1. Antibacterial Agents

2. Drugs have been used for the treatment of infectious diseases since the 17th century (e.g. quinine for malaria, emetine for amebiasis)
Chemotherapy as a science began with Paul Ehrlich in the first decade of the 20th century.
He formulated the principles of selective toxicity and recognized the specific chemical relationships between pathogens and drugs, the development of drug resistance, and the role of combined therapy.
Ehrlich's experiments led to the Arsphenamines for syphilis, the first planned chemotherapy.

3. The current era of antimicrobial chemotherapy began in 1935, with the discovery of sulfonamides.
Although antiseptics had been applied topically to prevent the growth of microorganisms, systemic bacterial infections had not as yet responded to any existing agents.
In this year, the red azo dye protosil was shown to protect mice against systemic streptococcal infections and to be curative in patients suffering from such infections.
It was soon found that protosil was cleaved in the body to release p-aminobenzene sulfonamide, or sulfanilamide.

4. Compounds (antibiotics) produced by microorganisms were eventually discovered to inhibit the growth of microorganisms (penicillin, streptomycin, tetracycline, … etc).
Despite the rapidity with which new chemotherapeutic agents are introduced, bacteria have shown a remarkable ability to develop resistance to these agents.
Thus antibiotic therapy will not be the predicted magical cure for all infections, rather it is only one weapon, albeit an important one, against infectious diseases

5. Fleming and Penicillin5

6. Microbial Sources of Antibiotics6

7. Definitions Selective toxicity Antimicrobial agents
- Antibiotics Vs chemotherapeutic agents
Antibacterial spectrum: range of activity
- Broad Vs Narrow
Bacteriostatic Vs bactericidal
Combination therapy
- Synergism Vs antagonism
MIC and MBC

8. Mechanisms of Action Inhibition of cell wall synthesis
- Beta lactams, Vancomysin, Teicoplanin, Bacitracin, Isoniazid, Ethambutol, and Cycloserine.
Alteration of cell membrane function
- Polymyxins, Amphotericin B, Imidazoles, Triazoles, Polyenes.
Inhibition of protein synthesis
- Chloramphenicol, Erythromycin, Lincomycins, Tetracycline, Aminoglycosides.
Inhibition of nucleic acid synthesis
- Rifampin, Quinolones, Metronidazole, Sulfonamides, Trimethoprim.

9. Modes of Antimicrobial Action9

10. Inhibitors of Cell Wall Synthesis

11. Beta Lactam Antibiotics
Penicillins (6-aminopenicillanic acid)
- composed of a beta- lactam ring, a thiazolidine
ring and side chains.
Cephalosporins (7-amino cephalosporanic acid)
- composed of a beta- lactam ring, a dihydrothiazine

12. 12

14. Cephamycin
- contains oxygen in place of sulfur in the
dihydrothiazine ring.
Carbapenems ( Imipenem)
Monobactams ( Aztreonam)
Beta- Lactamase inhibitors (sulbactam and clavulanic acid)

15. b-Lactam Antibiotics S R R R R Monobactams Cephalosporins R S S R R. R R R R
Monobactams
Cephalosporins R S S R R
Carbapenems
Penicillins

16. Beta Lactam Antibiotics
Bind to proteins known as penicillin binding proteins (PBPs)
PBPs are enzymes involved in cell wall synthesis (transpeptidases, transglycosylates, carboxypeptidases, and endopeptidases)
Inhibit cell wall synthesis and cause the release of autolytic enzymes that degrade the cell wall.

17. Penicillins Highly effective with an extremely low toxicity
Derivatives have variable pharmacologic properties.
Penicillin G, Penicillin V, methicillin, oxacillin, ampicillin, carbenicillin, ticarcillin, piperacillin.

18. Classes of Penicillins
Natural Penicillins: Pen G, Pen V, Benzathine and Procaine forms
For Streptococcal infection, syphilis, spirochetes
Highly susceptible to beta lactamases
Amino Penicillins: Ampicillin, amoxicillin, others…
Enhanced Gram negative coverage (due to amination)
Antistaphylococcal Penicillins: Methicillin, oxacillin, dicloxacillin…
More stable to beta lactamases, due to modifications near beta lactam ring
Poor entry into gram negatives
Antipseudomonal Penicillins: Carbenicillin, Ticarcilline, Piperacillin…
Enhanced Gram negative coverage (due to extensive side chain addition)

19. Beta lactam and beta lactamase inhibitor combinations
All have activity against staphylococci comparable to methicillin
Amoxicillin + clavulanate (Augmentin)
active against most respiratory tract, and some GI tract gram negative organisms
Ampicillin + sulbactam (Unasyn)
Very good activity against GI and respiratory tract gram negatives
Piperacillin + tazobactam (Zosyn)
Excellent activity against GI and respiratory tract gram negatives, including Pseudomonas aeruginosa

