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About Microorganisms: Microbiology
Microbiology is the study of small life forms, including bacteria, special fungi called molds and yeasts, protozoa, certain algae, and viruses. Commonly, the field of microbiology is divided into thematic elements, concentrating on specific types of microorganisms. Examples include bacteriology, mycology (study of fungi), protozoology, and virology. Microbiology can also be categorized by activities of selected microorganisms, such as medical microbiology, dental/oral microbiology, food microbiology, industrial microbiology, and environmental microbiology. An area closely related to microbiology is immunology, the study of the immune system.
Infection control, the controlling of microbial contamination, spread, infection and disease, is an essential part of microbiology. Microbiology, in fact, had its beginnings as a science concerned with the control and identification of microorganisms in an effort to explain and prevent disease.
Infection control is highly dependent on the understanding of the physical and chemical properties of microorganisms. It is important to know where microorganisms exist, how they grow, the influence of the environment on special physical and chemical agents, and how they cause specific diseases of concern. Such knowledge improves greatly our ability to kill or at least control microorganisms and prevent their spread from person to person. Also beneficial is working knowledge of immunology and body defense mechanisms. It is important to understand disease prevention achieved through immunization and our reliance on the body's natural barriers against infection.
Important Activities of Microorganisms
In reality, microorganisms are more beneficial than harmful to humans. Usually, we concentrate on the negative -- harmful activities such as causing diseases, spoiling food, occluding water lines, or destroying fabrics.
Yet microorganisms also are beneficial. Bacteria in the soil convert dead plants, animals, and insects into usable nutrients needed for survival of the live plants. Other soil microorganisms convert atmospheric nitrogen and carbon dioxide into forms that are required for growth by all plants. Bacteria forms the basis of modern sewage treatment by degrading the organic material in the sewage as it flows over or when added at treatment plants. In addition, bacteria are cultured in large vats to make several products, such as vinegar, vitamins, alcohol, organic acids, enzymatic cleaners, drain openers, antibiotics, insecticides, and special chemicals used in biomedical research.
The first approach to preventing the harmful activities of microorganisms is to attempt to keep them in their proper place by preventing contamination (e.g., infection control or exposure control). If they appear where they should not be, they must be removed, killed, or kept from growing to harmful numbers (e.g., by cleaning, sterilization, disinfection, growth inhibition, immunization, or antimicrobial therapy).
There are three groups of microorganisms with varying degrees of importance in the field of dentistry. These are bacteria, viruses and fungi. Although each group has different lifestyles and comprises many different types or shapes, they all share two common characteristics--they are too small to see with the naked eye, and many members of each group can live on or in the human body, which may result in the development of harmful infections.
Bacteria have different characteristics and activities that allow them to be distinguished from each other. These include cell morphology (size and shape), staining characteristics, colony characteristics (appearance during growth on agar media), metabolic properties, immunologic properties, and DNA characteristics.
A microscope that magnifies approximately 1,000 times normal size is required in order for us to see bacteria. In addition, bacteria require staining with special dyes for optimal visualization. An example is the gram stain. Gram staining differentiates bacteria into one of two groups. Those that appear blue or purple are grampositive, and those that appear pink or red are gramnegative. Both gram-positive and gram- negative bacteria are present in the oral cavity and are involved with the development of caries and periodontal disease. Oral bacteria can also cause infections in soft tissues and the bloodstream.
Bacteria are single cells with one of three basic shapes. Spherical cells are called cocci (singular: coccus); rod shaped cells are called bacilli (singular: bacillus); curved or spiral cells are called spirilla (singular: spirillum).
Cocci and bacilli may exist as single cells or they may exist in small clusters or in chains. The average diameter of a coccus is about one micrometer or micron (μm). A micrometer is one-millionth of a meter or onethousandth of a millimeter. A millimeter is the smallest division on a metric ruler. Common bacilli are about one mm wide and five to ten mm long.
Spirilla are 0.2 to 1.0 mm wide and up to 30 mm long. Approximately 25,000 cocci laid side by side would create a line just one inch long. In comparison, a human red blood cell is about seven mm in diameter.
Controlling bacterial growth involves preventing their multiplication or killing them outright. Such control is important to prevent damage they may cause by growing where they should not be (e.g., in our bodies, on food, in drinking water).
