Microbiology: A Definition, with a Brief History
Protozoa Bacteria Fungi: Molds and Yeasts
Viruses
Managing Biohazards
A Brief Word About Taxonomy
A Short Glossary of Useful Terms
When microorganisms `bloom' following an event such as a flood, sewer back up, roof leak or plumbing break, or with the buildup of condensation in building interiors, the microorganisms and their spores may produce health effects ranging from mild allergic reactions and asthma to brain abscesses and death. Rodents, insects, and birds can also present biohazards when they are in proximity to human activities. The following sections present information on microbiological hazards and their health effects.
Microbiology: A Definition, with a Brief History
Protozoa
Bacteria
Fungi: Molds and Yeasts
Viruses
Managing Biohazards
A Brief Word About Taxonomy
A Short Glossary of Useful Terms
More Information on Biohazards
The Bad Bug Book
Maryland's Communicable Disease Fact Sheet Index
A microbe, or microorganism, is an organism too small to be clearly seen by the unaided human eye. Organisms having a diameter of less than 0.1 mm are effectively invisible, while very little detail can be seen in an organism with a diameter of 1 mm or less. Microbes can thus be generally defined as organisms having a diameter of 1 mm or less. Microbiology is the study of microbes.
The science of microbiology was ushered in by Antony van Leeuwenhoek (1632 - 1723), a Dutch lens-grinder who first observed microbes in 1674. The impact of this discovery was enormous, opening an hitherto unsuspected world of life existing alongside our own. His skill as a microscope maker, coupled with his carefully recorded observations of the microbial world, ensured that this discovery would receive serious attention in the future.
With the optical improvements made to the compound microscope during the 19th century, the science of microbiology had truly arrived. Some significant discoveries made during this period were:
These are single-celled microbes, mostly aquatic, that reproduce by fission. The usual route of infection is through ingestion; specifically, the drinking of contaminated water. Unlike the other microbes discussed on this page, exposure to a single organism is often sufficient to cause infection and disease. Some examples of parasitic protozoa include:
Bacteria are single-celled microbes that lack an organized nucleus. The genetic material is often arranged as a single, circular strand of DNA. Most bacteria are parasites, saprophytes, or symbionts. Bacteria are subdivided into three major groups: the eubacteria, myxobacteria, and the spirochetes. Within these subgroups there are cylindrical, rod-shaped organisms, spherical organisms (the cocci), and spiral-shaped organisms (the spirilla). They are distinguishable from each other by their motility, type of cell wall, and their ability to form resting cells (spores or cysts).Eubacteria are the largest and most diverse of the bacteria groups, with wide variability between members and representatives of all three shapes. Some of these move with the aid of flagella, while others are immotile. The group is characterized by thick, rigid cell walls; some create endospores or cysts in their resting stage. Members of this group form chains of individual cells. Notable examples are the rod-shaped bacilli, and the spherical streptococci. Some of rod-shaped bacteria, and one of the cocci, create endospores that are extremely resistant to heat and other treatments lethal to animal and vegetative cells, and can remain dormant for years in some species.
Myxobacteria are rod-shaped and have thin, flexible cell walls. They exhibit a gliding movement, and form microcysts in their resting stage. The two principle subgroups are the fruiting myxobacteria and the cytophagas.
Spirochetes, as their name implies, are spiral-shaped organisms. They possess an axial filament, a thin, flexible cell wall, and do not form cysts or spores. Most of these organisms inhabit mud or water, and appear to be anaerobes. Many of the smaller spirochetes are parasites of man or other vertebrates, and are responsible for several human diseases, including syphilis and yaws (Treponema), relapsing fever (Borrelia), and one type of infectious jaundice (Leptospira).
There are two groups of microbes, the rickettsia and bedsonia, that are bacterial in nature, but which do not fit into any of the major bacterial groups. They are extremely difficult to cultivate outside of a host cell.
The rickettsia are obligate intracellular parasites found in arthropods such as fleas, lice, and ticks. They do not appear to produce disease symptoms in their arthropod hosts. However, when transmitted to a human host through a bite, a severe and often fatal infection may result. Diseases in this class include epidemic typhus, transmitted by lice, Rocky Mountain spotted fever, transmitted by ticks, and scrub typhus, transmitted by mites.
The bedsonia are also obligate intracellular parasites, but are transmitted directly between vertebrate hosts. They are roughly spherical and are slightly smaller than the rickettsia. Diseases in this class include psittacosis and trachoma.
Fungi are found virtually everywhere. The main reservoir for these organisms and their spores is the soil. Spores may remain viable for years, and sometimes for centuries. For this reason, these microbes and their spores are present wherever other creatures live. The number of organisms present in a particular location at any time is dependent on such factors as humidity, soil moisture, temperature, and the presence of plants, animals, and organic materials.Like plant cells, fungi have rigid cell walls. Some fungi grow into remarkably plant-like forms, such as the various mushrooms. However, fungi do not use sunlight as an energy source, and so are not considered to be true members of the plant kingdom.
There are four major classes of fungi: the ascomycetes, the basidiomycetes, the fungi imperfecti, and the phycomycetes.
The ascomycetes and basidiomycetes are differentiated by their reproductive processes. The ascomycetes include yeasts, molds, mildews, truffles, and morels. The basidiomycetes include rusts, smuts, mushrooms, and puffballs.
The phycomycetes are known collectively as water molds, and occur on the surfaces of decaying organic material in ponds and streams. Some are parasitic and attack algae or protozoa.
