MCQs PATHOLOGY: Disinfection and Sterilization

8.1 Which term is applied to chemicals used to kill bacteria on an inanimate object? A. Antibiotic B. Antiseptic C. Chemotherapeutic agent D. Disinfectant E. Sterilizing agent.


8.2 Which chemical disinfectant is incorporated into many antiseptic soaps used for pre-operative hand-washing? F. Cetrimide G. Chlorhexidine H. Crystal violet dye J. Hexachlorophane K. Mercuric chloride.


8.3 Which statement about the chemical disinfectant Cetrimide (e.g. Cetavlon) is FALSE? L. Chemically is a quaternary ammonium compound M. Does not kill spores N. Greatly inactivated by organic matter O. Totally inactivated by soap P. Wide antibacterial spectrum with bactericidal activity.


8.4Which type of chemical disinfectant is effective against Gram-positive bacteria but is ineffective against Gram-negative species? Q. R. S. T. u. Chloroxylenol Hypochlorites Iodine Iodophores Phenolics (e.g. Dettol) (e.g. Chloros) (e.g. Betadine) (e.g. Clearsol).


8.5Which of the following physical agencies is LEAST effective in killing bacteria of most genera suitably exposed to its effect? V. Desiccation by lyophilization W. Dry heat (160°C for 1 hour) X. Ionizing radiations (0-rays), ã-rays and X-rays) Y. Moist heat (121°C for 15 minutes) Z. Shortwave UV radiation (2,500 Â).


8.6What pressure, temperature and time combination is satisfactory for standard, autoclave usage? A. B. C. D. E. 0 lb/in2 0 lb/in2 15 lb/in2 15 lb/in2 low vacuum 100°C 100°C 121°C 121°C 80°C 20 min 60 min 20 min 60 min 20 min.


8.7What are the correct conditions for sterilization of glassware in a hot air oven? F. 60°C for 1 hour G. 100°C for 30 minutes on 3 successive days H. 121°C for 20 minutes at a pressure of 15 lbs/in2 J. 160°C for 1 to 2 hours K. 500°C for 20 minutes.



8.8 Which statement about ethylene oxide sterilization is FALSE? L. Efficiency tested by exposure of spore strip in chamber M. Kills bacteria by alkylating nitrogenous components N. Non-harmful to man and chemically inert in air O. Process requires controlled humidity, temperature and pressure P. Used to sterilize heat sensitive instruments.


8.9 Which is a satisfactory method for the sterilization of antisera before use? Q. Boil for 5 minutes R. Inactivate at 56°C for 30 minutes S. Pass through 0.2 ì membrane filter T. Steam at atmospheric pressure for 1 hour U. Steam at 15 lb pressure for 20 minutes. Answers overleaf 65


ANSWERS

8.1 D. Disinfection is any chemical treatment of an inanimate object with the intention of killing organisms. Disinfectants are influenced by concentration, time, nature of the microorganisms present, presence of protein which might protect the organisms or presence of organic matter which might inactivate the chemical. Spores and bacteria with a high lipid content (e.g. tubercle bacilli) are generally most resistant. Antisepsis is restricted to methods suitable for treatment of living tissues such as skin, open wounds, the membrane linings of the body cavities and certain hollow organs. The use of these terms depends entirely upon convention: some authors do not distinguish between them and simply apply the general term disinfection to both situations. Sterilization is any process which removes (i.e. filtration) or kills (e.g. heat, irradiation, etc) all forms of life. It is therefore "all or none".


8.2 J. Hexachlorophane is a chlorinated bi-phenol. It has a detergent action which disrupts the cell membrane and exerts a cumulative bacteriostatic activity. It is used in soaps, bath sachets and concentrates to suppress the growth of skin bacteria. It is.also incorporated into deodorants to suppress commensal organisms which decompose sweat.


8.3 P. Cetrimide has a narrow range of activity and is more bacteriostatic than bactericidal. As a cationic detergent it is useful for disinfection of surfaces and can be used for skin antisepsis as in Cetavlon or in combination with Hibitane in Savlon.


8.4 Q. With the exception of Chloroxylenol all these disinfectants have a wide antibacterial spectrum. Chloroxylenols and hypochlorites are greatly inactivated by organic matter. Particularly resistant species such as Ps. aeruginosa and other pseudomonads have been found contaminating Chloroxylenol solutions.


8.5 V. This is the process of freeze-drying used to preserve stock cultures of bacteria in a viable but dormant state. Organisms drying from the fluid phase, such as exhalation droplet-nucle1* formation, show a high initial death rate which is greatly reduced by drying from the frozen state as in lyophilization. The other physical agents are all lethal for bacteria given suitable exposure. Heat is used for laboratory sterilization, beta and gamma irradiation for sterilization of plastic and rubber articles and X-rays mainly for research on their biological effect. UV irradiation is used in safety cabinets for handling hazardous materials such as tubercle cultures and occasionally for reducing aerial transmission of infection in wards and operating theatres.


8.6 C. 121°C (15 lb/in2) for 20 minutes ensures the death of all spores, the most heat resistant of life forms. Increasing the exposure time to 60 minutes would simply damage relatively heat sensitive materials such as rubber gloves and culture media.


8.7 J. Sterilization by dry heat requires a higher temperature maintained for a longer time than the temperature-time ratio for steam under pressure. The use of the very high temperature of 500°C is unnecessary and might damage glassware.



8.8 N. Even small amounts of residual chemical remaining on a sterilized endoscope before use has a powerful toxic vesicant action on the tissues. The gas is highly explosive and to limit this reactivity it is usually provided in small canisters combined with 90 per cent CÖ2 or a fluorocarbon. Bacillus subtilis var globigi spore strips are the recommended biological test method. 8.9 S. Filtration through a bacterial filter will sterilize antisera without loss of potency. Steam at 15 lb pressure for 20 minutes is standard autoclave procedure for heat resistant articles but would result in loss of activity of antibody and is therefore unsuitable. Heating at 56°C for 30 minutes would merely inactivate complement.