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Transcript
ANTIBIOTICS
&
The Control of Bacterial Growth
Counting Chamber
Makes thin microscopic preparation in a known volume of liquid
Plate Counts
(keep track of the math)
Plate counts, continued
Plate counts, continued
Control of Bacterial Growth
Natural control of bacterial growth
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Nitrates, sulfur dioxide
Temperature (eg., Pasteurization, dry heat)
Salt, vinegar
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Heat
Filtration
Radiation
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Chemical methods of disinfection
Sites of action of germicidal chemicals
Sterilization: removal or destruction of all microorganisms on or in a product
Disinfection: elimination of most or all pathogens on or in a material
Decontamination: reducing pathogens to levels which are safe to handle.
The story of Penicillin
Late 1920s
Alexander Fleming
Scottish physician and bacteriologist
Discovered that a fungal metabolite could be used to control bacterial growth
Fungus: Penicillium notatum
Penicillin prevented growth of Staphylococcus aureus
The story of Penicillin
"the greatest
contribution medical
science ever made to
humanity."
Time magazine (1999)
The Great Minds Of The Century
“A spore that drifted into his lab and took root on a culture dish
started a chain of events that altered forever the treatment of
bacterial infections.”
Action of penicillin
Overlay plate
A colony of the fungus Penicillium notatum is allowed to grow on agar. The
plate is overlaid with a thin film of molten agar containing bacteria.
Penicillin production by the fungus creates a zone of growth inhibition of the
bacteria.
Penicillin rapidly became the "wonder drug“
One of the single most effective drugs of the last century
The extension of human lifespan
Antibiotics
Public health, Sanitation, Immunization
Antimicrobial Drugs
Antimicrobial drugs: Low-molecular weight substances, natural or synthetic,
which kill or inhibit the growth of microorganisms with relatively little harm to
the host.
Antibiotic: a compound naturally produced by molds or bacteria that kills or
inhibits the growth of other microorganisms.
Antiviral: a drug that interferes with the infection cycle of a virus.
Bactericidal: kills bacteria.
Bacteriostatic: inhibits the growth of bacteria.
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Chemical methods of disinfection
Features of antimicrobial drugs
Selective toxicity
Stability
Access to targets
Effectiveness of individual antibiotics
Varies with:
the location of the infection
the ability of the antibiotic to reach the site of infection
sensitivity of the bacterial target
Speed of action
Side effect on the host
the ability of the bacteria to resist or inactivate the antibiotic
Access to the world-wide population:
- should be inexpensive and easy to produce and administer
- should be chemically-stable (have a long shelf-life)
Location of the infection
Examples:
Certain antibiotics including some of the -lactams do not
function very well in the acidic environment of the stomach
Some antibiotics such as bacitracin may be too toxic for
internal use but can be used in topical creams for preventing
wound infections
Ability of the antibiotic to reach the site of
infection
Oral antibiotics - simplest approach when effective
Intravenous antibiotics - reserved for more serious cases
Examples:
Antibiotics vary in their ability to:
 be absorbed orally
 cross the blood brain barrier (BBB)
Vancomycin cannot cross the intestinal lining; administer intravenously
To overcome the BBB: inject large quantities of an antibiotic directly into a
patient's bloodstream
Sensitivity of target
Broad-spectrum
(kill or inhibit a wide range of Gram-positive and Gram-negative bacteria)
versus
Narrow-spectrum
(effective mainly against Gram-positive or Gram-negative bacteria)
Limited spectrum
(effective against a specific species)
Speed of action
Generally faster:
Bactericidal – drug kills bacteria (penicillin)
Generally slower:
Bacteriostatic – drug inhibits growth (tetracycline)
Side effects
Antimicrobial therapy:
Optimize the therapeutic index (ratio between
the effective dose and the toxic dose)
 Stomach Pain/diarrhea
 Nausea
 Yeast and other fungal infection
Allergic reactions
Disrupt function of liver, kidneys and other organs
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Chemical methods of disinfection
Features of antimicrobial drugs
Mechanisms of action of antibacterial drugs
Targets of antibiotic action
Drug target is present in bacteria, but absent in host cells
(preferred).
Drug targets a step in metabolism that is essential in bacteria but
not in host.
Drug target is present in both bacteria and host, but the bacterial
target is more sensitive to drug.
Targets of antibacterial medications
Peptidoglycan
Inhibition of peptidoglycan synthesis
Penicillin interferes with peptidoglycan cross-linking
through interaction with Penicillin-Binding Proteins (PBPs).
