Chemotherapy & Antibiotics

- Kalpesh Zunjarrao

Chemotherapy:The treatment of disease by means of chemicals that have a specific toxic effect upon the disease-producing microorganisms or that selectively destroy cancerous tissue

Antimicrobial Agents:Compounds that are used to kill or inhibit growth of microbial organisms

Antibiotics:Substances produced by some microorganisms that can kill or inhibit growth of other organisms.

Antibacterials refer to substances that act against bacteria

The ability of an antimicrobial to affect an invading microorganisms without harming the host is referred to as Selective Toxicity

Antimicrobials act by exploiting metabolic or structural differences between host and pathogens.

Father of Chemotherapy

Paul Ehrlich [1854-1915]:• Discovered Salvarsan 606 (derivative of arsenic) sometimes

called as ‘Magic Bullete’

• Salvarsan 606: capable of destroying spirochetes of syphilis.

• Gave rise to new branch of medicine: ‘Chemotherapy’

Discovery of 1st Antibiotic

Alexander Fleming [1928]:• Accidentally discovered Penicillin produced by a fungus

Penicillium

• Left his Staphylococcus culture on an agar plate for 2 weeks → went on vacation → came back & found mold on his plate which prevented bacterial growth

Important Terminologies:

• Antibacterial spectrum: Range of activity of an antibiotic

• Broad spectrum Antibiotic: that can inhibit wide range of Gram positive and Gram negative bacteria.

• Narrow spectrum Antibiotic: active only against a limited number of bacteria.

• Minimum inhibitory concentration (MIC):The lowest concentration of antimicrobial that inhibits the growth of bacterial population

Bacteriostatic Vs Bactericidal Antibiotic

Inhibit growth of microbes Kill microbes

Antibiotic combinations:

• Antibiotic synergism: Combination of antibiotics have enhanced activity when tested together compared with each antibiotic alone (e.g. 2 + 2 = 6)e.g. Ampicillin + Gentamicin in entercoccal carditis

• Additive effect: Combination of antibiotics has an additive effect (e.g. 2 + 2 = 4)

• Antibiotic antagonism: Combination in which the activity of one antibiotic interferes with the activity of the other (e.g. 2 + 2 < 4)

Mechanism of Action of Antibiotics

• Interfering with Cell Wall Synthesis• Acting on Cytoplasmic Membrane• Inhibiting Protein Synthesis• Inhibiting Nucleic Acid function• Metabolic antagonist

Peptidoglycan: Thick layer in Gram positiveThin layer in Gram negative

β-lactam bactericidal drugsThey inhibit bacterial cell wall peptidoglycan synthesis in growing bacteria.This leads to the death of the Bacteria

VancomycinThey kill Bacteria by interfering with peptidoglycan polymerization.

Interfering with Cell Wall Synthesis

Antibiotics that interfere with cell wall synthesis:

• Penicillin

• Cephalosporin

• Bacitracin

• Vancomycin

• Cycloserine

Certain Antibiotics bind to cell membrane

Semi-permeability of membrane is lost → loss of membrane integrity

Examples:PolymyxinNystatinAmphotericin B

Acting on Cytoplasmic membrane

Inhibitors of Protein Synthesis

Inhibitors of Translation

• Some antibiotics act on 30s or 50s subunits of ribosome

• Thus can affect Initiation, elongation or termination of peptide chain formation

• 30S inhibitors: Aminoglycosides, Tetracyclins• 50S inhibitors: Erythromycin, Chloramphenicol,

Lincomycin

Inhibitors of RNA synthesis: Rifampicin: They kill bacteria by inhibiting RNA

polymerase

Inhibitors of DNA synthesis:Examples: Novobiocin: inhibits DNA replication Metronidazole: damages DNA & inhibits

replication Quinolones: block DNA gyrase

Inhibitors of Nucleic acids

• Their structure resembles to essential metabolites & thus can compete with them

