ANTIMICROBIAL DRUGS
Chapter 20
CHEMOTHERAPY
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Use of drugs to treat diseases
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Inhibit growth or kill the pathogen
Used internally, not externally like disinfectants
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SELECTIVE TOXICITY
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ANTIBIOTICS - naturally produced by bacteria & fungi
SYNTHETIC DRUGS - synthesized in a lab
ANTI-VIRAL DRUGS
GOAL: Kill pathogen and do not harm the host
THREE CLASSES
HISTORY #1
• 1910: Paul Ehrlich --> “MAGIC BULLET”
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Some dyes stain eukaryotes & prokaryotes differently
POISON + DYE ----> Kill microorganism
ARSENIC & its derivatives --> SALVARSAN
SALVARSAN effective vs syphilis (T. pallidum)
• 1928: Alexander Fleming ---> PENICILLIN
– S. aureus did not grow near the mold Penicillium notatum
– P. notatum inhibitory substance secreted was toxic
• “Penicillin” was unstable ----> discarded idea
– ANTIBIOTIC from ANTIBIOSIS ie against life
HISTORY #2
• 1930s: Sulfa drugs were discovered
– Prontosil ---> metabolized to SULFANILAMIDE
• 1940: Florey & Chain --> resumed penicillin studies
– Isolated and purified penicillin
– USA became involved and purified high-producing strains
• Able to increase the stability of penicillin
– Was used during WWII ---> “AGE OF ANTIBIOTICS”
ANTIBIOTICS
• A substance that is produced by one microorganism (a
bacterium or fungus) which in small amounts will kill or inhibit
the growth of another microorganism
• MAJOR PRODUCERS - most are soil organisms
– MOLDS
• Penicillium
• Cephalosporium
– BACTERIA
• Streptomyces
• Bacillus
SPECTRUM of ANTIMICROBIAL ACTIVITY
• SELECTIVE TOXICITY
– Drug should only kill/inhibit the pathogen not the host
• DIFFERENCES BETWEEN PATHOGEN & HOST
– Eukaryote host vs prokaryote pathogen
– Eukaryote host vs eukaryote pathogen
– Eukaryote host vs viruses
• NARROW SPECTRUM
– Effective only against Gram +ve OR Gram -ve bacteria
• BROAD SPECTRUM
– Effective against both Gram +ve AND Gram -ve bacteria
• Normal flora may also be affected
PROPERTIES of ANTIBIOTICS
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Antibiotics should be:
Selectively toxic
Bactericidal vs bacteriostatic
No resistance to the drug emerging
Broad spectrum of activity
No toxic side effects
Active in vivo (in the body)
Active at low concentrations
MECHANISMS of ACTION
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INHIBIT CELL WALL SYNTHESIS
INHIBIT PROTEIN SYNTHESIS
INJURE PLASMA MEMBRANE
INHIBIT NUCLEIC ACID REPLICATION &
TRANSCRIPTION
• INHIBIT METABOLITE SYNTHESIS
1. INHIBITION of CELL WALL SYNTHESIS
• Excellent selective toxicity
– Peptidoglycan found only in prokaryotes
• Penicillins & cephalosporins
– Prevent cross linking of peptidoglycan units  stops new cell wall
synthesis
– More effective against Gram +ve organisms
• Bacitracin & vancomycin
– Also inhibit cell wall synthesis
2. INHIBITION of PROTEIN SYNTHESIS
• DIFFERENT RIBOSOME STRUCTURE
– Eukaryotes = 80S (40S + 60S)
– Prokaryotes = 70S (30S + 50S)
– Mitochondrial = 70S
• Bind to 50S subunit
