Transcript Control of microbial growth: Sterilization and disinfectants

Antimicrobial Therapy
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• Chemotherapy: any treatment of patient with chemicals to
treat a condition.
– Now word associated with cancer treatment
– Our focus is on antimicrobial agents
• Antimicrobials: synthetic, antibiotics, or semi-synthetic
– Antibiotics: natural products made by microbes, effective against
other microbes. Some drugs are wholly synthetic.
– Semi-synthetic antibiotics: use natural antibiotic as base, but
modified chemically; most of our new “antibiotics”
– Antibiotics are “small molecules”, 300 - 1500 MW, not to be at all
confused with antibodies which are proteins (MW 150,000)
Spectrum
• Some antibiotics are considered “broad spectrum”
– By definition, these are effective against many types of
bacteria, both Gram negative and Gram positive
– Broad spectrum antibiotics can sometimes cause
problems because of damage to normal microbiota of
host
– “Superinfection” may result from this situation
• Overgrowth of “normal” microbes that cause disease
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Selective Toxicity:
the key to antibiotic therapy
• 3 concentration ranges:
ineffective, effective, and
toxic. A drug needs to have a
wide effective (therapeutic)
range.
Selective toxicity is the ability of the drug to harm the target
without harming the host. Bacteria have many targets that are
biologically different from us that the drugs can hit. As the
target becomes more like us, there are fewer and fewer drugs
that are selectively toxic: fungi, protozoa, worms, viruses,
cancer.
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Selective Toxicity and side effects
•Drugs may fail to be selectively toxic and interfere with
mammalian biochemistry. They may cause allergies. They
may destroy too many normal bacteria.
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Actions of antimicrobials
• Drugs work against microbes by these basic mechanisms:
– Inhibition of cell wall synthesis
• Causes bacterium to commit suicide, but only during
growth when cells are cutting their own PG.
– Disruption of membrane function
• Often toxic to humans because we have membranes
too, cause leakage of vital molecules.
– Inhibition of protein synthesis – many antibiotics
• Bind to ribosomal RNAs, proteins.
– Inhibition of nucleic acid synthesis
• Attack transcription, DNA unwinding enzymes
– Act as anti-metabolites – competitive inhibitors, inhibit
function of enzymes, usually bacteriostatic.
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Ideal Antibiotic
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Good drug properties (e.g. soluble in body fluids)
Selectively toxic, obviously
Easily administered
Non-allergenic
Stable in vivo, slowly broken down and excreted
Difficult for microbe to become resistant to.
Long shelf life (chemically stable)
low $
http://catshospital.com/2095_160.gif
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Measurement of Efficacy-1
Disk diffusion assay
•Paper disks with antibiotic
applied to lawn in Petri dish
•Zone of inhibition indicates
susceptibility to drug
http://faculty.mc3.edu/jearl/ML/20-17.jpg
[drug]
Distance from disk
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Measurement of Efficacy-2
• Broth dilution test to measure MIC
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Minimum inhibitory concentration
Drug is diluted in broth which is inoculated
Clear broth indicates no growth or bacteria killed.
That concentration of drug that first inhibits: MIC
http://www.bmb.leeds.ac.uk/mbiology/ug/ugteach/icu8/images/antibiotics/mic.gif
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Why bacteria might be resistant
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• Innate or behavioral: Evasion, bacteria may hide in
cells or organelles; Gram – have OM; some bacteria
have no cell walls.
• Mutations: change in transport protein, ribosome,
enzyme, etc. Mutations that are normally harmful
are selected FOR in the presence of antibiotic.
• New genetic information from elsewhere
– Plasmids or other DNA from
conjugation, transduction, etc.
http://www.mun.ca/biochem/courses/3107/
images/Stryer/Stryer_F32-13.jpg
Mechanisms of drug resistance
• Alteration of target: active site of enzyme changes,
ribosome changes.
• Alteration of membrane permeability: transport
protein changes, drug no longer enters; drug that
does enter is actively pumped out.
• Enzymatic destruction of drug: penicillinases (beta
lactamases)
• “End around” inhibitor: bacteria learns to use new
metabolic pathway, drug no longer effective.
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Attack by penicillinase
http://dwp.fcroc.nl/microbiologie/images/antibiotica/de_wer4.gif
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Human behavior and antibiotic resistance
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• Bacteria once under control are making a comeback due to
antibiotic resistance:
– S. aureus, Enterococcus, M. tuberculosis, et al.
• Human behavior:
– Most diseases caused by viruses, non-cellular, not
treatable with antibiotics (but Doctor, do something)
– last bacteria left are the most resistant, if they aren’t
killed, they become “normal”; don’t stop regimen
because you feel better.
• Social behavior
– resistance in homeless/poor
– growth stimulants in agriculture
http://www.dkp-ml.dk/images/homeless.jpg
Fighting antibiotic resistance
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• Use drug as prescribed
– Use drug at correct concentration, correct amount of time
– Don’t allow the least sensitive bacteria to survive
• Drugs in combination
– Odds of mutating to resist 2 drugs: 1 in 106 x 106
– Synergism: e.g. amoxicillin and clavulanic acid
• Limit antibiotic use
– >50% of infections are viral; not affected by antibiotics
– Constant exposure breeds resistance, selects for resist.
• New drugs