Mohammad Emaneini, PhD
Assistant Professor in Microbiology
Department of Microbiology
School of Medicine
Tehran University of Medical Sciences
emaneini@tums.ac.ir
http://tums.ac.ir/faculties/emaneini
Learning Objectives
 Identify five modes of action of antimicrobial drugs
 Explain why the antibiotics are specific for bacteria
 List the advantages of each of the following:
Semisynthetic penicillin, Cephalosporins, & Vancomycin
 Describe how each of the following inhibits protein synthesis:
Aminoglycosides, Tetracyclines, Chloramphenicol, Macrolides
 Compare the mode of action of polymyxin B, bacitracin
 Describe how rifamycins and quinolones kill bacteria
 Describe how sulfa drugs inhibit microbial growth
Emaneini M. PhD.
The era of chemotherapy

1910

Paul Ehrlich

The German chemist

Discovered Salvarsan

Effective against Treponema pallidum
Emaneini M. PhD.
The era of chemotherapy
 In 1928, Alexander Fleming's discovery of penicillin
 In 1935, Gerhard Domagk's discovery of Sulfonamidochrysoidine
 In 1943, Selman Waksman's discovery of streptomycin
Emaneini M. PhD.
The Spectrum of Antimicrobial Activity
Narrow spectrum (limited spectrum)
 Antimicrobials effective against a (limited spectrum)
of microbial types
 A drug effective on G+ or G- bacteria
Broad spectrum (extended spectrum)
 Antimicrobials effective against a (extended spectrum) wide
variety of microbial types
 A drug effective against both G+& G- bacteria
Emaneini M. PhD.
The Action of Antimicrobial Drugs
Bactericidal
Kill microbes directly
Bacteriostatic
Prevent microbes from growing
Emaneini M. PhD.
Mechanisms of Antibiotics Action
Emaneini M. PhD.
Mechanisms of Antibiotics Action
1- Inhibition of Cell Wall Synthesis
2- Injuring the Plasma Membrane
3- Inhibition of Protein Synthesis
4- Inhibition of Nucleic Acid Synthesis
5- Inhibiting the Synthesis of Essential Metabolites
Emaneini M. PhD.
Inhibition of Cell Wall Synthesis
Penicillin-binding proteins (PBPs(
Transpeptidases, Carboxypeptidases, Transglycosylases
β-Lactam antibiotics: generally are bactericidal agents
Emaneini M. PhD.
Inhibition of Cell Wall Synthesis
1- Beta-Lactam Antibiotics
 Penicillins
 Cephalosporins
2- Glycopeptides
 Vancomycin
3- Lipopeptides
 Daptomycin
4- Polypeptides
 Bacitracin
Emaneini M. PhD.
Beta-Lactam Antibiotics
Penicillins
 6-aminopenicillanic acid
Penicillium chrysogenum
R
β-lactam
Thiazolidine
Emaneini M. PhD.
Natural Penicillins
Penicillin G
 Is incompletely absorbed
 Inactivated by gastric acid
 An intravenous drug
Penicillin G
Penicillin V
 Resistant to acid
 Oral form
Active against
All β-hemolytic & most other streptococci
Meningococci & most G+ anaerobes
Emaneini M. PhD.
Penicillin V
Penicillinase resistant penicillins
Nafcillin, Oxacillin, Methicillin, Cloxacillin, Dicloxacillin
Similar to natural penicillins
Enhanced activity against staphylococci
Nafcillin
Methicillin
Oxacillin
Dicloxacillin
Cloxacillin
Emaneini M. PhD.
Broad-spectrum penicillins
1- Aminopenicillins: Ampicillin, Amoxicillin
Ampicillin was limited primarily to Escherichia & Proteus species
Amoxicillin
Ampicillin
2- Carboxypenicillins: Carbenicillin, Ticarcillin
Are effective against a broader range of G- bacteria
Klebsiella, Enterobacter, & Pseudomonas species
Ticarcillin
Carbenicillin
Emaneini M. PhD.
Broad-spectrum penicillins
3- Ureidopenicillins: Azlocillin, Piperacillin, Mezlocillin
Azlocillin
Piperacillin
Emaneini M. PhD.
Mezlocillin
Analogues
Clavulanic acid, Sulbactam, Tazobactam
 β- lactamase inhibitors
 Irreversibly inactivate susceptible bacterial β- lactamases
 Are relatively inactive by themselves
 When combined with some penicillins are effective
(ampicillin, amoxicillin, ticarcillin, piperacillin)
Amoxicillin/clavulanic acid (Co-amoxiclav)
Ampicillin/sulbactam (Sultamicillin)
Clavulanic acid
Sulbactam
Emaneini M. PhD.
Tazobactam
Cephalosporins
 7-aminocephalosporanic acid
 Originally isolated from the mold Cephalosporium
Cephamycins
 Contain O in place of S
 More stable to β-lactamase hydrolysis
R3
R1
β-lactam ring
Dihydrothiazine
ring
R2
Emaneini M. PhD.
First-generation (narrow-spectrum)
Cefazolin, Cephalexin, Cephalothin, Cephapirin, Cephradine
Escherichia coli
Klebsiella species
Proteus mirabilis
Oxacillin-susceptible gram-positive cocci
Cefazolin
Cephalexin
Emaneini M. PhD.
Cephradine
Second-generation (expanded-spectrum)
Cefamandole, Cefaclor, Cefuroxime, Cefotetan, Cefoxitin