– Chloramphenicol - inhibits peptide bond formation
– Erythromycin - prevents movement along mRNA
• Bind to 30S subunit
– Tetracyline - prevents tRNA attachment to ribosome
– Streptomycin - changes shape of 30S reads mRNA incorrectly
3. INJURY to PLASMA MEMBRANE
• Bind to and alter membrane permeability and/or cause leakage
– Polypeptide antibiotics - polymxyin B
• Antifungal drugs bind to membrane sterols and disrupt the
membrane
– Fungal cell membranes contain ergosterol
– Animal cell membranes contain sterol
• Examples
– Ketoconazole & Miconazole
– Amphotericin B & Nystatin
• May be toxic to the host
4. INHIBITION of NUCLEIC ACID
REPLICATION and TRANSCRIPTION
• Selective toxicity is POOR
• STOP DNA replication
– Many antiviral drugs act at this level
• STOP RNA synthesis
5. INHIBITION of ESSENTIAL
METABOLITE SYNTHESIS
• Many act as competitive inhibitors of enzymes
– Drug mimics normal substrate (metabolite)
– Enzyme binds drug and is inhibited
– Pathway stops, normal metabolites are not produced
• Example
– Sulfanilamide (a sulfa drug) replaces PABA (para-aminobenzoic acid) a
substrate in the folic acid pathway
– Selectively toxic for microbes that synthesize PABA
– Humans do not synthesize PABA
INHIBITORS of CELL WALL SYNTHESIS
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ANTIBACTERIAL DRUGS
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Penicillins
Cephalosporins
1. PENICILLINS #1
• All have similar structure - “Penicillin nucleus”
– Beta lactam ring = 4 member ring
– Thiazolidine ring = 5 member ring with N & S atoms
• Together these rings are called 6-aminopenicillanic acid
– Unique side chain group that differentiates the different penicillins (R
group)
• Bactericidal - stops cell wall synthesis
– Binds to a transpeptidase --> prevents crosslinking of peptides of
peptidoglycan
• Some are produced naturally
• Some are produced semi-synthetically
1. PENICILLINS #2
• R group affects
– Spectrum of penicillin
– pH stability
– Sensitivity to penicillinase
• Penicillinase (-lactamase)
– Enzyme that breaks down -lactam rings
– Secreted by some microorganisms
– Alters activity of the penicillin
• Many people may have allergic reactions and anaphylactic shock
to Penicillins
NATURAL PENICILLINS
• Penicillin G = prototype
– Effective vs Gram positive bacteria
• Staph & strep as well as gonoccoci and some spirochetes
– Sensitive to low pH  not taken orally
– Often given with other drugs to increase retention time
• Procaine and benzathine (I.M. injection)
• Penicillin V - acid stable
– Given orally
• Both susceptible to penicillinase
• Relatively narrow spectrum of activity
SEMISYNTHETIC PENICILLINS
• Produced two ways:
– Grow Penicillium so only the nucleus is produced then attach R group in
the lab OR 2. grow Penicillium, extract the natural penicillin, remove
the “natural” R group & replace with new R group
• Tend to have a broader spectrum of activity
– Effective vs Gram -ve bacteria as well (methicillin & oxacillin)
• Aminopenicillins: ampicillin & amoxicillin
– Resistance has developed against these
• Carboxypenicillins: carbenicllin & ticarcillin
– More effective vs Gram –ve & some P. aeruginosa sp.