20. Cephalosporins
More stable to beta lactamase hydrolysis and have a wider antibacterial spectrum.
First generation: E.coli, Klebsiella, gram-positives
Second generation: also H-influenza, Enterobacter, Serratia, Citrobacter, and some anaerobes (Bacteriodes Fragilis).
Third generation: most enterobacteriacae and P. aeruginosa
Fourth generation: enterobacter and citrobacter

21. Generations of Cephalosporins
1st Generation: Cephalexin, Cefadroxil, Cefazolin
Good gram positive, limited gram negative activity
2nd Generation: Cefaclor, cefuroxime, Cefoxitin, cefotetan
Good gram positive, moderate gram negative activity
Cefoxitin and cefotetan have reasonable anaerobic activity
3d Generation : Ceftriaxone, cefotaxime, ceftazidime, cefoperazone…
Only Ceftriaxone has usable activity against S. aureus
All have very good activity against gram negatives, BUT activity against pseudomonas is limited to ceftazidime and cefoperazone.
4th Generation: cefepime
Very good activity against gram postives, including S. aureus
Very good activity against gram negatives, including P.aeruginosa

22. Semisynthetic Penicillins
Penicilinase-resistant penicillins
Carbapenems: very broad spectrum
Monobactam: Gram negative
Extended-spectrum penicillins
Penicillins + -lactamase inhibitors

23. Carbapemems
Imipenem
Extremely broad activity, including S.aureus and other gram positives , Pseudomonas, and anaerobic organisms.
Toxicities: hypersensitivity; seizures
Induces b-lactamases in many gram negatives

24. Resistance to b-lactams
Degradation by beta-lactamases
Example: nearly all Staphylococcus aureus
Decreased permeability
Example: most gram negative bacteria refractory to Penicillin G
Altered Penicillin binding proteins
Examples: Methicillin resistant Staphylococcus aureus
Penicillin resistant Streptococcus pneumoniae

25. Penicillinase (b Lactamase)
Figure 20.8 25

26. Extended-Spectrum -lactamases
Plasmid-mediated ESBLs:
predominantly K. pneumoniae, K. oxytoca and E. coli but also P. aeruginosa
encodes high-level resistance to ceftazidime, cefotaxime, and aztreonam
some susceptible to  -lactam/  -lactamase inhibitors, few susceptible to Ciprofloxacin.
Treatment with carbapenems recommended. 18 28 18

27. Inducible chromosomal cephalosporinases:
Serretia, P. aeruginosa, Providencia, Acinetobacter,
Citrobacter, Enterobacter.
induced by exposure to 3rd generation cephalosporins
single step mutation in repressor gene leads to continuous
high-level resistance to all cephalosporins and -lactam/  -
lactamase Inhibitors
Carbapenem is first line Rx, but note Cefepime has low
affinity for Bush group 1 b lactamases (eg, Enterobacter)

28. Glycopeptides Vancomycin and Teicoplanin
Complex glycopeptides that disrupt cell wall synthesis in growing gram-positive bacteria at a later stage (bridge formation).
For methicillin resistant staphylococci, C. difficile and other gram-positives resistant to beta lactams.

29. Others
Bacitracin
-A mixture of polypeptides used in topical applications for the treatment of skin infections caused by gram-positive bacteria.
Isoniazid, Ethionamide, Ethambutol, and cycloserine
- Antimycobacterial (inhibition of mycolic acid
synthesis)

30. Inhibitors of Cell Membrane Function
Polymyxins (B, E)
- Cationic branched cyclic decapeptides that
destroy bacterial cell membranes.
- Nephrotoxic: limited to topical use.
Imidazoles and polyenes (antifungal)
- Oral: ketoconazole, Itraconzole, Fluconazol
- Parentelral: Microconazole, Clotrimazole, Econazole

31. the first cyclic lipopeptide
Daptomycin
the first cyclic lipopeptide
Binds to the bacterial cell membrane, causes rapid
membrane depolarization without lysis
Indications:
Complicated skin and skin structure infections
Staphylococcus aureus bloodstream infections, including right-sided endocarditis
NOT for pneumonia, has no activity in lung

32. Inhibitors of Protein Synthesis
Aminoglycosides
- Amino sugars linked through glycosidic bonds to
an aminocyclitol ring
- Streptomycin, Neomycin, Kanamycin
- Tobramycin, Gentamicin, Netilmicin, Amikacin
- Netilmicin is less ototoxic than gentamicin and tobramycin

33. - Bind to the 30's subunit of ribosomes; which has two effects:
1) Production of aberrant proteins as the result of
misreading of mRNA.
2) Interruption of protein synthesis by causing the
premature release of the ribosome from mRNA.
- Bactericidal because binding is irreversible
- Streptococci and anaerobes are resistant to aminoglycosides (entrance is oxygen dependent)

34. Resistance to aminoglycosides develops in one of three ways:
1) Mutation of the ribosome binding site (uncommon)
2) Decreased uptake of the antibiotic into the
bacterial cell (anaerobes)
3) Enzymatic modification of the antibiotic
(phosphorylation, adenylation, acetylation of the
amino and hydroxyl groups of the antibiotic)

35. Tetracyclines Broad spectrum Bacteriostatic
Inhibit protein synthesis by blocking the binding of aminoacyl tRNA to the 30's ribosome – mRNA complex.
Staining of teeth

37. Resistance to the tetracyclines can stem from
- decreased penetration of the antibiotic into the bacterial cell
- Active efflux of the antibiotic out of the cell (most common)
- Mutation of the chromosomal gene encoding the outer membrane porin protein (OmpF)

38. Chloramphenicol Broad spectrum
Binds to the peptidyl tranferase component of the 50's ribosomal subunit, thus blocking peptide elongation.
Chloramphenicol can disrupt protein synthesis in human bone marrow cells and can produce blood dyscrasias such as aplastic anemia (1/24000).

