Microbiology Department General Bacteriology Antimicrobial Chemotherapy Prof Dr

Microbiology Department General Bacteriology Antimicrobial Chemotherapy Prof. Dr. Amany Mostafa

Intended Learning Outcomes By the end of this lecture , the student will be able to: 1. . Define antibiotics , broad spectrum , narrow spectrum , bacteriostatic and bactericidal 2. . know the mechanism of action of antibiotic 3. . Describe an ideal antibiotic 4. . Study antifungal drugs ©

Intended Learning Outcomes By the end of this lecture , the student will be able to: 1. . define Properties of an Ideal Antimicrobial Agent 2. . Describe Prophylactic Use of Antimicrobials 3. . enumerate Complications of Chemotherapy 4. . Explain Mechanisms of Drug Resistance 5. . know Antibiotic Combination. ©

• "Antimicrobial agents" are substances that kill or inhibit the growth of microorganisms and are suitable for systemic use. • If this antimicrobial substance is synthesized in the laboratory it is named "Chemotherapeutic"

• The term "antibiotic" refers to a substance produced by a living microorganism and is effective against other microorganisms. Since several of these antibiotics are now chemically synthesized, the term antibiotic is generally used

Some important definitions "Bactericidal agent" is applied to any antimicrobial agent that affect bacteria by killing them e. g. penicillin. "Bacteriostatic agent" acts through inhibition of growth and multiplication of microorganisms e. g. Sulfonamide.

• Narrow-spectrum antibacterial agents: these are chemotherapeutics that are effective only against a narrow range of bacteria (either Gram negative or Gram positive bacteria). • Broad-spectrum antibacterial agents: antibiotics that affect a wide variety of bacteria.

Mechanisms of Action of Antimicrobial drugs 1. Inhibition of Bacterial Cell Wall Synthesis: These drugs inhibit the synthesis of peptidoglycan which is found only in the cell wall of the bacteria without harming the host cell (selective toxicity).

Since the internal osmotic pressure of the bacteria is high, they will take up fluid rapidly and explode Bacteria that lack cell wall as mycoplasmas and L-forms are resistant to these drugs

• Examples of these antimicrobial agents: Penicillin and Cephalosporins They act by inhibiting cross-linking of peptidoglycan strands which is the last steps of peptidoglycan synthesis that give the cell wall its strength.

Example of Penicillins methicillin- nafcicillin -amoxicillin ampicillin Example of cephalosporins cefazolin, cefoxtin , cefotaxime Monobactam e. g. aztreonam

• Glycopeptides Vancomycin , teicoplanin, cycloserine and bacitracin inhibit early steps in peptidoglycan synthesis. Vancomycin has restricted spectrum to Gram positive bacteria used successfully in infections caused by methicillinresistant Staphylococcus aureus (MRSA).

Interference with Cell Membrane Function • The cytoplasmic membrane of bacteria and fungi has a structure different from that of mammalian cells and can be more readily disrupted by certain agents (selective toxicity).

These drugs lead to disruption of the functional integrity of the cytoplasmic membrane. As a result, macromolecules and ions escape from the cell resulting in cell damage.

Examples: • Antibacterial drugs e. g. Polymyxin B, and Colistin (polymyxin E). They are used for topical applications only because of their toxicity. • Antifungal drugs e. g. Amphotricin B, Imidazole).

Inhibition of Bacterial Protein Synthesis Bacterial ribosomes are greatly different from those of mammalian cells this allows selective inhibition of bacterial protein synthesis. • These drugs inhibit protein synthesis by acting on the 30 S or 50 S subunits of the bacterial ribosomes

Examples: 1 - Aminoglycosides (streptomycin – garamycin – amikacin -tobramycin ) and tetracyclines ( doxycycline , minocycline) act on the 30 S ribosomal subunit. 2 - Chloramphenicol and Macrolides (Erythromycin – lincomycin) act on the 50 S subunit of bacterial ribosome.

