11:680:390 Lecture Notes - Lecture 20: Pyrimidine Dimer, Dna Repair, Electromagnetic Radiation
26 Apr 2018
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20: Control of Microbial Growth
I. Physical Control Methods
A. Growth Control by Heat
1. The autoclave is a sealed device that uses steam under pressure
a) Allow temperature of water to get above 100oC
b) Kills endospores
c) Not the pressure but the high temperature that kills the microbes
2. Pasteurization
a) The process of using precisely controlled heat to reduce the
microbial load in heat-sensitive liquids
b) Does not kill al organisms, so it is different from sterilization
c) UHP: ultra-high pasteurization; increases temp to 160oC for 30
seconds, enough to kill more microbes → extended shelf life
B. Radiation - UV radiation has sufficient energy to cause modifications and breaks
in DNA; useful for decontaminating surfaces; Disadvantage is it cannot
penetrate through plastic and paper
1. Non-ionizing Radiation (UV)
a) Ultraviolet range
b) 260 nm effectively absorbed by DNA → thymine dimers
c) Photolyase repair in light (photoreactivation) and excision repair in
dark
d) INDUCES MUTATIONS IN THYMINE DIMERS
2. Ionizing Radiation (X-rays)
a) Electromagnetic radiation that produces ions and other reactive
molecules upon collision
b) Some microorganisms are more resistant to radiation than others
c) Used for diverse items including surgical supplies, plastic labware,
drugs, fresh produce, meat
d) BREAKS THE PHOSPHODIESTER BONDS (BACKBONE) IN
DNA
C. Filtration
1. Used for heat-sensitive solutions (e.g., sugar solutions). Why? Solution
might get burned (sugar solution might turn to caramel).
2. Depth filters made of overlapping paper or glass fibers; functions like a
sieve (pores are ~0.2-0.4 um)
3. HEPA filter
a) Used in rooms that have to be cleaned
b) Air is sucked away into filter and traps microbes so that air
released is clean
c) High efficiency
II. Chemical Control of Microbial Growth
A. Antimicrobial agents - chemicals that kill or inhibit growth
1. Types
a) Bacteriostatic - inhibits growth; when agent is removed, growth
continues
b) Bactericidal - kills cells; cells stay behind
c) Bacteriolytic - kills and lyses cells, so nothing is left behind
2. Assays
a) Minimum inhibitory concentration (MIC): smallest amount of an
agent needed to inhibit growth of a microorganism
b) Disc diffusion assay uses solid media
○ Antimicrobial agent added to filter paper disc, diffuses into
agar
○ MIC is reached at some distance
○ Zone of inhibition: area of no growth around disc
c) Microbes behave differently to antibiotics due to physiology and
structures
○ When microbes are in between sensitive and resistant: you
could make an “antibiotic cocktail”
○ Synergism: combined effect is greater than individual
antibiotic effect
B. Sterilants: kill all microbes
C. Disinfectants: doesn’t guarantee that everything is killed; used on non-living
surfaces (e.g., bleach at 10%)
1. Bleach degrades over time → fresh solution needed
D. Sanitizers: cleaning something used for personal purposes (plates, hands, etc.)
E. Antiseptic: can be used on living surfaces (e.g., alcohol at 70%)
1. Alcohol at 100% starts preserving microbes
2. Alcohol can also be used for disinfecting
F. Virus resistance
1. Viruses with lipid envelope is less resistant than viruses without
envelopes
a) Dependent on consistency of envelope - lipid envelope is easier to
dissolve and degrade than capsomeres
2. Prions - most resistant (Incubation period: ~60 years)
a) Misfolded proteins (already sorta denatured); no target
b) Can go through the blood-brain barrier → degrades brain matter
c) Example: Mad Cow Disease - cows have no sense of orientation;
mad cows were killed and bones were fed to other cows, which
caused it to spread
Document Summary
Dna: filtration, used for heat-sensitive solutions (e. g. , sugar solutions). Antimicrobial agent added to filter paper disc, diffuses into agar. Zone of inhibition: area of no growth around disc: microbes behave differently to antibiotics due to physiology and structures. When microbes are in between sensitive and resistant: you could make an antibiotic cocktail . Beta-lactamase presence makes it resistant (can break down beta-lactam: n-acyl group can be modified: Methicillin - makes penicillin acid-stable and beta-lactamase-resistant. Ampicillin - broadened spectrum of activity; acid-stable, beta-lactamase-sensitive: allergy: allergic to fungus found in drug. In order to be useful, antibiotics need to : exhibit selective toxicity low risk to patient, high risk to microbes, be administered properly. Target cells (a) broad spectrum: affects lots of microbes (good and bad); leads to drug resistance by picking up plasmids (b) narrow spectrum: if drug prescribed is specific to certain type of microbe. Drug stability shelf-life has to be long.