The effect of penicillin on a cell
Antibacterial
medications that
interfere with cell
wall synthesis
The -lactam rings of penicillin and
cephalosporin
The penicillin
family
Vancomycin
Glycopeptide antibiotic
Given intravenously – commonly used for
nosocomial infections
Vancomycin binds to the D-ala-D-ala terminal
peptide on peptidoglycan precursors and
prevents chain elongation
- last resort
Antibiotics that inhibit prokaryotic protein
synthesis
Anti-ribosomal antibiotics
– second largest class of
antibiotics
- selective; structural differences
between ribosomes of
bacteria and host cells
- affect different stages of
protein synthesis
Block initiation
Block chain elongation
oxazolidinones
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Chemical methods of disinfection
Features of antimicrobial drugs
Mechanisms of action of antibacterial drugs
Determining susceptibility to antimicrobial drugs
Determining susceptibility to
antimicrobial drugs
Minimum inhibitory concentration (MIC)
Minimum bactericidal concentration (MBC)
Dilution assays and disc sensitivity assays
Determining the minimum inhibitory
concentration (MIC) of an antimicrobial
drug
The Kirby-Bauer method for determining
drug susceptibility
Size of the zone reflects sensitivity to the drug
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Chemical methods of disinfection
Features of antimicrobial drugs
Mechanisms of action of antibacterial drugs
Determining susceptibility to antimicrobial drugs
Resistance to antimicrobial drugs
Antibiotic resistance
Clatworthy et al Nature Chemical Biology, 2007
Antibiotic resistance
Resistance to more than 15 drugs
has become one of the world’s most pressing public health
problems
70 percent of nosocomial bacteria - resistant to at least
one of the drugs used to treat infections
Some organisms - resistant to all approved antibiotics and
must be treated with experimental and potentially toxic
drugs
The selective
advantage of drug
resistance
Mechanisms
of acquired
antimicrobial
resistance
Resistance to antibiotics
For an antibiotic to exert its effect it must:
 be transported to the site of action
 associate with bacteria and penetrate their cell envelope
 bind to their specific molecular target
Resistance to drug can occur at any step in this process.
Intrinsic resistance
Acquired resistance: resistance that develops through mutation
or acquisition of new genes.
Mechanisms of resistance
Synthesis of enzymes that break down the drug
Chemical modification of the drug
Prevention of access to the target site by blocking uptake
Prevention of access to the target by increasing efflux of the drug
Increased activity of an alternative pathway
Modification of the target site
-lactams and resistance
The principal mechanism of resistance to -lactams
(penicillin) – inactivating enzymes called -lactamases.
They hydrolyze the -lactam ring.
Gram-positives produce extracellular -lactamases
Gram-negatives make periplasmic -lactamases
Vancomycin
Resistance to anti-ribosomal antibiotics
Enzymes modify antibiotics
Pumps actively excrete antibiotics
Methylation of ribosomal RNA
Mutations in ribosomal RNA or ribosomal S50 subunit
Quinolones (nalidixic acid)
Inhibit the action of topoisomerases. They trap these enzymes in the
act of cutting DNA thus promoting the formation and persistence of
double strand breaks in the bacterial chromosome.
Resistance I: mutations in the topoisomerases that prevent/reduce
the binding of the drugs.
Resistance II: efflux pumps remove quinolones from the bacterial cells.
Rifampicin
Binds to the RNA polymerase to prevent transcription
One of the few drugs that can treat tuberculosis
Resistance: point mutations in RNA polymerase
Multidrug therapy
Advantage:
The chance that a single bacterium becomes resistant to both antibiotics
is small
The effectiveness of the drugs together is greater than that of either drug
alone (synergism)
Disadvantage:
The action of one drug reduces the
efficiency of the other (antagonism).
Indifference: Alone or in combination
- no better no worse
Control of Bacterial Growth
Natural control of bacterial growth
Commercial control of bacterial growth
Physical methods of disinfection
Chemical methods of disinfection
Features of antimicrobial drugs
Mechanisms of action of antibacterial drugs
Determining susceptibility to antimicrobial drugs
Resistance to antimicrobial drugs
New approaches to discovery of antimicrobial drugs
Efforts to identify and synthesize new antibiotics
Continue modification of existing antibiotics
Continue screening of soil isolates
Screening of organisms from other
environments
Rational drug design – use crystal structure
of target molecule as guide for design and
synthesis of chemicals that bind to and inactivate the target molecule
Combinatorial chemistry – create an array of chemical derivatives and test
this library against particular bacterial targets
Inhibitor screen
High-throughput screen of ~ 50,000-100,000 chemical compound libraries
New paradigm for antibiotic development
Traditional antibiotics: kill or inhibit growth of bacteria
New way: target virulence properties
Target bacterial proteins that promote infection
- Prevent colonization
- Block the activity of toxins or other virulence factors
Prevent their binding to receptors or intracellular targets
Prevent their secretion
Target virulence properties
Three petri dishes
Inhibitors of folic acid
metabolism
Folic acid