antimicrobial alpana

ANTIMICROBIAL AGENTS AND CHEMOTHERAPY

BY:DR ALPANA VERMA

Head,Department of Microbiology

and Parasitology

1

2

•Chemotherapeutic agent is a chemical agent that is used to treat the diseases.

•Chemotherapeutic agents destroy pathogenic microbes or inhibit their growth, at concentrations low enough, to avoid undesirable damage to the host.Most of these agents are antibiotics

•Role of antibiotics is to destroy the pathogens residing within the body of host is of great importance.

CHEMOTHERAPEUTIC AGENTS

• The modern era of chemotherapy began with the work of the German physician Paul Ehrlich.

• He realized that a chemical with selective toxicity that would kill pathogens and not human cells might be effective in treating disease.

• He tried to find a toxic dye molecule, a “magic bullet,” that would specifically bind to pathogens and destroy them; therefore he began experimenting with dyes.

• In1904 Ehrlich found that the dye trypan red was active against the trypanosome that causes African sleeping sickness and could be used therapeutically.

3

• Ehrlich and a young Japanese scientist named Sahachiro Hata tested a variety of arsenicals on syphilis-infected rabbits and found that compound number 606, arsphenamine, was active against the

• syphilis spirochete.• Arsphenamine was made available in 1910 under the

trade name Salvarsan. Ehrlich’s successes in the chemotherapy of sleeping sickness and syphilis established his concept of selective toxicity .

• This led to the testing of hundreds of compounds for their therapeutic potential.

4

• In 1927 the German chemical industry giant, I. G. Farbenindustrie, began a long-term search for chemotherapeutic agents under the direction of Gerhard Domagk.

• The company provided vast numbers of dyes and other chemicals that Domagk tested for activity against pathogenic bacteria and for toxicity in animals.

• These results were published in 1935, and in the same year the French scientists Jacques and Therese Trefouel showed that Prontosil Red was converted in the body to sulfanilamide, the true active factor.

• Domagk had actually discovered sulfonamides or sulfa drugs and for this discovery he received the Nobel Prize in 1939.

5

• First antibiotic to be used, therapeutically was penicillin discovered by the Scottish physician Alexander Fleming.

• Fleming, Florey, and Chain received the Nobel Prize in 1945 for the discovery and production of penicillin.

6

• The discovery of penicillin stimulated the search for other antibiotics.

•

7

Later on for screening of about 10,000 strains of soil bacteria and fungi by Waksman Nobel Prize was given to him in 1952, and his success led to a worldwide search for Microorganisms producing chloramphenicol, neomycin, terramycin, and tetracycline etc.

– It must kill or inhibit the pathogen without damaging the host as little as possible.

– Degree of selective toxicity must be expressed in terms of ---

– Therapeutic dose ,that is the drug level, required for the clinical treatment of a particular infection.

– Toxic dose the drug level at which an agent becomes too toxic for the host

– Therapeutic index is the ratio of toxic dose to the therapeutic dose.

– Larger the therapeutic index better the chemotherapeutic agents.

8

Characteristics of Antimicrobial Drugs or

Agents

Characteristics• The drug that disrupts a microbial function not found in Eukaryotic

animal cells often has greater selective toxicity and higher therapeutic index.

• For ex. Penicillin inhibits bacterial cell wall peptidoglycan synthesis but has little or no effect on host cells, because cell wall is absent in eukaryotic cell. thrrefore ,penicillin has higher therapeutic index.

• Some times a drug may have low therapeutic index because it inhibits the same process in the host cells or damages the host in other ways , these undesirable effects on hosts are called side effects.

• Side effects may be of many kinds and involve almost any organ system.

• Because side effects may be severe, the chemotherapeutic agents should be administered with great care.

• an antibiotic or chemotherapeutic agent should have minimum side effects on host .

9

Characteristics:

• All antibiotics and chemotherapeutic agents vary considerably in their effectiveness.

• Some are narrow spectrum• Some are broad spectrum• On the basis of pathogen----• Antibacterial, Antifungal• Antiprotozoan, Antiviral• Some agents can be used against more than one

group Of pathogens for ex.Sulfonamide drugs are used against bacteria as well as against protozoa.

• Action may be cidal or static.