Changing or eliminating a physical or nutritional requirement for growth or using a chemical agent that interferes with cell division can prevent growth. Agents or conditions that prevent bacterial growth without killing them are bacteriostatic. Treating bacterial diseases with antibiotics can result in inhibiting growth or in killing the cells, depending on the bacterium and the antibiotic used. Antibiotics are chemicals that interfere with some metabolic activity of the bacterium but usually do not affect the metabolic activity of our body cells. This is an example of selective toxicity. For example, penicillin prevents the formation of the cell wall in certain bacteria, which retards their growth and usually kills the bacterium. Penicillin does not affect the growth of our body cells because our cells do not have cell walls.
Killing bacteria requires physical or chemical means and is a very important aspect of disease prevention and infection control. Agents or conditions that kill microorganisms rather than just prevent their growth are bactericidal, virucidal, or fungicidal. Probably the surest way to kill bacteria (or any other type of microorganism) in the shortest amount of time is to expose them to high temperatures, such as those achieved in a steam, dry heat, or unsaturated chemical vapor sterilizer. The heat may destroy proteins, break down DNA and RNA, or cause structural damage to the cell membrane.
Several chemicals can kill bacteria and other microorganisms, and the type of chemical used depends on the location of the microorganisms. Strong chemicals (disinfectants) are appropriate for inanimate objects, but not body tissues. Chemicals used to kill bacteria on tissues are antiseptics. Chemicals kill microorganisms by several mechanisms, depending on the properties of the chemical.
Viruses cause many different diseases and can infect humans, lower animals, plants, fungi, bacteria, algae, and protozoa. Some viruses have been associated with the development of cancer (oncogenic). Viruses are important in dentistry because they can cause the following:
• specific oral diseases, such as herpes;
• diseases elsewhere in the body that may result in lesions in the mouth;
• bloodborne diseases, such as acquired immunodeficiency syndrome (AIDS) and hepatitis B and C;
• other types of diseases that may be transmitted in the dental office without use of proper infection control procedures.
Human viruses are smaller than bacteria, ranging from 0.02 to 0.3 mm. They may have many different shapes but their general structure consists of a nucleic acid core (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer structure of lipids, proteins, and polysaccharides called an envelope. The entire virus is a virion or a viral particle, not a cell.
Unlike most bacteria, viruses are not free-living in that they do not have the metabolic machinery to synthesize new protein coats and nucleic acids. They must use both the nutrients and the metabolic machinery of living cells to multiply (replicate themselves). Viruses are obligate intracellular parasites. Table 1 summarizes the life cycle of a virus.
Fungi include mushrooms, molds, and yeasts. Some yeasts and fungi can cause diseases in humans. Such diseases may involve the lungs or other organs or tissues, or infections of the skin, nails, scalp, and hair.
In dentistry, the most important fungal infection is oral candidiasis (e.g., thrush, denture stomatitis). This causative agent is a fungus Candida albicans, which may exist as a yeast cell or as a filamentous fungus (mold). This organism also may cause skin infections, vaginal infections, or widespread infection in the body.
Candida albicansoccurs as a member of the normal oral flora in approximately 40% of adults. It is an opportunistic pathogen in that it usually causes harmful infections only when given a special opportunity resulting from depressed body defenses, immature body defenses as in the newborn, changes in the body's physiology, trauma to tissues (e.g., poor-fitting dentures), debilitating systemic diseases, or long-term antibacterial therapy. Application of topical antifungal agents usually is effective against Candida albicans. Yeast cells outside the body usually die after exposure to heat and certain chemicals. Molds are much more environmentally stable.
Infectious Human Diseases
Human disease has several causes. One important cause is the presence and activity of microorganisms. An infectious disease occurs when microorganisms enter the body, multiply and, in time, cause damage. Pathogens are microorganisms capable of causing infectious diseases. Pathogens spread in specific ways and often affect only specific species and tissues.
There are two types of infectious diseases - endogenous and exogenous. The two terms refer to the source of infection. Microorganisms that are normally present on or in the human body generally do not cause harm. However, they can on occasion cause endogenous diseases. Commonly, a change in a person's health (e.g., trauma, burns, onset of diabetes or immune suppression) allows endogenous microbes to become opportunistic pathogens. An example is a staph infection caused by members of the normal human flora.
Microorganisms that normally are not present on or in the human body cause exogenous diseases. These organisms come from external sources - food, drink, animals, air, fomites and other humans. Examples of exogenous infections include hepatitis B and C viruses, HIV/AIDS, herpes viruses, the common cold, influenza and strep throat.
Exogenous diseases develop through six basic steps. Each disease modifies steps to its advantage. Disease prevention or treatment can occur by interfering with any of the six stages.
