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The Hazards of Laboratory Microbiology

Scientific exploration is not without hazards. Each year’s these explorers contract infectious disease because of accidental exposure to pathogens in laboratories. Over 4000 cases of laboratory associated infections have been recorded with more than 160 fatalities.

The Centers for Disease Control and Prevention has defined which microbes currently represent the greatest hazard I the laboratory and has classified etiologic agents into four categories. Class 1 contains agents that pose little risk of serious disease. These include Staphylococcus epidermidis and many other members of the normal flora. Microbes in class 4 are the most dangerous and require the highest degree of containment. The plague bacillus is a class 4 bacterium.
The greatest number of laboratory infections has occurred among persons engaged in research activities. Fewer than 33% of these infections have been reported as occurring in diagnostic laboratories. This perhaps reflects the unanticipated hazards of handling newly discovered, poorly understood or previously unencountered microbes. In a dramatic incident that occurred in Marburg, Germany in 1967, 31 persons handling the tissues of African green monkeys were infected by a previously unknown virus in the tissue. Six of these victims died. Researchers are microbiogy’s “test pilots”, and many unexpected hazards will likely surface during their preliminary investigations. All persons who handle pathogenic microorganisms are at risk of infection.
Most laboratory associated infections are acquired by contact with infections aerosols. Random air sampling demonstrates that common laboratory manipulations release microorganisms into the atmosphere. Infectious aerosols are created by blowing out a pipette or by removing a stopper.
Aerosols probably account for the many laboratory associated infections that occur in the absence of an identifiable accident. The centrifuge is also an important source of infection and has accounted for at least four fatalities. Infected experimental animals may discharge contaminated respiratory droplets into air. In addition bites and scratches from these animals occasionally cause infection. Mouth pipetting is one of the most hazardous of all laboratory manipulations, resulting in numerous cases of typhoid fever, tularemia, scarlet fever, hepatitis and influenza. Accidental inoculation with needles and syringes accounts for many laboratory associated infections, such as HIV. Spills of infectious materials are also implicated.
Laboratory associated infections can be reduced by immunizing employees, establishing and enforcing rules for safe laboratory practices, and using protective equipments. Good hygiene, especially hand washing after exposure to clinical specimens or laboratory cultures is essential. Biological safety cabinets are designed to contain infectious aerosols at the source. Sealable centrifuge tubes of unbreakable plastic minimize the possibility of leaks or breakage during high speed spins. Hand operated suction devices eliminate the need for mouth pipetting. Experimental animals can be properly housed to control airborne infection. Properly designed laboratories provide adequate space, restricted access to areas where highly infectious systems that being used and ventilation systems that encourage the flow of air into areas where infectious aerosols may be generated. Such ventilation systems however protect only persons outside the laboratory. Adherence to universal precautions will also minimize transmission of infectious agents.
Gloves should be worn by all workers exposed to blood, body substances, or cultures of highly infectious pathogens. Masks, gowns and protective eyewear may be necessary if aerosols or spills are likely. In the final analysis, biological safety depends on the attitude and conduct of the individual worker.


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