10

• Chemotherapeutic agent -- any chemical irrespective of its origin, used for the treatment or to destroy the pathogen from the host .

• Chemotherapeutic agents synthesized by microbes are Natural antibiotics.

• synthesized by chemical processes-Synthetic antibiotics .

• A number of antibiotics are semi synthetic also. • Semi synthetic antibiotics are natural antibiotics

that have been chemically modified by the addition of extra chemical groups to make them less susceptible to inactivation by pathogens. Ampicillin, carbenicillin, and methicillin.

11

What is the ideal antibiotic?

• Have the appropriate spectrum of activity for the clinical setting.

• Have no toxicity to the host,

• Low propensity for development of resistance.

• Not induce hypersensitive reactions in the host.

13

What is the ideal antibiotic?

• Has rapid and extensive tissue distribution

• Has a relatively long half-life.

• Be free of interactions with other drugs.

• Be convenient for administration.

• Be relatively inexpensive

14

Principles / Definitions

• Spectrum of Activity: Narrow spectrum - drug is effective against a limited number of species

Broad spectrum - drug is effective against a wide variety of species

• Gram negative agentGram positive agentAnti-anaerobic activity

15


• Minimum Inhibitory Concentration (MIC)- minimum concentration of antibiotic required to inhibit the growth of the test organism.Minimum Bactericidal Concentration (MBC)- minimum concentration of antibiotic required to kill the test organism.Bacteriostatic

• Bactericidal• Time dependent killing• Concentration dependent killing

16


• Treatment vs prophylaxis

• Prophylaxis - antimicrobial agents are administered to prevent infection

• Treatment - antimicrobial agents are administered to cure existing or suspected infection

17

Combination Therapy

Why not use 2 antibiotics all the time?• Antagonism• Cost• Increased risk of side effects• May actually enhance development of resistance

inducible resistance• Interactions between drugs of different classes• Often unnecessary for maximal efficacy

18

• Determining the Level of Antimicrobial Activity

• Determination of antimicrobial effectiveness against specific pathogens is essential for proper therapy.

• Testing can show which agents are most effective against a pathogen and give an estimate of the proper therapeutic dose.

19

• Dilution Susceptibility Tests• used to determine MIC and MLC values. • In the broth dilution test, a series of broth tubes

(usually Mueller-Hinton broth) containing antibiotic concentrations in the range of 0.1 to 128 micro g/ml is prepared and inoculated with standard numbers of the test organism.

• The lowest concentration of the antibiotic resulting in no growth after 16 to 20 hours of incubation is the MIC (minimum inhibitory concentration).

20

21

Measurement of antimicrobial activity

Minimal inhibitory concentration: the lowest concentration of an antimicrobial compound that prevents growth of a microbe in vitro

0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50µg/ml

No drug:control

Add growth

medium

Add dilutions of test

compound

Inoculate &

incubate cultures

MIC

22

Inoculate small

samples on drug-

free plates

0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50

Minimum cidal concentration: the lowest concentration of an antimicrobial compound that kills a microbe in vitro

Incubate plates

Minimal inhibitory concentration: the lowest concentration of a antimicrobial compound that prevents growth of a microbe in vitro

• The MIC.(minimum lethalconcentration) can be obtained if the tubes showing no growth are sub cultured into fresh medium lacking antibiotic.

• The lowest antibiotic concentration from which the microorganisms do not grow when transferred to fresh medium is the MLC.

• The agar dilution test is similar to the broth dilution test. Plates containing Mueller-Hinton agar and various amounts of antibiotic are inoculated and examined for growth.

• Recently several automated systems for MIC determination with broth or agar cultures have been developed

23

• Disk Diffusion Tests• If a rapidly growing aerobic or facultative

pathogen like Staphylococcus or pseudomonas is being tested, a disk diffusion technique may be used to save time and media.

• technique is fairly simple. When an antibiotic-impregnated disk is placed on agar previously inoculated with the test bacterium,

• the disk picks up moisture and the antibiotic diffuses radially outward through the agar, producing an antibiotic concentration gradient.

24

• The antibiotic is present at high concentrations near the disk and affects even minimally susceptible microorganisms (resistant organisms will grow up to the disk).

• As the distance from the disk increases, the antibiotic concentration drops and only more susceptible pathogens are harmed. A clear zone or ring is present around an antibiotic disk after incubation if the agent inhibits bacterial growth.

