Antimicrobial Chemotherapy Chapter 34 Antimicrobial Chemotherapy

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

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

Figure 34.1

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

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

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

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

Table 34.2

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

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

A multiple antibiotic disk dispenser. Figure 34.2 (b)

A multiple antibiotic disk dispenser. Table 34.3

A multiple antibiotic disk dispenser. Figure 34.3

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

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

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

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

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

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

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

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

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

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

Erythromycin and Other Macrolides inhibit protein synthesis Erythromycin Figure 34.9

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

Chloramphenicol inhibits protein synthesis Figure 34.10

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

Sulfanilamide and Its Relationship to Folic Acid Structure antimetabolite Figure 34.11

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

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

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

Quinolones Ciprofloxacin and norfloxacin are newer generation fluoroquinolones. Figure 34.15

DNA Gyrase Action and Quinolone Inhibition Figure 34.16

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

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

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

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

Figure 34.18

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

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

Figure 34.19

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

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

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

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

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

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

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

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

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

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

Reverse transcriptase inhibitors Figure 34.21

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

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