Steps in the establishment of an exogenous disease in dental environments include:
• an external source of microorganisms;
• a mechanism for the microorganism to escape from this source;
• spread of microorganisms to a new person;
• a pathway for entry into the new person;
• infection (survival and growth of the organism);
• injury/damage to the new person.
Spread of Microorganisms
In dentistry, microorganisms that escape from patient oral cavities spread through four basic modes of disease transmission. These include direct contact, indirect contact, droplet infection and airborne infection.
Touching oral soft and hard tissues in a patient's oral cavity can result in direct contact with microorganisms, which immediately can spread through skin defects such as abrasions and around fingernails of ungloved hands. Indirect contact results from contact with contaminated items, such as needles, instruments, surfaces and pieces of equipment.
Droplet infection involves large particles (spatter), which are larger than five microns. Spatter can affect any part of the human body by direct contact; however, spread is limited to a small area immediately surrounding the patient. Airborne infection involves smaller particles (often called droplet nuclei or aerosol particles) usually smaller than five microns. These particles can remain suspended in the air for extended periods and can travel throughout an office or even a building. Disease spread via the airborne route is through inhalation. Examples of airborne diseases include influenza, chicken pox and tuberculosis. Bloodborne disease transmission is not associated with airborne particles.
Modes of Microbial Entry
There are four basic routes of entry for microorganisms. These include inhalation, ingestion, mucous membranes and breaks in the skin.
Infection is the entry, survival and even multiplication of microorganisms in or on a human body. Infection does not always result in disease. A person can be infected with a microorganism (meaning it is present in their body), and not develop active disease. Actually, disease is rather uncommon. Usually, during some step of disease development, the microbial challenge is defeated. Also, parts of our bodies are infected with large numbers of microorganisms, without the presence of disease. The organisms reside in these areas because they are vulnerable to exposure to the outside world through breathing, eating, touching, etc. Examples include the oral cavity, nose, intestine and skin.
Emerging diseases are new infectious diseases not recognized before or known infectious diseases with changing patterns. Infectious diseases are a continuing danger to everyone. Effective control of some diseases can come with the help of modern technology. Yet new diseases-such as Zika virus, SARS, MERS, Chikungunya, avian flu, and swine flu-are constantly appearing. Others, such as malaria, tuberculosis, staphylococcus and bacterial pneumonias, are now appearing in forms that are resistant to drug treatments. Diseases emerge because conditions change that bring microorganisms and human beings together in new ways. These changes involve six categories (Table 4).
Sometimes new or recurring (recrudescent) disease occurs without explanation. Ebola hemorrhagic fever occurs in random outbreaks in Africa. To date, outbreaks have been limited to rural areas involving a limited number of victims. One limiting factor is that Ebola progresses very rapidly, with symptoms occurring within about two days of exposure and causing death within a few days. Spread usually involves family members and healthcare providers. However, the exact mode or modes of emersion and spread are not yet defined. Ebola only affects humans, so the mechanism of disease reservoirs is not known.
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ABOUT THE AUTHORS
Eve Cuny, MS, RDA
Eve Cuny is the Director of Environmental Health and Safety and Assistant Professor in the department of Pathology and Medicine at the University of the Pacific School of Dentistry. She is a nationally recognized expert in infection control in dentistry, publishing and lecturing widely throughout North America. She was a member of the working group that developed the 2003 CDC Infection Control Guidelines for Dentistry and acts as an advisor for numerous regulatory agencies. She is past chairperson of OSAP and currently serves on its board of directors.
Charles J. Palenik, PhD, MS, MBA
Charles J. Palenik has held over the last 30 years a number of academic and administrative positions at Indiana University School of Dentistry. These include Professor of Oral Microbiology, Director/Human Health & Safety, Director/Central Sterilization Services, Director/Dental Informatics and Chairman/Infection Control and Hazardous Materials Management Committees. Currently he is Director/Infection Control Research & Services. Dr. Palenik has published 160 articles, over 300 monographs, three books and seven book chapters, the majority of which involve infection control and human safety and health. In addition, he has provided 120 continuing education courses throughout the United States and nine foreign countries. He is past chairperson of OSAP and currently serves on its board of directors.
REVIEWED AND REVISED BY ADAA COUNCIL ON EDUCATION AND PROFESSIONAL DEVELOPMENT MEMBERS:
Tracey A. Green, BS, CDA; Christina Ross, MS, EDFA, CDA; and Roxanne Terranova, MSM, CDA, RDA