• The wider the zone surrounding a disk, the more susceptible the pathogen is.

• Zone width also is a function of the antibiotic’s initial concentration, its solubility, and its diffusion rate through agar.

25

Currently the disk diffusion test most often used is the Kirby-Bauer method.• An inoculating loop is touched to four or five isolated

colonies of the pathogen growing on agar and then used to inoculate a tube of broth medium

• The culture is incubated for 6 hours at 35°C until it becomes slightly turbid. (bacterial test suspension)

• A sterile cotton swab is dipped into the standardized bacterial test suspension and used to evenly inoculate the entire surface of a Mueller-Hinton agar plate.

• After the agar surface has dried for about 5 minutes, the appropriate antibiotic test disks are placed on it, either with sterilized forceps or with a multiple applicator device). The plate is immediately placed in a 35°C incubator.

• After 16 to 18 hours of incubation, the diameters of the zones of inhibition are measured to the nearest mm.

26

Explanation of pictures• The Kirby-Bauer Method.

multiple antibiotic Disc dispenser (A)

• Result of disc diffusion tests. (B)

29

• Kirby-Bauer test results are interpreted using a table that relates zone diameter to the degree of microbial resistance.

• MIC values and zone diameters for many different microbial strains are determined then-

• A plot of MIC (on a logarithmic scale) versus zone inhibition diameter (arithmetic scale) is prepared for each antibiotic

• . These plots are then used to find the zone diameters corresponding to the drug concentrations actually reached in the body..

• pathogen with too high an MIC value (too small a zone diameter) is

resistant to the agent at normal body concentrations

30

Interpretation of Kirby-Bauer Test Results.

• The relationship between the minimal inhibitory concentrations with a hypothetical drug and the size of the zone around a disk in which microbial growth is inhibited.

• As the sensitivity of microorganisms to the drug increases, the MIC value decreases and the inhibition zone grows larger. Suppose that this drug varies from 7–28micro g/ml in the body during treatment. Dashed line A shows that any pathogen with a zone of inhibition less than 12 mm in diameter will have an MIC value greater than 28 g/ml and will be resistant to drug treatment.

• A pathogen with a zone diameter greater than 17 mm will have an MIC less than 7 g/ml and will be sensitive to the drug (see line B). Zone

• diameters between 12 and 17 mm indicate intermediate sensitivity and usually signify resistance.

32

MODE OF ACTION• All antibiotics have the common property of

interfering in some way with a normal, critical function of the target bacterial cell by one of the following methods:

• 1 Inhibition of cell wall synthesis (group I)• 2 Disruption of cell membranes (group II)• 3 Interference with protein synthesis (group III)• 4 Interference with nucleic acid synthesis (group IV)• 5 Interference with metabolic pathways (group v)

33

34

50S30S

50S30S

Mechanisms of antibacterial action

50S30S

DNA

mRNA

ribosomes

THF

DHF

pABA

folate synthesissulphonamidestrimethoprim

DNA topoisomerasesquinolones,novobiocin

RNA polymeraserifampicin

cell membranepolymyxins

protein synthesis (30S)tetracyclinesaminoglycosidesfusidic acid and others

protein synthesis (50S)macrolideslincosamideschloramphenicoloxazolidinones

cell wall-lactamsvancomycinbacitracin

Some commonly used antibiotic classes---

• Inhibitors of cell wall synthesis - Penicillin, cephalosporin

• Disrupters of cell membranes- Polymixins, polyenes• Inhibitors of protein synthesis -Streptomycin,

tetracycline• Inhibitors of nucleic acid synthesis Rifamycins• Inhibitors of metabolic pathways sulphonamide or

sulfur drugs

35

GROUP I INHIBITORS OF CELLWALL SYNTHESIS• antibiotics, of this group are called beta lactam antibiotics

because they contain a β-lactam ring in their structure. • penicillin's and the cephalosporin are the common

examples.• bacterial cell wall's peptidoglycan component Which is the

cross-linking of chains by transpeptidation. • this process is target for the β-lactams antibiotics; • they bind irreversibly to the transpeptidase enzyme’s active

site. • The cell wall continues to form, but becomes progressively

weaker, as the wall weakens, water enters the cell, leading to swelling and then lyses.