39. Resistance is observed in bacteria producing chloramphenicol acetyltransferase (plasmid- encolded)
It catalyzes the acetylation of the 3-hydroxy group of chloramphenicol. The product is incapable of binding to the 50's subunit.
Less commonly, chromosomal mutations alter the outer membrane porin proteins causing decreased permeability.

40. Macrolides (Erythromycin)
Broad spectrum and bacteriostatic
Bind to the 50's ribosome (reversible) which blocks polypeptide elongation.
Resistance stems from the methylation of the 23's ribosomal RNA which prevents binding of the antibiotic.
Other mechanisms include destruction of the lactone ring by an erythromycin esterase and the active efflux from the bacterial cell.

41. Clindmycin and Lincomycin
Block protein elongation by binding to the 50's ribosome.
It inhibits peptidyl tranferase by interfering with the binding of the amino acid- acyl -tRNA complex.
Inactive against aerobic gram-negative bacteria.
Methylation of the 23's ribosomal RNA is the source of bacterial resistance (cross resistance with erythromycin).

42. Inhibitors of Nucleic Acid Synthesis
Quinolones (Nalidixic acid, Norfloxacin, Ciprofloxacin, Ofloxacin, Levofloxacin, Sparfloxacin)
Inhibit bacterial gyrases (topoisomerases) which are required to supercoil DNA (binds to alpha subunit)
- Resistance develops rapidly in Pseudomonas, Oxacillin -resistant Staphylococci, and Entrococci
- Resistance is chromosomal, mutation in alpha subunit or decreased uptake

43. Rifampin and Rifabutin
Bind to transcriptase inhibiting initiation of RNA synthesis
Bactericidal to M. tuberculosis and very active against gram positive cocci
Resistance develops rapidly due to a mutation in the chromosomal gene encoding the beta subunit of RNA polymerase
Decreased uptake by gram negative bacteria

44. Metronidazole Has no activity against aerobes or facultative anaerobes
Its nitrogroup is reduced by bacterial nitroreductase producing cytotoxic compounds that disrupt DNA
Resistance due to decreased uptake or elimination of cytotoxic compounds before interacting with DNA

45. Antimetabolites Sulfonamides
Structural analogues of P- aminobenzoic acid thereby preventing folic acid synthesis.
It interferes with the ATP –dependent condensation of petridine with PABA to yield dihydropetroic acid which is a substance converted to folic acid
Sulfonamides inhibit dihydropetroate synthetase

48. Trimethoprim
Inhibits dihydrofolic acid rductase which reduces dihydrofolate to tetrahydrofolate a stage in the sequence leading to the synthesis of purines (thymidine).
Sulfonamides, 5 parts and trimethoprim, 1 part
( Co- Trimoxazole) are synergistic and bacteridical but each alone is bacteriostatic.
Resistance is common due to permeability barriers or decreased affinity of the enzyme and the ability to use exogenous thymidine (enterococci)

49. Measuring Antimicrobial Sensitivity: Disk Diffusion

50. Measuring Antimicrobial Sensitivity
E Test
MIC: Minimal inhibitory concentration

51. Antimicrobial Resistance
Relative or complete lack of effect of antimicrobial against a previously susceptible microbe
Increase in MIC

52. Mechanisms of Antibiotic Resistance
Enzymatic destruction of drug
Prevention of penetration of drug
Alteration of drug's target site
Rapid ejection of the drug

53. Antibiotic Selection for Resistant Bacteria

54. What Factors Promote Antimicrobial Resistance?
Exposure to sub-optimal levels of antimicrobial
Exposure to microbes carrying resistance genes

55. Inappropriate Antimicrobial Use
Prescription not taken correctly
Antibiotics for viral infections
Antibiotics sold without medical supervision
Spread of resistant microbes in hospitals due to lack of hygiene

56. Inappropriate Antimicrobial Use
Lack of quality control in manufacture or outdated antimicrobial
Inadequate surveillance or defective susceptibility assays
Poverty or war
Use of antibiotics in foods

57. Consequences of Antimicrobial Resistance
Infections resistant to available antibiotics
Increased cost of treatment

59. Multi-Drug Resistant TB

60. Proposals to Combat Antimicrobial Resistance
Speed development of new antibiotics
Track resistance data worldwide
Restrict antimicrobial use
Direct observed dosing (TB)
Use more narrow spectrum antibiotics
Use antimicrobial cocktails