Haemophilus influenzae
Enterobacter species
Citrobacter species
Serratia species
Some anaerobes, such as Bacteroides fragilis
Cefoxitin
Cefaclor
Cefamandole
Emaneini M. PhD.
Third-generation (broad-spectrum)
Cefixime, Cefoperazone, Cefotaxime, Ceftazidime,
Ceftizoxime, Ceftriaxone
Most Enterobacteriaceae
Pseudomonas aeruginosa
Ceftriaxone
Cefixime
Ceftazidime
Emaneini M. PhD.
Fourth-generation
Cefepime, Cefpirome
Activity = oxacillin against gram-positive bacteria
Improved gram negative activity
Cefpirome
Emaneini M. PhD.
Fifth-generation
Ceftobiprole, Ceftaroline
Ceftobiprole
Ceftaroline acetate
Emaneini M. PhD.
Carbapenems
Imipenem, Meropenem, Ertapenem
 Broad-spectrum antibiotics
 Active against virtually all organisms
 Resistance has been reported
All oxacillin-resistant staphylococci
Selected Enterobacteriaceae
Pseudomonas
Ertapenem
Meropenem
Imipenem
Emaneini M. PhD.
Monobactams
Aztreonam
 Narrow-spectrum antibiotics
 Are active only against aerobic, G- bacteria
 Anaerobic bacteria and G+ bacteria are resistant
Aztreonam
Emaneini M. PhD.
Resistant to β-lactam antibiotics
1) Prevention of the interaction of the antibiotic & the target PBP
Only in G- particularly Pseudomonas species
Changes in the porins
Alter the size or charge of channels
2) Modification of the binding of the antibiotic to the PBP
I- A mutation in the PBP gene
Penicillin resistance in Enterococcus faecium
II- Modification of an existing PBP through recombination
Penicillin resistance in Streptococcus pneumoniae
III- Acquisition of a new PBP
Escherichia coli , MRSA
IV- An overproduction of PBP
3) Hydrolysis of the antibiotic by β-lactamases
Emaneini M. PhD.
β-lactamases
 Serine proteases as the PBPs
> 200 different β-lactamases
Penicillinases: specific for penicillins
Cephalosporinases: specific for cephalosporins
Carbapenemases: specific for carbapenems
 Four classes (A to D)
Emaneini M. PhD.
β-lactamases
Class A
 The most common are SHV-l & TEM-l
 Found in G- rods (e.g., Escherichia, Klebsiella)
 Minimal activity against cephalosporins
 Point mutations: Extended-spectrum β-lactamases [ESBLs]
Are commonly encoded on plasmids
Emaneini M. PhD.
β-lactamases
Class B
 Zinc dependent metalloenzymes
 Broad spectrum of activity against all β-lactam antibiotics
Class C
 Are primarily cephalosporinases
 Are encoded on the bacterial chromosome
Class D
 Are penicillinases
 Found primarily in G- rods
Emaneini M. PhD.
Glycopeptides: Vancomycin
 Obtained from Streptomyces orientalis
 Interacts with the D-alanine-D-alanine in the pentapeptide
 Is inactive against G- bacteria
 Intrinsically resistant
D-alanine-D-lactate
Lactobacillus, Erysipelothrix
D-alanine-D-serine
Enterococcus gallinarum, E. casseliflavus
 Acquired resistance: vanA & vanB
Emaneini M. PhD.
Vancomycin
Polypeptides: Bacitracin
 Bacillus licheniformis
 Interfering with dephosphorylation of the lipid carrier
 Damage cytoplasmic membrane and inhibit RNA transcription
 The treatment of skin infections caused by
Staphylococcus & group A Streptococcus
 Used in creams, ointments, sprays
 G- bacteria are resistant
 Resistance: failure of the antibiotic to penetrate into the cell
Bacitracin
Emaneini M. PhD.
Inhibition of Cell Wall Synthesis
Isoniazid, Ethionamide, Ethambutol, & Cycloserine
Used for the treatment of mycobacterial infections
Isoniazid
Isonicotinic acid hydrazide [INH])
Bactericidal; Blocks mycolic acid synthesis
Ethionamide
Derivative of INH
Blocks mycolic acid synthesis
Ethambutol
Interferes with the synthesis of arabinogalactan in the cell wall
Cycloserine
Inhibits D-alanine-Dalanine synthetase & Alanine racemase
Emaneini M. PhD.
Injuring the Plasma Membrane
1- Lipopeptides
Daptomycin
2- Polypeptides
Polymyxins
Emaneini M. PhD.
Lipopeptides: Daptomycin
A naturally cyclic lipopeptide
 Streptomyces roseosporus
 Binds irreversibly to the CM.
 Disruption of the ionic gradients
 Active against G+ bacteria
 G- bacteria are resistant
Emaneini M. PhD.
Daptomycin
Polypeptides: Polymyxins
 Cyclic polypeptides
 Bacillus polymyxa
 Interacting with LPS & the phospholipids in the OM
 Increased cell permeability
 Polymyxin B & E (Colistin) causing serious nephrotoxicity
 Localized infections: external otitis, eye & skin infections
Colistin
Polymyxin B
Emaneini M. PhD.
Inhibition of Protein Synthesis
Emaneini M. PhD.
Inhibition of Protein Synthesis
Emaneini M. PhD.
Aminoglycosides