36

• Mechanism of Action (group Icontd)• Cell Wall Synthesis Inhibition• Penicillin ,Ampicillin, Carbenicillin, Methicillin and

Cephalosporin Inhibit transpeptidation enzymes involved in the cross-linking of the polysaccharide chains of the bacterial cell wall's peptidoglycan.

• Activate cell wall lytic enzymes.• Vancomycin Binds directly to the D-Ala-D-Ala terminus

and inhibits transpeptidation.• Bacitracin Inhibits cell wall synthesis by interfering

with action of the lipid carrier that transports wall precursors across the plasma membrane.

37

• Penicillin• Penicillin G or benzyl penicillin, the first antibiotic to be

widely used in medicine, the characteristic of penicillin family.

• All penicillins are derivatives of 6-aminopenicillanic acid and differ from one another only with respect to the side chain attached to its amino group.

• The most crucial feature of the molecule is the beta-lactam ring, which appears to be essential for activity.

• Penicillinase, the enzyme synthesized by many penicillin-resistant bacteria, destroys penicillin activity by hydrolyzing a bond in this ring .

38

• Penicillin (contd,)• benzyl penicillin or penicillin-G,(naturally occurring) action restricted to Gram-positive bacteria, only. unable to penetrate the Gram-negative cell wall. effective against Gram-positive bacteria when administered

intramuscularly, but cannot be taken by mouth because it is broken down in the acid conditions of the stomach.

Another, naturally occurring penicillin-- penicillin-V, an advance form it is less acid-labile.

39

• Semi synthetic penicillin are based on the core structure of the naturally occurring molecule, with the addition of chemically synthesized side chains.

• Can be taken orally (extensive research has led to the development of many variants semi synthetic penicillin.

• Ampicillin is a semi-synthetic penicillin that has a broader specificity

40

• penicillin G • High activity against most gram-positive

bacteria, low against gram negative;• destroyed by acid and Penicillinase

• Penicillin V • More acid resistant than penicillin G

• Ampicillin• Active against gram-positive and• gram-negative bacteria; acid stable

41

• Carbenicillin• Active against gram-negative bacteria

(Pseudomonas and Proteus;)• acid stable; not well absorbed by small

intestine• Methicillin• Penicillinase-resistant, but less• active than penicillin G ,acid-labile• Ticarcillin• Similar to carbenicillin,• but more active against Pseudomonas

42

43

L-ala

D-glu

L-lys

D-ala

NAMNAG

–lys –lys –lys –lys –lys

L-ala

D-glu

L-lys

D-ala

NAMNAG


L-ala

D-glu

L-lys

D-ala

NAMNAG

D-ala

-lactams* inhibit this crosslinking step

*penicillins and cephalosporins

44

Inhibitors of peptidoglycan synthesis*

D-ala—D-ala

NAG NAM P P C55 lipid

L-ala

D-glu

L-lys

D-ala

D-ala


*steps shown are forStaphylococcus aureus

46

Action of β-lactamase on penicillin. A number of bacteria, especially staphylococci,possess the enzyme β-lactamase (penicillinase), which inactivates penicillin by cleavageof the β-lactam ring at the point marked by the arrow

• Other antibiotics that affect the cell wall • Carbapenems are β-lactam antibiotics produced naturally by a

species of Streptomyces. • A semisynthetic form, imipenem, is active against a wide range of

Gram-positive and -negative bacteria, and is used when resistance to other β-lactams has developed.

• Bacitracin and vancomycin are two other antibiotics that exert their effect on the cell wall, but by a different mechanism.

• Bacitracin is derived from species of Bacillus and• acts on bactoprenol pyrophosphate, the lipid carrier molecule

responsible for transporting units of peptidoglycan across the cell membrane

• But its use internally can cause kidney damage.

47

• Vancomycin is a highly toxic antibiotic with• a narrow spectrum of use against Gram-positive organisms

such as streptococci and staphylococci.

• It is particularly important in its use against infections caused by those organisms which are resistant to methicillin and the cephalosporin, for ex.methicillin-resistant Staphylococcus aureus (MRSA)).

• It is not absorbed from the gastrointestinal tract and is therefore most commonly administered intravenously.

