Control of Microbial Populations: Chapter 7

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Control of Microbial Populations Chapter 7 ---
Bacteria are ubiquitous, we cant beat them (and
in many ways we would not want to even if we
could), but we can learn to control where and at
what rates they do grow if we keep in mind that
bacteria are constantly evolving. Methods of
Sterilization Dry Heat Boiling Moist Heat
(Autoclave) Radiation Chemical
Disruption Filtration --- Keep in mind that
in sterilization you must kill all bacteria, so
the technique must be harsh enough to kill even
the toughest bacteria that could possibly be
present, generally that means the endospore
forming bacteria (Bacillus subtilis, et. al.)
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Dry Heat Basically baking bacteria to
death, Not very efficient Works for glass or
metal surfaces No good for media or most
chemicals Boiling As the name implies,
treatment with boiling water Will kill most
pathogenic bacteria, viruses, and fungi Wont
kill many of the soil endospore formers Moist
Heat An autoclave is basically a big pressure
cooker (at 15 psi water boils at 121 C) Very
effective for most liquid and dry materials Keep
in mind that heat transfer limits how fast
materials reach 121 C autoclave rule of
thumb 20 min/ liter
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Radiation Any type of radiation that causes
molecular damage (particularly to DNA) can be
used to sterilize material, the important things
to keep in mind are exposure time and depth
of penetration Ultraviolet (UV) good for
sterilizing surfaces, will kill most
bacteria very effective at damaging DNA (Thymine
dimers) Ionizing Radiation much more
penetrating power may cause chemical changes in
over-exposed material
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Filtration Physically remove bacteria and
or viruses can separate viruses from
bacteria good for heat/ radiation sensitive
materials (drugs, antibiotics, etc.) may
leave some soluble materials behind (LPS
?endotoxins)
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Not So Sterile, Sterilization Pasteurization B
rief heating kills many, but not all
bacteria Most pathogens are less hardy and/or
do not grow well outside of the host and if
enough are killed they will be too few
to initiate a new infection has been very
effective for wine, beer, and milk Oddball
aside (when aesthetics trump health) CO
treatment of meat
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Chemical Warfare Disinfectant kills cells
on contact, generally through chemical
reaction non-specific will kill any
cell Antiseptic Kills bacteria but does less
damage to eukaryotic cells ( example H2O2),
still relies mostly on basic chemistry
generally not used internally Antibiotic A
chemical compound that kills bacteria
specifically, usually without reactive
chemistry, but by blocking some essential
cellular function, can be used internally
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Antibiotics Sulfa drugs A class of
molecules that inhibit biosynthetic
reactions developed during the 1920 30s can
be taken internally, but many were not well
tolerated some still used today Modern
Antibiotics Penicillin was the first discovered
in 1929 but was not brought to production
until the early 1940s Many come from the
conflict between different bacteria and
between bacteria and fungi Over 100 are known,
not all are completely selective against
bacteria
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The Penicillin Story First discovered by
Alexander Fleming (a British doctor and
researcher) in 1929. Fleming had an interest
in natural products that could inhibit
bacterial infection, he is also known for the
discovery of human lysozyme. Fleming was not a
chemist and was not successful in producing a
molecule that would be useful in medicine. It
took Fleming a decade to interest a biochemist in
his penicillin project. Howard Florey and his
group started on the project in 1938, and by
1940 had a therapeutically useful molecule. It
is estimated that penicillin has saved over 200
million lives
Collaboration in Science
Chance favors the prepared mind --- Louis
Pastuer
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Aminoglycosides (Streptomycin, Gentamycin)
Inhibit protein synthesis by binding to a portion
of the bacterial ribosome. Most of them are
bacteriocidal (i.e., cause bacterial cell
death).Bacitracin Inhibits cell wall
production by blocking the step in the process
(recycling of the membrane lipid carrier) which
is needed to add on new cell wall
subunits.Beta-lactam antibiotics A name for
the group of antibiotics which contain a specific
chemical structure (i.e., a beta-lactam ring).
This includes penicillins, cephalosporins,
carbapenems and monobactams.Cephalosporins
Similar to penicillins in their mode of action
but they treat a broader range of bacterial
infections. They have structural similarities to
penicillins and many people with allergies to
penicillins also have allergic reactions to
cephalosporins.Chloramphenicol Inhibits
protein synthesis by binding to a subunit of
bacterial ribosomes (50S).Glycopeptides (e.g.,
vancomycin)  Interferes with cell wall
development by blocking the attachment of new
cell wall subunits (muramyl pentapeptides).
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Macrolides (e.g., erythromycin) and Lincosamides
(e.g., clindamycin) Inhibit protein synthesis by
binding to a subunit of the bacterial ribosome
(50S).Quinolones (Novobiocin) Blocks DNA
synthesis by inhibiting one of the enzymes (DNA
gyrase) needed in this process.Rifampin
Inhibits RNA synthesis by inhibiting one of the
enzymes (DNA-dependent RNA polymerase) needed in
this process. RNA is needed to make
proteins.Tetracyclines Inhibit protein
synthesis by binding to the subunit of the
bacterial ribosome (30S subunit).Trimethoprim
and Sulfonamides Blocks cell metabolism by
inhibiting enzymes which are needed in the
biosynthesis of folic acid which is a necessary
cell compound.
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Using antibiotics b-lactams are often not
prescribed for Gram (-) bacteria, Why
not? In general it is best to have at least
a good idea of what group of bacteria is
involved in the disease. Also, these days, one
must consider patterns of resistance that may
be common in your geographic area.
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Antibiotic Resistance Bacteria evolve, adapting
to rapidly changing conditions is the broadest
description of their niche. So it should not
surprise us that within ten years of its
introduction, resistance to penicillin was
well documented and widespread. Overuse and
misuse of antibiotics plays to the strengths of
bacterial adaptation. What does not completely
kill them only makes them stronger. The
Problem Antibiotics are seen as a magic
bullet that cures all bacterial problems. We
need to focus more on other ways to control
bacteria and save antibiotics as a treatment of
last resort for acute infections.
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Annul Antibiotic use in the United States
(2002) 35,000,000 pounds of antibiotics used
annually 13 in human medicine 6
therapeutics use in agriculture 78
non-therapeutic use in agriculture 6 use in
pets from Shea., M., K Pediatrics, vol.112,
No.1, July 2003.
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Mechanisms of Antibiotic Resistance 1.)
Cleavage (penicillinase) 2.) Chemical
modification (kanamycin methyl transferase) 3.)
Efflux pumps (pump it back out of the cell) 4.)
Mutation in the affected protein (mutations in
ribosomal proteins can lead to resistance to
erythromycin) Potentially any of these
mechanism are transferable by HGT
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Solutions to Antibiotic Resistance 1.)
Work on overuse (in medicine and agriculture)
--- reduce selective pressure 2.) Prevent
spread of resistance mechanisms 3.) Massive
overkill strategies in medicine (no survivors
no adaptation) 4.) Work with bacterial
ecology (competition with innocuous strains,
Probiotics, or natural bacterial predators) 5.)
In the short term, develop more truly new
antibiotics