1. Antimicrobial Chemotherapy
Chapter 34
Antimicrobial Chemotherapy

2. Chemotherapeutic agents
chemical agents used to treat disease
destroy pathogenic microbes or inhibit their growth within host
most are antibiotics
microbial products or their derivatives that kill susceptible microbes or inhibit their growth

3. Penicillin accidentally discovered by Alexander Fleming (1928)
observed penicillin activity on contaminated plate
did not think could be developed further
effectiveness demonstrated by Florey, Chain, and Heatley (1939)
Fleming, Florey and Chain received Nobel Prize in 1945 for discovery and production of penicillin

4. Figure 34.1

5. Later discoveries
Streptomycin, an antibiotic active against tuberculosis, was discovered by Selman Waksman (1944)
Nobel Prize was awarded to Waksman in 1952 for this discovery
by 1953 chloramphenicol, neomycin, and tetracycline isolated

6. General Characterisitics of Antimicrobial Drugs
selective toxicity
ability of drug to kill or inhibit pathogen while damaging host as little as possible

7. General Characteristics of Antimicrobial Drugs
side effects – undesirable effects of drugs on host cells
narrow-spectrum drugs – attack only a few different pathogens
broad-spectrum drugs – attack many different pathogens
cidal agent - kills microbes
static agent - inhibits growth of microbes

8. Antibiotic Production
although bacteria and fungi are able to naturally produce many commonly used antibiotics, many important chemotherapeutic agents are synthetic
semisynthetic antibiotics are natural antibiotics that have been chemically modified to make them less susceptible to pathogen inactivation
e.g., ampicillin semisynthetic whereas penicillin is naturally produced

9. Table 34.2

10. Disk Diffusion Tests
disks impregnated with specific drugs are placed on agar plates inoculated with test microbe
drug diffuses from disk into agar, establishing concentration gradient
observe clear zones (no growth) around disks

11. Kirby-Bauer method
standardized method for carrying out disk diffusion test
sensitivity and resistance determined using tables that relate zone diameter to degree of microbial resistance
table values plotted and used to determine if concentration of drug reached in body will be effective

12. A multiple antibiotic disk dispenser.
Figure 34.2 (b)

13. A multiple antibiotic disk dispenser.
Table 34.3

14. A multiple antibiotic disk dispenser.
Figure 34.3

15. Antimicrobial Drugs inhibitors of cell wall synthesis
protein synthesis inhibitors
metabolic antagonists
nucleic acid synthesis inhibition

16. Inhibitors of Cell Wall Synthesis
penicillins
most are 6-aminopenicillanic acid derivatives and differ in side chain attached to amino group-
most crucial feature of molecule is the b-lactam ring
essential for bioactivity
many penicillin resistant organisms produce b-lactamase (penicillinase) which hydrolyzes a bond in this ring

17. Penicillins inhibit peptidoglycan synthesis
All are derivatives of 6-aminopenicillanic acid; in each case the purple shaded portion of penicillin G is replaced by the side chain indicated. The β-lactam ring is also shaded (blue), and an arrow points to the bond that is hydrolyzed by penicillinase.
penicillinase-enzyme produced by penicillin-resistant bacteria
Figure 34.5

18. More on Penicillins
Mode of action thought to be inhibition of last step (transpeptidation) in bacterial cell wall synthesis
prevents the synthesis of complete cell walls leading to lysis of cell
acts only on growing bacteria that are synthesizing new peptidoglycan

19. More…
semisynthetic penicillins have a broader spectrum than naturally occuring ones

20. Vancomycin glycopeptide antibiotics inhibit cell wall synthesis
vancomycin has been important for treatment of antibiotic resistant staphylococcal and enterococcal infections
previously considered “drug of last resort” so rise in resistence to vancomycin is of great concern

21. Protein Synthesis Inhibitors
many antibiotics bind specifically to the procaryotic ribosome
binding can be to 30S (small) or 50S (large) ribosomal subunit
other antibiotics inhibit a step in protein synthesis
aminoacyl-tRNA binding
peptide bond formation
translocation

22. Tetracyclines
all have a four-ring structure to which a variety of side chains are attached
are broad spectrum, bacteriostatic
combine with 30S ribosomal subunit
inhibits bind of aminoacyl-tRNA molecules to the A site of the ribosome

23. The Tetracyclines inhibit protein synthesis Figure 34.8
Tetracycline lacks both of the groups that are shaded. Chlortetracycline (aureomycin) has the shaded chlorine atom; doxycycline has the shaded hydroxyl group.
Figure 34.8

24. used for patients allergic to penicillin
e.g., erythromycin
broad spectrum, usually bacteriostatic
binds to 23S rRNA of 50S ribosomal subunit
inhibits peptide chain elongation
used for patients allergic to penicillin

25. Erythromycin and Other Macrolides
inhibit protein synthesis
Erythromycin
Figure 34.9

26. Chloramphenicol now is chemically synthesized
binds to 23s rRNA on 50S ribosomal subunit and inhibits peptidyl transferase reaction
toxic with numerous side effects so only used in life-threatening situations