48

• Cephalosporin• have also a structure based on a β-lactams ring. • exert their effect on transpeptidase, but have a

broader specificity and are more resistant to the action of β-lactamases.

• Ceftriaxone, for example, is now used in the treatment of gonorrheal infections, caused by penicillin-resistant strains of Neisseria gonorrhoeae.

• In addition, patients who are allergic to penicillin are often treated with cephalosporin.

49

• isolated from a marine fungus called Cephalosporium acremonium,

second, third and fourth generation cephalosporins have been developed to widen the spectrum of activity to include many Gram-negative organisms,.

• Both penicillins and cephalosporins are also used prophylactic ally, that is, in the prevention of infections, prior to surgery in particularly vulnerable patients.

50

Cephalosporins

• 1st generation- mainly gram pos, some gram neg (cefazolin)

2nd generation- weaker gram pos, better gram neg (cefuroxime)

3rd generation - excellent gram neg, some gram pos (Ceftriaxone)

4th generation - excellent gram neg, good gram pos (cefepime)

51

First-Generation Cephalosporins: What do they cover?

• Cefazolin (Kefzol) and cephalexin (Keflex)Activity includes:

• Methicillin susceptible staphylococci• Streptococci excluding enterococci• E. coli, Klebsiella sp., and Proteus

mirabilis• Many anaerobes excluding B. fragilis

52

second generation cephalosporins?

• Cefuroxime– Haemophilus in addition to

1st generation specturm – A respiratory drug– Active against

enterobacter,citrobacter and serratia sp.

• Cefoxitin/cefotetan– 1st generation plus-

anaerobes– A mixed, non-serious

infection surgeon drug

53

Third-Generation Cephalosporins• Cefotaxime, ceftriaxone (IV)

Enhanced activity against Enterobacteriaceae– Enhanced activity against streptococci, including

penicillin resistant S. pneumoniae.– Long half life favors ceftriaxone– Less diarrhea favors cefotaxime

Ceftazidime (IV)

– Active against P. aeruginosa.– Decreased activity against gram positive cocci.

54

Fourth generation cephalosporins

• Cefepime – Marginal improvements– Activity equivalent to oxacillin against gram

positive bacteria.– Active against most of the enterobacteriaceae and

pseudomonas aeruginosa.– Gram negative bacteria have developed resistance

against cephalosporins.as a result of beta lactamase production.

– This has compromised the use of these agents.

55

Cell Wall Active Agents ,resistance problem

• B-lactam resistance1. Production of a B-lactamase (most common)2. Altered PBP (S.pneumoniae)3. prevention of interaction between antibiotic and target PBP4. Altered permeability

• Glycopeptide resistance- primary concern is Enterococcus / S.aureus- altered target- bacteria substitutes D-lac for D-ala- vancomycin can no longer bind

56

Group ii Inhibitors of protein synthesis• Ribosomes are the site of protein synthesis• many classes of antibiotics inhibit protein

synthesis by binding to the ribosome• binding may be reversible or irreversible

• Macrolides, ketolides, lincosamides, streptograminsTetracyclinesAminoglycosides

57

Inhibitors of protein synthesis

• Macrolides (erythromycin,clarithromycin, azithromycin) - primarily gram positive, mycoplasma, chlamydia. - bacteriostatic,

• Lincosamides (clindamycin) - gram positive, anaerobic activityresistance (acquisition of a gene) - M phenotype: macrolides only - MLSB phenotype: macrolides, lincosamides, streptogramins target site modification

58

Inhibitors of protein synthesis

• Aminoglycosides: gentamicin, tobramycin, amikacin - excellent gram negative, moderate gram positive - bactericidal, concentration dependent

• ResistancePrimarily due to aminoglycoside modifying enzymes

59

• Protein Synthesis Inhibition• Streptomycin and Gentamicin Binds with the 30S

subunit of the bacterial ribosome to inhibit protein synthesis and causes misreading of mRNA.

• Chloramphenicol Binds to the 50S ribosomal subunit and blocks peptide bond formation through inhibition of peptidyl transferase.

• Tetracyclines Bind to the 30S ribosomal subunit and interfere with aminoacyl-tRNA binding.

• Erythromycin and clindamycin Bind to the 50S ribosomal subunit and inhibit peptide chain elongation.