• Mix with clavulanic acid that inhibits penicillinase
– Amoxicillin + clavulanic acid = Augmentin
3. CEPHALOSPORINS
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Similar in structure and range of activity to penicillins
Inhibit prokaryotic cell wall synthesis
Relatively resistant to penicillinases
More effective vs some Gram -ve bacteria
Not sensitive to pH changes
First generation cephalosporins are active vs Gram +ve bacteria
– EXCEPT: MRSA - methicillin resistant S. aureus
• 2nd and 3rd generation cephalosporins have a broader range of
activity
• Can have allergic reaction and superinfections
IINHIBITORS of PROTEIN SYNTHESIS
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ANTIBACTERIAL DRUGS
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Aminoglycosides
Tetracyclines
Chloramphenicol
Macrolides
1. AMINOGLYCOSIDES
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Bactericidal - stop protein synthesis by binding to the 30S
ribosomal subunit
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Effective vs Gram -ve & some mycobacteria
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Alternative treatment for tuberculosis
Side effect --> damages 8th cranial nerve --> loss of hearing
STREPTOMYCIN - discovered in 1944
GENTAMICIN
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Effective vs Pseudomonas infections
NEOMYCIN
– Used topically
Side effects: Hearing loss and kidney damage
2. TETRACYCLINES
• Produced by Streptomyces
• Very broad spectrum antibiotic
– Gram +, Gram -, rickettsia & chlamydia
• Bacteriostatic - inhibits protein synthesis
• Many derivatives - oxytetracycline, chlorotetracycline
– Doxycycline, minocycline, clorotetracycline
• Used to treat urinary tract infections (UTI), chlamydial and
spirochetal diseases
– Can lead to GI upset & superinfections
– Long term use  tooth discoloration & kidney damage
• Not used in pregnant women or young children
3. CHLORAMPHENICOL
• Broad spectrum antibiotic
• Bacteriostatic - inhibits protein synthesis
– Binds to the 50S ribosomal subunit  stops elongation of the protein
– Small molecule  diffuses readily (brain, CSF etc)
– Used to treat meningitis & typhoid fever
• Can suppress the bone marrow activity  aplastic anemia
4. MACROLIDES
• LARGE molecules
– Contain a macrocyclic LACTONE RING
• Bacteriostatic - inhibits proteins synthesis
– Binds to the 50S subunit
• ERYTHROMYCIN
– Fairly narrow spectrum
• Not very effective against Gram -ve
– Alternate to penicillin (allergic patients)
– Used to Legionella, Neisseria & mycoplasmas
• Newer macrolides include
– Azithromycin and clarithromycin
INJURY to PLASMA MEMBRANE
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ANTIBACTERIAL DRUGS
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Polymyxin B
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Polyenes
Imidazoles and Triazoles
ANTIFUNGAL DRUGS
1. POLYMXIN B
• Polypeptide antibiotic
– Produced by Bacillus
• Bactericidal - injures plasma membrane
• Primarily effective vs Gram -ve bacteria
• Used topically due to high toxicity
2. POLYENES
• Antibiotic produced by Streptomyces
– Fungicidal - interacts with membrane ergosterols to destroy selective
permeability leading to death of the fungi
• NYSTATIN
– Treat localized fungal infections in the vagina and skin such as Candida
albicans
• AMPHOTERICIN B
– Treat systemic fungal infections such as histoplasmosis but
nephrotoxic
3. IMIDAZOLES & TRIAZOLES
• Inhibit synthesis of ergosterols interferes with fungal
growth
– Used to treat dermatophytes, dimorphic fungi & yeasts
• IMIDAZOLES
– Clotrimazole and miconazole – used topically
• Cutaneous mycoses (Athlete’s foot, vaginal yeast infections)
– Ketoconazole – given orally (systemic mycoses)
• TRIAZOLES
– Fluconazole & itraconazole (Diflucan)
• Less toxic, used to treat systemic and opportunistic infections
INHIBITORS of NUCLEIC ACID SYNTHESIS
• ANTIBACTERIAL DRUGS
– Quinolones and fluoroquinolones
– Rifamycins
• ANTIVIRAL DRUGS
– Very few available, ANTIBIOTICS do not act on viruses
• Often given to treat secondary bacterial infections
• SELECTIVE TOXICITY = DIFFICULT
– Usually numerous side effects
– Amantadine
– Ribavarin
– Acylcovir
– Ganciclovir
– Idoxuridine and fluridine
– Zidovudine (AZT)
3. AMANTADINE
• First antiviral to be approved
– A tricyclic amine
• MECHANISM of ACTION
– May prevents virus from entering the cell OR
– May prevent uncoating of the virus after enters the cell
• Effective against prevention of influenza A viruses