Amino sugars --- Glycosidic Bond--- Aminocyclitol ring
Bactericidal

Bind irreversibly to ribosomal proteins
Misreading of the messenger RNA (mRNA)
Premature release of the ribosome from mRNA

Streptomycin, Neomycin, Kanamycin, & Tobramycin
Streptomyces species

Gentamicin & Sisomicin
Micromonospora species

Amikacin from kanamycin

Netilmicin from sisomicin

Systemic infections caused by many G- rods
Emaneini M. PhD.
Streptomycin
Aminoglycosides
Resistance
1- Mutation of the ribosomal binding site
2- Decreased uptake of the antibiotic (Anaerobic bacteria)
3- Increased expulsion of the antibiotic from the cell
4- Enzymatic modification
The most common mechanism of resistance
 Phosphotransferases (APHs; 7 described)
 Adenyltransferases (ANTs; 4 described)
 Acetyltransferases (AACs; 4 described)
Emaneini M. PhD.
Tetracyclines
 Broad-spectrum
 Bacteriostatic
 Tetracycline, Doxycycline, Minocycline
 Binding reversibly to the 30S
 Blocking the binding of aminoacyl-tRNA
 Chlamydia, Mycoplasma, Rickettsia
Emaneini M. PhD.
The staining of teeth associated
with tetracycline use
Tetracyclines
Resistance
1- Decreased penetration of the antibiotic
2- Active efflux of the antibiotic out of the cell
3- Alteration of the ribosomal target site
4- Enzymatic modification of the antibiotic
Emaneini M. PhD.
Glycylcycline
Tigecycline
 Semisynthetic derivative of minocycline
 Inhibits protein synthesis as the tetracyclines
 Broad spectrum of activity: G+, G- & anaerobic bacteria
 Resistant Bacteria
Proteus
Morganella
Providencia
Pseudomonas aeruginosa
Emaneini M. PhD.
Oxazolidinones
Linezolid
 Narrow-spectrum
 Block initiation of protein synthesis
(70S initiation complex)
 Binds to the 50S ribosomal subunit
 Mechanism of resistance
Target site modification
Emaneini M. PhD.
Chloramphenicol
 Broad spectrum
 Bacteriostatic
 Blocking peptide elongation
 Binding reversibly to the peptidyl transferase (50S)
 Only for the treatment of typhoid fever
 Can produce aplastic anemia (1 per 24,000 treated patients)
 Resistance: plasmid-encoded chloramphenicol acetyltransferase
Emaneini M. PhD.
Macrolides
Erythromycin, Azithromycin, Clarithromycin
 Streptomyces erythreus
Desosamine
 Broad spectrum
 Bacteriostatic
 Blocks polypeptide elongation
Reversible binding to the 23S rRNA
 Used to treat pulmonary infections
Mycoplasma, Legionella, & Chlamydia species
 Infections caused by Campylobacter species
Emaneini M. PhD.
Cladinose
Macrolides
Resistance
1- Alteration of the ribosomal target site
Methylation of the 23S rRNA
2- Enzymatic modification of the antibiotic
Destruction of the lactone ring by an erythromycin esterase
3- Mutations in the 23S rRNA & ribosomal proteins
Emaneini M. PhD.
Ketolides
Telithromycin
 Semisynthetic derivatives of erythromycin