• Fusidic acid Binds to EF-G and blocks translocation.

60

• Antibiotics that act by affecting protein synthesis generally have a relatively broad spectrum of action.

. It proved to be particularly useful in the treatment of tuberculosis, the causative agent of which, Mycobacterium tuberculosis.

61

• Streptomycin (Acting on protein synth.) belongs to a group of antibiotics called amino

glycosides, which act by binding to the 30S subunit of the bacterial ribosome, preventing attachment of the 50S subunit to the initiation complex).

• They can thus discriminate between procaryotic (70S) and eucaryotic (80S) ribosomes, and consequently have a relatively high therapeutic index

• Gentamicin, kanamycin and neomycin. (‘wonder drugs’)are other members

62

• Tetracyclines• Acting on protein synthesis.• also work by binding to the 30S ribosomal subunit,

preventing the attachment of aminoacyl tRNA, and therefore extension of the peptide chain

• They are yet another group of antibiotics produced by Streptomyces spp.

• Both natural and semi synthetic Tetracyclines are easily absorbed from the intestine,allowing them to be taken orally.

• sometimes complications caused by the destruction of the normal resident microflora.

• Tetracyclines are still used for a number of applications, notably to treat a variety of sexually transmitted diseases.

63

• Two important antibiotics which act on the larger, 50S, subunit of the prokaryotic ribosome are erythromycin and chloramphenicol.

• Both combine with the subunit in such a way as to prevent the assembly of amino acids into a chain thus affecting protein synthesis of pathogen (bacteria).

• Chloramphenicol was the first antibiotic to be discovered with a broad spectrum of activity;

• it also derives originally from Streptomyces spp., but is nowadays produced synthetically.

• Its use has become severely restricted since it was shown to have some serious side-effects, notably on the bone marrow, but it remains the agent of choice for the treatment of typhoid fever.

64

• Erythromycin (protein synthesis inhibitor)• best known antibiotics of the macrolide group.• it has a large hydrophobic molecule and is unable to gain

access to most Gram-negative bacteria, thus restricting its spectrum of activity.

• Erythromycin can be taken orally and has a similar spectrum of activity to penicillin G; it is often used instead of penicillin in the treatment of staphylococcal and streptococcal infections in children.

• It is particularly appropriate for this application as it is one of the least toxic of all

65

Group iii Nucleic Acid Synthesis Inhibition

• Ciprofloxacin and other quinolones Inhibit bacterial DNA gyrase and thus interfere with DNA replication, transcription, and other activities involving DNA.

• Rifampin Blocks RNA synthesis by binding to and inhibiting the DNA-dependent RNA polymerase activity.

66

Inhibitors of nucleic acid synthesis group iii (cont.)

• Rifampin belongs to a group of agents called rifamycins.

• It acts by inhibiting the enzyme RNApolymerase, thereby preventing the production of mRNA.

• Rifampin is used against the mycobacterium that cause tuberculosis,

• its ability to penetrate tissues makes it well suited.

67

• Inhibitors of nucleic acid synthesis• Unlike most antibiotics, Rifampin interacts with

other drugs,• often reducing or nullifying their effect.• When used in high doses, it has the unusual

side effect of turning secretions such as tears, sweat and saliva, as well as the skin, an orange red colour.

• quinolone group of synthetic antimicrobial drugs act by disrupting DNA replication.

68

Inhibitors of nucleic acid synthesis• Fluoroquinolones (ciprofloxacin, norfloxacin,

levofloxacin, moxifloxacin)- bactericidal, concentration dependent

•- bind to 2 essential enzymes required for DNA replication–

• DNA gyrase and topoisomerase IV- gram pos and gram neg- a typical bacteria, some have anaerobic activity

• Resistance - altered permeability (porin channels) - altered target site

69

70

single bacterial chromosome

Topoisomerase inhibitors

RNA core

RNA core

DNA gyrase

quinolones

• The quinolones are broad-spectrum drugs. • Highly effective against enteric bacteria such as E. coli and

Klebsiella ,pneumoniae. • They can be used with Haemophilus,

Neisseria,Pseudomonas aeruginosa, and other gram-negative pathogens.