Inhibition of Nucleic Acid Synthesis A number of antibacterial agents act directly or indirectly on DNA or RNA synthesis • Inhibitors of RNA synthesis: they act by inhibiting RNA polymerase of bacteria; so inhibit transcription of RNA from DNA e. g. rifampicin

Inhibitors of DNA synthesis Drugs act directly by blocking DNA replication, e. g. nitroimidazoles, nitrofurans and Quinolones. (ciprofloxacin)

• Drugs act indirectly, e. g. Sulfonamides. P-amino benzoic acid (PABA) is an essential metabolite for many organisms, as it is involved in the synthesis of folic acid (an important precursor to the synthesis of nucleic acids).

• Sulfonamides are structural analogues of PABA. They enter into the reaction in place of PABA and compete for the active center of the enzyme (competitive inhibition). As a result, prevent the formation of folic acid, and nucleic acid synthesis is inhibited.

Antifungal Agents 1 - Polyene Antibiotics: Examples: Nystatin, amphotericin B. • They interact with the sterol of the cell membrane of fungi (ergosterol). This increases the permeability of cell membrane causing loss of the cellular constituents that result in lyses. • Amphotericin is less toxic than other polyenes, because it binds more avidly to ergosterol than cholesterol (the main membrane sterol in human cell).

2 - Flucytosine (5 -flurocytosine): • Fungi static agent. It is an analogue of uracil, leading to its incorporation into fungal RNA. Also impaired DNA synthesis. 3 - Imidazoles: • Imidazoles act on the cell membrane, causing increased permeability and loss of intracellular phosphate and potassium. .

Properties of an Ideal Antimicrobial Agent • Have a selective toxicity, i. e. toxic to microorganisms, but not toxic to the host. • Can not be affected by body fluids. • Have bactericidal rather than bacteriostatic effect.

• Have a broad spectrum activity. • Easily administered, oral water-soluble. • Reach effective level in all body fluids, slowly excreted. • Does not induce allergy or hypersensitivity. • Microorganisms do not acquire resistance against it.

Prophylactic Use of Antimicrobials Chemoprophylaxis is the administration of antimicrobial drugs to prevent infection Indications: 1 - Rheumatic fever: the use of long acting penicillin (penicillin G) every 3 -4 weeks for patients with rheumatic fever to prevent recurrent Streptococcal throat infections which may lead to another attack of rheumatic fever. .

2 - Sub acute bacterial endocarditis: A single large dose of amoxicillin given prior to dental manipulation to patients with congenital or rheumatic heart disease, to prevent sub acute bacterial endocarditic. 3 - Meningococcal meningitis: immediate prophylaxis for close contacts by rifampicin

4 -Cholera: Tetracyclines are used as a prophylaxis to contacts by oral administration. 5 -Tuberculosis: Rifampin and isoniazide (INH) are given as a prophylaxis to close contacts of open pulmonary tuberculosis for six months. 6 -Surgical antimicrobial prophylaxis: To prevent the spread of infection to a primarily clean surgical field.

Criteria for Use of Surgical antimicrobial prophylaxis • Systemic preventive antibiotics should be used when: Ø High risk of infection is associated with the procedure (e. g. , colon resection) Ø Consequences of infection are unusually severe (e. g. , total joint replacement) Ø Patient has a high Risk.

• The antibiotic should be administered preoperatively, before induction of anesthesia in most situations. • The antibiotic selected should have activity against the pathogens likely to be encountered in the procedure. • Postoperative administration of preventive systemic antibiotics beyond 24 hours has not been demonstrated to reduce the risk of surgical site infection

Complications of Chemotherapy • Super infection: Antibiotic will be effective against pathogenic bacteria and at the same time suppresses normal bacterial flora. This allows the overgrowth of drug resistant members of normal flora which may be potentially pathogenic, e. g.

Example of potentially pathogenic bacteria • a) overgrowth of Candida leads to oral thrush or vulvovaginitis and • b) overgrowth of Clostridium difficile causes pseudomembranous colitis (Antibiotic associated colitis).