• Usually used in the elderly to prevent spread of the disease
– Especially in nursing homes & institutions to prevent spread of infection
– Not effective once the disease has been contracted
– Many side effects
5. ACYCLOVIR
• ACYCLOVIR (Zovirax) - guanine analog
– Used to treat herpesviruses infections
• Genital herpes, chickenpox and shingles
• Reduces pain & promotes healing of primary lesions in a new case of
genital herpes
• BASE ANALOG - structurally similar to nucleic acid bases
– Stops viral DNA synthesis
– Activated by a herpesvirus enzyme herpesvirus specific
• Can be administered as an ointment, orally or by injection
8. ZIDOVUDINE (AZT)
• AZIDOTHYMIDINE (AZT) - thymidine analog
– Stops the RNA dependent DNA polymerase of HIV
• Reverse transcriptase
– Used to treat AIDS patients
– Sides effects = immunosuppression and anemia
• Other nucleoside analogs used to block RT
– Dideoxyinosine (ddI)
– Dideoxycytosine (ddC)
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INHIBITORS of ESSENTIAL METABOLITES
ANTIBACTERIAL DRUGS
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Sulfonamides
1. SULFONAMIDES
• 1 of 1 synthetic drugs used to treat infections
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– Discovered in the 1930s
• Bacteriostatic: interferes with folic acid synthesis
– Folic acid needed for synthesis of purines & pyrimidines
– Sulfanilamide similar to PABA (para-aminobenzoic acid)
– Sulfanilamide is an ANTIMETABOLITE
• Sulfamethoxazole usually given with trimethoprim as TMP-SMZ
– Sulfamethoxazole – inhibits formation of dihydrofolate
– Trimethoprim – inhibits synthesis of tetrahydrofolate
• Usually effective vs Gram –vs M/O
– Especially in urinary tract infections
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TEST of CHEMOTHERAPY EFFECTIVENESS
Several different techniques have been developed
Disk-diffusion technique
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Kirby Bauer Technique = standardized method
Pure culture of pathogen on agar plate
Paper discs containing antibiotic on agar plate
Incubate plates
Observe ZONES of INHIBITION of GROWTH
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Automated systems also exist
DRUG RESISTANCE
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One of the most serious threats affecting the world today.
Mechanisms of resistance:
– Alteration of drug target inside the cell
• DNA mutation leads to change in target
• Usually affects ribosomes, drug can no longer bind.
– Alteration of membrane permeability
• Prevention of entry of drug into the cell
• Change a membrane transport protein or increase ability
to pump out of the membranes
– Inactivation of the drug by enzymes.
• Penicillinases or β lactamases.
4. Alteration of an enzyme, so a metabolic pathway is no longer
affected by the drug.
How Resistance Occurs
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Bacteria may become resistant to drugs through: genetic
changes.
Antibiotics do not induce mutations but create environments
that favors the resistant bacteria to grow.
– MUTATION creating a resistant strain that will eventually
be the only type of bacterium in the host. Change in bacterial
chromosome. Usually causes resistance to one type of
antibiotic.
– ACQUIRING A PLASMID (R factors). Some can carry as
many as six or seven genes for resistance to many classes of
different antibiotics. Transferred from one strain and one
species to another by conjugation or transduction.
How is Resistance promoted?
• Overuse and misuse of antibiotics. Selective pressure.
• Patient non compliance
• Use of bacteriostatic drugs. Need to use drugs that are
bacteriocidal and at concentrations that are high enough to kill
most bacteria.
• Use of antibiotics in animal feeds. Promotes transfer of
resistant bacteria from animal to humans.
Limiting Drug Resistance
• Responsibility of healthcare workers to target causative agent,
identify it and treat it appropriately.
• Patient compliance.
• Decreasing or banning antibiotic use in animal feeds.
• Administer a narrow spectrum antibiotic to treat specific
organism.
• Use of multiple drugs together.
SIGNIFICANCE of DRUG RESISTANCE
• Resistant cells may make up a small percentage of the
population
• Resistant cells may be able to overgrow once sensitive cells in
the population die
• RESULT - large number of resistant cells
• So drugs should not be used indiscriminately
– Used at optimal concentrations/strengths