• The quinolones also are active against gram-positive bacteria such as Staphylococcus aureus, Streptococcus pyogenes,and

• Mycobacterium tuberculosis.• Currently they are used in treating• urinary tract infections, sexually transmitted diseases

caused by Neisseria and Chlamydia, gastrointestinal infections, and respiratory infections.

71

.• Cell Membrane Disruption Group iv• Polymyxin B Binds to the plasma membrane and

disrupts its structure and permeability propertiy.

72

Antibiotics that disrupt cell membranes –group iv• Polymyxin are a class of antibiotic Produced

naturally by a species of Bacillus,• that act by disrupting the phospholipids of the

cytoplasmic membrane and causing leakage of cell contents.

Effective against pseudomonad infections of wounds and burns, often in combination with Bacitracin and neomycin

• Their toxicity makes them unsuitable for internal use. • Polyene antibiotics such as amphotericin and

nystatin are antifungal agents that act on the sterol components of membranes;

73

• Metabolic Antagonism group v• Sulfonamides Inhibit folic acid synthesis by

competition with p-aminobenzoic acid.• Trimethoprim Blocks tetrahydrofolate

synthesis through inhibition of the enzyme dihydrofolate reductase.

• Dapsone Interferes with folic acid synthesis.

74

• Group v (contd) metabolic inhibitors• A good way to inhibit or kill pathogens is by use of

compounds that are structural analogues, • molecules structurally similar to metabolic

intermediates. These analogues compete with metabolites in metabolic processes because of their similarity,.

• Sulfonamides or sulfur drugs are structurally related to sulfanilamide, an analogue of p-aminobenzoic acid. which is used in the synthesis of folic acid as the cofactor.

• When sulfanilamide or another sulfonamide enters a bacterial cell, it competes with p-aminobenzoic acid for the active site of an enzyme involved in folic acid synthesis, and the folate concentration decreases.

75

• The decline in folic acid is detrimental to the bacterium because folic acid is essential to the synthesis of purines and pyrimidines,the bases used in the construction of DNA, RNA, and other important cell constituents.

• The resulting inhibition of purine and pyrimidine synthesis leads to cessation of bacterial growth or death of the pathogen.

• Sulfonamides are selectively toxic for many• pathogens because these bacteria manufacture their own

folate and cannot effectively take up the cofactor. • In contrast, humans cannot synthesize folate ,therefore

sulfonamides will not affect the host.

76

Mechanism of action of TMP-SMX (sulfur drugs)

77

78

Sulphonamides: inhibitors of folate synthesis

p-aminobenzoic acid + pteridine

dihydropteroatesynthase

dihydropteroic acid

sulphonamides

dihydrofolic acid

tetrahydrofolic acid

trimethoprim

Inhibitors of metabolic pathways group v contd--

• Trimethoprim/sulfamethoxazole (Septra, TMP/SMX) active against - gram negative,and some gram positive organisms.

• block folic acid synthesis at two different pointsTMP and SMX act synergistically

• Resistance may arise if the organism can “bypass” the pathway making it redundant

79

Targets of antibacterial agents

• Inhibit cell wall production - penicillin binding proteins

• Inhibit protein synthesis - bind 30s or 50s ribosomal subunits

• Inhibit nucleic acid synthesis - binding topoisomerases / RNA polymerase

• Block biosynthetic pathways - interfere with folate metabolism

• Disrupt bacterial membranes - polymixins

80

81

Mechanisms of antimicrobial resistance

1. inactivate antimicrobial molecule

e.g. -lactamases: produced by many G+ and G– bacteria

genes for -lactamases often carried on plasmids, so easily spread between bacteria

2. alter antimicrobial target

specific mutations in genes encoding target proteins can cause resistance without loss of target’s function

3. prevent access of antimicrobial to target

e.g. reductions in permeability to drug (porins)

drug efflux mechanisms — multi-drug resistance pumps

4. overexpress antimicrobial target

5. use alternative pathways to the one including the target

82

Spread of antimicrobial resistance

transposon

mobile genetic element carrying a drug resistance gene expressed from its own promoter: hops from chromosome to plasmid

drug resistance plasmids

can carry more than one drug resistance-encoding gene

easily transferred between organisms

integrons

groups of drug resistance genes in a large array, under control of a single promoter

responsible for rapid spread in Gram-positive bacteria

antimicrobial alpana

Business