The fungi imperfecti include the various fungi which have not been studied sufficiently to allow their classification as ascomycetes or basidiomycetes.
Another group of organisms that are not true fungi, but possess some of the characteristics of fungi, are the slime molds. The best known of these are the myxomycetes, commonly found growing on decaying logs and stumps in damp woodlands. Their vegetative structure is a plasmodium (distinct from the genus Plasmodium; see the section on Protozoa), which flows in an amoeboid manner over the surface of the substrate.
Fungi exist in two basic forms: molds and yeasts. Molds are the more complicated of the two, forming specialized structures to support their life-cycles, such as hyphae and sporangia. Yeasts, on the other hand, are small single cells, usually ovoid, that multiply by forming buds.
Many fungi are beneficial, performing functions necessary to the maintenance of all life on earth. Others, including some of the natural skin flora of humans, plants, and animals, lack any obvious beneficial or harmful effects. A few of these organisms are utilized to process products in everyday use, such as breads, beer, and soy sauce. Still others produce disease in plants and animals; some of these include:
Viruses represent a unique class of biological entity, different from cellular organisms. In its extracellular phase, the virion, a virus consists of a single kind of nucleic acid, either DNA or RNA, contained within a complex protein capsid. In addition, it may have one or more enzymes which are instrumental in the initiation of infection, and may assist in the replication of the viral nucleic acid within a host cell. The virion is much smaller than other microbial cells, often by as much as two or three orders of magnitude.A new cell always arises directly from a pre-existing cell. By contrast, a virus cannot arise directly from a pre-existing virus in its extracellular phase. A virus must invade an existing eukaryotic host cell. The virus then uses the host cell's resources to replicate its genetic material and build new virions. At this point, the new virions either burst through the cell membrane of the host, killing the cell, or "bud off" from the cell membrane without killing the host. In the latter case the host cell will continue its own life processes, while the virus continues to multiply.
It can be seen, therefore, that all viruses are parasites. Perhaps due to their relative simplicity, viruses tend to be much hardier than cells. Treatments that avail against other microbes are often less effective or completely ineffective against viruses. Some viruses that cause disease in humans are:
Of all environmental hazards, biological hazards are the most elusive and difficult. Physical hazards, such as structural problems, poor lighting, obstructions, slippery footing and the like are easy to identify, and can be dealt with in a permanent fashion. Identifying toxic chemicals and wastes is more involved, but the process is relatively straightforward, following which the hazard can be isolated, treated, or removed. The process may be involved and/or costly, but will usually result in a permanent mitigation of the hazard. Biohazards, on the other hand, are often difficult to identify and resolve.
Far and away the most common biohazard problem for building and home owners is extensive growth of fungi. This is often the result of a transient event, such as flooding or a sewer line back-up. Fortunately, this is the easiest problem to identify, as fungi will usually grow in visible colonies. The bad news is that the problem is often not noticed or considered noteworthy until symptoms of exposure begin to develop.
Excessive concentrations of spores will often provoke an allergic reaction of greater or lesser severity in one or more of the building occupants. Once an individual becomes sensitive to a particular fungus, normal or seasonal concentrations of the spore may continue to produce an allergic response. In many such cases, regular medical treatment over a long period of time will be required to alleviate the symptoms. Excessive concentrations of spores may also result in acute infections, requiring medical treatment.
Viruses, protozoa, and most bacteria do not produce direct visible evidence of their presence. Evaluation of these hazards must be based on other factors, such as the past and present uses of the site, evidence of potential disease reservoirs and vectors such as mice, rats, mosquitos, stagnant water, etc, and interviews with persons living or working in the area to determine the presence of a common disease (epidemiological investigation).
Any site investigation may involve sampling of ambient conditions, to determine the types and relative concentrations of microbes present. Sampling may include soil, air, surfaces, and water. There are a variety of methods used for the sampling of each medium. The type and method used will depend on the site conditions, and the sampling objectives.
Once the hazards have been identified and assessed, a strategy can be developed to mitigate them. Proper use of disinfectants will reduce the growth of fungi and bacteria, and kill most of the spores. Elimination of harborage for disease vectors will reduce the risk of exposure. Maintaining a clean water supply and food preparation area is also desirable.
Realize foremost that biohazards can be minimized, but never eliminated. Surfaces can be cleaned and disinfected; convenient harborage can be eliminated; stagnant water can be drained. All of these methods, and others as well, can be taken to ensure that ideal conditions for harmful microbial growth are minimized. However, microbes (and their vectors) are living organisms; any condition livable for humans will also be livable for countless other organisms. In most cases, the natural defenses of the body can deal with normal levels of harmful microbes. To prevent conditions for abnormal growth and exposure to these hazards, we must rely on education and good hygienic practices.
In biology, taxonomy is the classification of organisms according to their natural relationships. Classical (or Evolutionary) taxonomy groups all living organisms into one of three kingdoms: plant, animal, or protist; organisms are further classified into subgroups within these kingdoms. Recent genetic research has given rise to a new, phylogenetic taxonomy, that may well supplant classical taxonomy. For the time being, however, the taxonomic terms used on this page are those of classical taxonomy. Anyone interested in phylogenetic taxonomy may wish to investigate the NCBI Taxonomy Homepage
.Specialized terminology from microbiology is used in the topics discussed on this page. Some of these terms are defined in the following list:
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