucleic acid for its reproduction. The nucleic acid core may be made up of either RNA (ribonucleic acid) or DNA (deoxyribonucleic acid), which represents the basic infectious material. When viruses invade a host cell, the genetic material takes over the host cell's own reproduction process and causes the cell to produce more viruses. Viruses cannot replicate themselves in the extracellular state and are highly specific in selecting cells that they will infect. Some viruses replicate within the cytoplasm of the host cells, and others replicate within the nucleus of the cell. Viruses can be released from the host cell when the cell ruptures, a process which is lethal to the host cell. Other viruses are released from the host cell gradually through a "budding-off" process. In this process, each newly formed virus particle released is surrounded by a host-derived membrane called an envelope. This process does not result in the death of the cell. The severity of the disease caused by viruses varies considerably in terms of their effect on susceptible individuals. Pulmonary diseases caused by viruses include influenza, the common cold, and bronchiolitis.
Fungi are a diverse group of saprophytes that occur in many forms, inhabit air, soil, water, and vegetation, and also live on the bodies of humans and animals. It has been estimated that there are more than 50,000 species of fungi, but fortunately only about 50 are associated with human disease. Fungi are considered one of the most common forms of life on earth. They vary in shape and size from a single-celled microbe to giant multicellular mushrooms. Fungi reproduce by a variety of methods, including budding, fission, and spore formation. Pathogenic fungi generally produce no toxins. All fungi are heterotropic, requiring organic nutrients for existence, and most are obligate aerobes. Within the protoplasm of fungi are enzymes which, when diffused into the surrounding environment, change complex substances into simpler substances useful as nutrients for the cell. Human disease caused by fungi can vary from superficial infections of the skin to severe diseases that involve the respiratory system and other internal tissues and organs. The pulmonary diseases associated with fungal infection include asthma, aspergillosis, coccidioidomycosis, histoplasmosis, and various types of hypersensitivity pneumonitis. Fungi have a special tendency to cause infections in people with compromised immune systems. Fungal pneumonia may be dormant in the host respiratory system for months or years and cause disease only when the host's defenses are compromised.
Yeasts are filamentous, unicellular forms of fungi that are usually 3 to 5 mm in diameter. In view of the ubiquitous distribution of yeast in air, dust, and soil and also on surfaces of the body, it is not surprising that these forms have been associated with a variety of pathological processes. Morphologically they are oval or ellipsoidal and rarely form long filaments. Yeast cells are, on average, larger than bacteria and may have a volume thousands of time that of some bacteria. Yeast can be frequently found inhabiting the throats of healthy individuals. Many types of yeast infections are superficial, but serious systemic diseases may occur.
Molds are multicellular fungi found practically everywhere. The term mold is a general one which is used to describe the woolly, cobweb-like, cottony or powdery, black, green, yellowish, or white growths, seen on a variety of surfaces. Molds are very conspicuous in nature and are recognized by their filamentous and branching structures and macroscopic size. Molds have branching tubular structures that can be 2 to 10 mm in diameter and are associated with nutrition and reproduction. Molds produce enzymes that cause rapid fermentation, proteolysis, and other biochemical changes in a variety of substances. Some allergic diseases caused by the inhalation of molds include farmer's lung disease from the inhalation of moldy hay, bagassosis from dried stalks of sugarcane, grain fever from handling grain in all stages of production from harvest to storage, sequoiosis from moldy sawdust, and coffee worker's lung from coffee bean dust (5, 15).
Rickettsias are closely related to viruses. They are smaller than bacteria, and their growth occurs within the cytoplasm (e.g., typhus) or in the nucleus of the infected cell (e.g., Rocky Mountain spotted fever). These organisms have a diameter of about 0.3 mm and lengths seldom exceeding 2.0 mm. They are pleomorphic and may be found singly, in pairs, in chains, or in long filaments. No spores are produced. The normal natural reservoir and primary hosts of these organisms are a variety of infected insects (lice, fleas) or arachnids (ticks and mites). When these organism invade the human, they attack the reticuloendothelial system and colonize in the lining of the blood vessel walls. With such infection, there is hyperplasia of endothelial cells and a localized thrombus formation that leads to obstruction of blood flow and results in the escape of red blood cells into the surrounding tissue. The rickettsia that causes Q fever is resistant to drying and can survive in the dust for months until it becomes airborne and infects an individual. 2.5 Nonliving Contaminants of Biological Origin
The products of microbial metabolism are complex and numerous. Chemically, they can consist of a wide variety of proteins, lipoproteins, and mucopolysaccharides. The amounts of these materials produced are determined by the particular substrate available to the organism and the environmental conditions (i.e., temperature, humidity). Exotoxins are proteins produced within the cell and excreted into the surrounding environment and may be fatal in quantities of micrograms or less by causing diseases such as botulism, tetanus, gas gangrene, and food poisoning. Bacteria may also liberate endotoxins, which are lipolysarcharides intimately associated with the cell wall of certain gramnegative bacteria that can produce diseases such as typhoid fever, cholera, and brucellosis. They are not released until the cell disintegrates. Endotoxins are usually water soluble, relatively heat-stable but less potent than the exotoxins, and less specific in their action. Certain fungi may also produce toxins that may have health effects in sufficient doses. For example, the Aspergillus organism elaborates aflatoxins that can cause hepatic necrosis, liver cancer, and immunotoxic effects.
Microorganisms such as bacteria, actinomycetes, and fungi may also produce a variety of volatile organic compounds (VOCs) and semi-VOCs during metabolism. VOC metabolites such as higher alcohols, ketones, and organic acids are responsible for the odor often associated with the growth of these microbes.
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