 Increase stability in acid
 Blocks protein synthesis as Macrolides
 Broad-spectrum antibiotic
 Active against some macrolide
resistant staphylococci & enterococci
Emaneini M. PhD.
Lincosamide
Clindamycin
 Derivative of lincomycin (Streptomyces lincolnensi)
 Inhibits peptidyl transferase
 Block the binding of the amino acid-acyltRNA complex
 Resistance: Methylation of the 23S ribosomal RNA
E
CD
‫حساس‬
E
CD
E
CD
‫مقاومت پيوسته‬
‫مقاومت القايي‬
Emaneini M. PhD.
Streptogramin
Streptogramin
 Cyclic peptides
 Streptomyces species
 Group A and group B
 Quinupristin-dalfopristin (Synercid)
 Dalfopristin prevents peptide chain elongation
 Quinupristin initiates premature release of peptide
Emaneini M. PhD.
Inhibition of Nucleic Acid Synthesis
1- Quinolones
2- Rifampin
3- Metronidazole
Emaneini M. PhD.
Quinolones
 Synthetic
 Inhibit bacterial DNA gyrases (II) or topoisomerases (IV)
Nalidixic acid
Fluoroquinolones:
Ciprofloxacin
Levofloxacin
Gatifloxacin
Resistance: mutations in chromosomal genes of DNA gyrases (II)
or topoisomerases (IV)
Emaneini M. PhD.
Rifampin
 Semisynthetic derivative of rifamycin B
 Streptomyces mediterranei
 Inhibits the initiation of RNA synthesis
 Bactericidal
 Mycobacterium tuberculosis
Staphylococci
Streptococci
Resistance: a mutation in the chromosomal gene
that codes for the β subunit of RNA polymerase (In G+)
Emaneini M. PhD.
Metronidazole
 Reduction of its nitro group by bacterial nitroreductase
 Producing cytotoxic compounds that disrupt the host DNA
 Anaerobic bacterial infections (B. fragilis)
Resistance
1- Decreased uptake
2- Elimination of the cytotoxic compounds
Emaneini M. PhD.
Antimetabolites
Sulfonamides
 Preventing the synthesis of the folic acid
 Compete with p-aminobenzoic acid
 Mammalian organisms do not synthesize folic acid
Treatment of Nocardia, Chlamydia, & some protozoa infections
Sulfacetamide
R Group
Emaneini M. PhD.
Sulfadiazine
Sulfisoxazole
Antimetabolites
Trimethoprim
 Blocks the conversion of dihydrofolate to tetrahydrofolate
 Inhibiting dihydrofolate reductase
 Trimethoprim + sulfamethoxazole: Synergistic combination
 Treatment of acute and chronic urinary tract infections
 Resistance
Permeability barriers : Pseudomonas
Decreased affinity of dihydrofolate reductase
Emaneini M. PhD.
Mechanisms of Resistance
1- Enzymatic Destruction or Inactivation of the Drug
2- Prevention of Penetration to the Target Site
3- Alteration of the Drug's Target Site
4- Rapid Efflux (Ejection) of the Antibiotic
Emaneini M. PhD.
Antibiotic Assays
The disk agar diffusion (DAD) method involves
Different antibiotics diffusing from paper disks in a bacterial colony
Emaneini M. PhD.
Antibiotic Assays
The tube dilution method determines the minimum inhibitory
concentration (MIC)
Emaneini M. PhD.
Emaneini M. PhD.