Complications of Chemotherapy • Drug Toxicity: Some drugs are potentially toxic especially if they are administered to children, pregnant women and old people. Over dosage or prolonged use of such drugs may lead to toxic effects on the host, e. g. :

• Tetracycline causes permanent staining of the teeth, if it is given to pregnant women or to very young children. • Streptomycin is toxic to the 8 th cranial nerve leading to deafness. • Chloramphenicol may cause bone marrow depression. • Aminoglycosides may cause nephrotoxicity. • Polymyxins are nephrotoxic and neurotoxin

Allergy (hypersensitivity) penicillin cause allergic reactions which vary from simple urticaria to anaphylactic shock. Local application of sulfonamides may result in contact dermatitis.

Complications of Chemotherapy Emergence of resistant microorganisms is due to the abuse of antibiotics (low dosage, interrupted course, not actually indicated, wrong choice of the antibiotic). These resistant microorganisms will overgrow and replace the originally susceptible population.

Mechanisms of Drug Resistance 1 - Microorganisms produce enzymes that destroy the drug, e. g. -lactamase (penicillinase) produced by Staphylococci destroy penicillin and cephalosporins. 2 - Microorganisms change their permeability to the drug due to outer membrane change that impairs active transport into the cell, e. g. tetracycline.

Mechanisms of Drug Resistance 3 - Microorganisms develop an altered structural target (receptor) for the drug, e. g. chromosomal resistance to amino glycosides is associated with alteration of the 30 S subunit of the ribosome.

Mechanisms of Drug Resistance 4 - Microorganisms develop an alternative metabolic pathway, that bypass the reaction inhibited by the drug, e. g. bacteria resistant to sulfonamides are capable of using preformed folic acid and so do not require to synthesize it out of PABA.

5 - Microorganisms develop an altered enzyme that can still perform its metabolic function, but less affected by the drug, e. g. sulfonamide-resistant bacteria produce enzyme which has a much higher affinity for PABA than sulfonamides.

Origin of Drug Resistance A- Non-genetic origin: 1 - Active replication of bacteria is usually required for most antibacterial drug action. Thus metabolically inactive bacteria may be phenotypically resistant to the drug (e. g. Mycobacterium T. B. ).

A- Non-genetic origin • 2 - Bacteria lacking the target site for the action of the drug (e. g. L-forms of bacteria are resistant to antibiotics which inhibit cell wall synthesis).

B- Genetic Origin: 1 - Chromosomal: this occurs as a result of spontaneous mutation in a gene that control the susceptibility to a given drug. This results in change in the structural receptor for the drug. 2 - Extrachromosomal: resisrance occurs through plasmids or transposons.

Example: R-factors are a class of plasmids that carry genes usually code for enzymes that are capable of destroying antimicrobial drug, e. g. -lactamases. Genetic material of plasmid can be transferred between bacteria by transduction.

Antibiotic Combination Indications: • Severe undiagnosed infections. • Prolonged course of treatment to prevent the emergence of resistant strains. • Suspected mixed infections; each drug is aimed at an important pathogen. • For complete eradication of organisms to avoid the occurrence of complications

Effects: • Indifference: the combined effect of the two drugs is no greater than that of the more effective agent when used alone. • Addition: the combined effect is equivalent to the sum of the effect of each drug when used alone. • Synergism: The combined effect is greater than the sum of both effects. • Antagonism: Combined action is less than that of the more effective agent when given alone.

Application in Treatment • Combination of two or more agents has been long accepted in the treatment of tuberculosis, so limiting the selection of mutants resistant to the individual components

• The use of beta lactam antibiotics with an aminoglycoside in the treatment of streptococcal endocarditis is also accepted, since the mixture is more bactericidal than the individual components

• A combination of a beta lactam antibiotic with beta lactamase inhibitor may prevent destruction of the antibiotic. For example: combination of amoxicillin with the enzyme inhibitor clavulanic acid restores the activity of the antibiotic against many beta lactamase-producing bacteria