Page 115 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART I Basic Bacteriology
104
content. X-rays are used in medicine for sterilization of
heat-sensitive items, such as sutures and surgical gloves,
negative bacteria caused fever in recipients of the solutions.
and plastic items, such as syringes.
Therefore, solutions are now filtered to make them pyro-
gen-free prior to autoclaving.
The most commonly used filter is composed of nitrocel-
FILTRATION
all bacteria and spores. Filters work by physically trapping
Filtration is the preferred method of sterilizing certain
particles larger than the pore size and by retaining some-
solutions (e.g., those with heat-sensitive components). In
what smaller particles via electrostatic attraction of the
the past, solutions for intravenous use were autoclaved, but lulose and has a pore size of 0.22 μm. This size will retain
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mebooksfree.com mebooksfree.com mebooksfree.com • Chemical agents kill bacteria by one of three actions: disrup- mebooksfree.com
particles to the filters.
PEARLS
• Sterilization is the killing of all forms of microbial life, includ-
ing bacterial spores. Spores are resistant to boiling, so steril-
tion of lipid in cell membranes, modification of proteins, or
modification of DNA.
ization of medical equipment is typically achieved at 121°C for
15 minutes in an autoclave. Sterilization of heat-sensitive mate-
• Physical agents kill (or remove) bacteria by one of three pro-
rials is achieved by exposure to ethylene oxide, and liquids can
cesses: heat, radiation, or filtration.
be sterilized by filtration.
• Disinfection is reducing the number of bacteria to a level
to kill spores, but heat-sensitive materials such as milk are
low enough that disease is unlikely to occur. Spores and some • Heat is usually applied at temperatures above boiling (121°C)
exposed to temperatures below boiling (pasteurization) that
bacteria will survive. For example, disinfection of the water
kill the pathogens in milk but do not sterilize it.
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mebooksfree.com mebooksfree.com mebooksfree.com • Filtration can sterilize liquids if the pore size of the filter is small mebooksfree.com
supply is achieved by treatment with chlorine. Disinfection of
• Radiation, such as ultraviolet light and X-radiation, is often
the skin prior to venipuncture is achieved by treatment with
used to sterilize heat-sensitive items. Ultraviolet light and
70% ethanol. Disinfectants that are mild enough to use on skin
X-radiation kill by damaging DNA.
and other tissues, such as 70% ethanol, are called antiseptics.
• The killing of microbes by either chemicals or radiation is pro-
enough to retain all bacteria and spores. Heat-sensitive liquids
portional to the dose, which is defined as the product of the
(e.g., intravenous fluids) are often sterilized by filtration.
concentration multiplied by the time of exposure.
SELF-ASSESSMENT QUESTIONS
ANSWERS
1. Regarding sterilization and disinfection, which one of the
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following is the most accurate statement?
2. (C)
(A) Seventy percent alcohol is a better antiseptic than iodine,
so 70% alcohol should be used to disinfect the skin prior
PRACTICE QUESTIONS: USMLE &
to drawing a blood culture rather than iodine.
COURSE EXAMINATIONS
(B) Disinfectants kill both bacterial cells and bacterial spores.
(C) During sterilization by autoclaving, the temperature must
Questions on the topics discussed in this chapter can be
be raised above boiling in order to kill bacterial spores.
(D) Transmission of milk-borne diseases can be prevented by
found in the Basic Bacteriology section of Part XIII:
pasteurization, which kills both bacterial cells and spores.
USMLE (National Board) Practice Questions starting on
(E) Ultraviolet light used in the operating room to disinfect
page 709. Also see Part XIV: USMLE (National Board)
the room kills bacteria primarily by causing oxidation of
lipids in the cell membrane.
2. Which one of the following chemicals is used to sterilize
heat-sensitive materials, such as surgical instruments, in the Practice Examination starting on page 751.
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hospital?
(A) Benzalkonium chloride
(B) Cresol (Lysol)
(C) Ethylene oxide
(D) Thimerosal
(E) Tincture of iodine

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