27. Chloramphenicol
inhibits protein synthesis
Figure 34.10

28. Sulfonamides or Sulfa Drugs
structurally related to sulfanilamide, a para aminobenzoic acid (PABA) analog
PABA used for the synthesis of folic acid and is made by many pathogens
sulfa drugs are selectively toxic for these pathogens because they compete with PABA for the active site of an enzyme involved in folic acid synthesis, resulting in a decline in folic acid concentration
pathogen dies because folic acid is a precursor to purines and pyrimidines which are nucleic acid building blocks

29. Sulfanilamide and Its Relationship to Folic Acid Structure
antimetabolite
Figure 34.11

30. Two Sulfonamide Drugs Figure 34.12
Shaded areas are side chains substituted for a hydrogen in sulfanilamide.
Figure 34.12

31. Nucleic Acid Synthesis Inhibition
a variety of mechanisms
block DNA replication
inhibition of DNA polymerase
inhibition of DNA helicase (unwind DNA)
block transcription
inhibition of RNA polymerase
drugs not as selectively toxic as other antibiotics because procaryotes and eucaryotes do not differ greatly in the way they synthesize nucleic acids

32. Quinolones
broad-spectrum, synthetic drugs containing the 4-quinolone ring
nalidixic acid was first quinolone to be synthesized (1962)
Ciprofloxfacin (Cipro) used to treat anthrax in 2001 U.S. bioterror attacks
act by inhibiting bacterial DNA-gyrase complex
disrupts many cell processes involving DNA
DNA gyrase – negtive twists in DNA and separates DNA.
alidixic

33. Quinolones
Ciprofloxacin and norfloxacin are newer generation fluoroquinolones.
Figure 34.15

34. DNA Gyrase Action and Quinolone Inhibition
Figure 34.16

35. Drug Resistance an increasing problem
once resistance originates in a population it can be transmitted to other bacteria
a particular type of resistance mechanism is not confirmed to a single class of drugs
resistance mutants arise spontaneously and are then selected

36. Drug Resistant “Superbug”
a Staphylococcus aureus that had acquired a new antibiotic resistance
a meticillin-resistant S. aureus (MSRA) that developed resistance to vancomycin.

37. Drug Resistant “Superbug”
a methicillin-resistant Staphylococcus aureus (MRSA) that developed resistance to vancomycin
this new vancomycin-resistant S. aureus (VRSA) was also resistant to most other antibiotics
isolated from foot ulcers on a diabetic patient
vancomycin-resistant enterococci (VRE) were isolated from same patient
these drug resistant organisms are a serious threat to human health

38. Mechanisms of Drug Resistance
prevent entrance of drug
drug can’t bind to or penetrate pathogen
pump drug out
inactivation of drug
chemical modification of drug by pathogen
alteration of target enzyme or organelle
use of alternative pathways or increased production of target metabolite

42. Figure 34.18

43. The Origin and Transmission of Drug Resistance
resistance genes can be found on
bacterial chromosomes
plasmids
transposons
when found on mobile genetic elements they can be freely exchanged between bacteria

44. Origin and Spread of Resistance Genes
chromosomal genes
resistance results from (rare) spontaneous mutations which usually result in a change in the drug target
R plasmids
resistance plasmids
can be transferred to other cells by conjugation, transduction, and transformation – horizontal gene transfer
can carry multiple resistance genes

45. Figure 34.19

46. Origin and spread… composite transposons
contain genes for antibiotic resistance – some have multiple resistance genes
can move rapidly between plasmids and through a bacterial population

47. Preventing emergence of drug resistance
give drug in high concentrations
give two or more drugs at same time
use drugs only when necessary
possible future solutions
continued development of new drugs
use of bacteriophages to treat bacterial disease

48. Antifungal Drugs
fewer effective agents because of similarity of fungal cells and human cells
easier to treat superficial mycoses than systemic infections

49. Treating superficial mycoses
polyene antibiotic from
Streptomyces
disrupt
membrane
permeability
and inhibit
sterol
synthesis
disrupts
mitotic spindle;
may inhibit protein
and DNA
synthesis
Figure 34.20

50. Treating systemic infections
binds sterols in membranes
disrupts RNA function
Figure 34.20

51. Antiviral Drugs
drug development has been slow because it is difficult to specifically target viral replication
drugs currently used inhibit virus-specific enzymes and life cycle processes

52. Amantadine used to prevent influenza infections
blocks penetration and uncoating of influenza virus
Figure 34.21

53. inhibits herpes virus enzymes
involved in DNA and RNA
synthesis and function
inhibits herpes
virus and
cytomegalovirus
DNA polymerase
inhibits herpes
virus DNA
polymerase
Figure 34.21

54. Broad-spectrum anti-DNA virus drugs
inhibits
viral DNA
polymerase ,
herpesviruses
Figure 34.21

55. Anti-HIV drugs
two main targets
HIV reverse transcriptase
HIV protease

56. Reverse transcriptase inhibitors
Figure 34.21

57. that is normally attacked
Protease inhibitors
Inhibits – protease
Single poly peptide
Broken into smaller
Can be used for capsid
mimic peptide bond
that is normally attacked
by the protease
Figure 34.21

58. Antiprotozoan Drugs
the mechanism of drug action for antiprotozoan drugs is not known
examples of available drugs
some antibiotics that inhibit bacterial protein synthesis are used against protozoa
chloroquine and mefloquine – malaria
metronidazole – Entamoeba infections