• PABA (Para-aminobenzoic acid): essential co-factor for bacterial cell growth

• Sulphonamides: structure similar to that of PABA• Other examples:

SulphonesTrimethoprim

Metabolic Antagonist

Antimicrobial Drugs

PENICILLINS Belong to β-lactam drugs Mode of action – Inhibit cell wall synthesis (bind transpeptidase

enzyme involved in cross-linking of peptidoglycans) Spectrum:

act against G +ve aerobes and anaerobesSemi-synthetic penicillins are effetcive against some G –ve bacteria

Preparations (Natural Penicillins)Penicillin GPenicillin CPenicillin V

Penicillinase-stable penicillinsMethicillinOxacillinCloxacillinDicloxacillin

Broad spectrum PenicillinsAmpicillinAmoxicillinHetacillin

CEPHALOSPORINS

Modes of Action – Inhibit cell wall synthesis

Preparations 1st Generation cephalosporins (G +ve aerobes)

- Cephalexin, Cefadroxil, Cephaprin, Cephalothin, Cefazolin 2nd Generation cephalosporins (G +ve & some G –ve)

- Cefaclor, Cefoxitin 3rd Generation cephalosporins (G +ve, G –ve, resistance to beta-

lactamase, penetrate BBB)- Ceftiofur, Moxalactam

AMINOGLYCOSIDES Mode of action – Interferes protein synthesis

Preparations Natural: Streptomycin & Dihydrostreptomycin

Neomycin

Extended spectrum:Gentamicin and amikacinTobramycinKanamycin

TETRACYCLINESMode of action – Inhibit Protein synthesis (bind to 30s ribosome)

Spectrum – Broad

Preparations:

Tetracycline

Chlortetracycline

Oxytetracycline

Doxycycline

CHLORAMPHENICOL

Mode of action - Bind to 50s of ribosome

Spectrum – it is a broad-spectrum antibiotic, and it is effective against most anaerobic bacteria

MACROLIDES

Mode of action – Inhibit protein synthesis by binding to 50s of ribosome

Spectrum – Effective against G +ve aerobes and anaerobes and Mycoplasma speices

Examples:ErythromycinTylosinTilmicosin

FLUOROQUINOLONES

Mode of Action – inhibit DNA replication. They are bactericidal

Preparation –Enrofloxacin Ciprofloxacin

Spectrum of activity - Broad

SULFONAMIDES Mode of action – interferes Folate synthesis by inhibiting

dihydropteroate synthetase, that incorporates PABA in making folate

Spectrum of Action - Broad

Preparations - Sulfamethazine

- Sulfadimethoxine

- Sulfathiazole

- Sulfachlorpyridazine

- Sulfasoxasole and sulfamethaxazole

- Sulfacetamide

- Sulfasalazine

Antibiotic Resistance

Permeability:- Some microbes → alteration in chemical nature of outer membrane → change cell wall permeability to drug- Eg: Tetracyclin resistance by Pseudomonas aeruginosa

Production of enzymes:- enzymes which can act on drug- Eg: β-lactamase produced by certain bacteria destroy penicillin

Altered structure target:- Aminoglycosides act by attaching to 30S subunit but resistant bacteria develop altered receptor

Altered metabolic pathway:- Drugs inhibit certain pathways- Resistant bacteria → bypass the reaction

Mechanism

Genetic basis of Resistance

Chromosomal Resistance:

Result of spontaneous mutation

Antimicrobial drug:• Suppress susceptible microbe• But resistant mutant unnoticed

Eg:Mutational resistance in Tuberculosis

↓Two or more anti-tuberculous drugs used for treatment

↓Multiple drug therapy

Extra-chromosomal Resistance:

• Occurs by transfer of plasmid & genetic material

• Drug resistance can be transferred by R-factor

• R-factor: plasmid containing drug resistant genes

• Plasmid codes for enzyme which inactivates drug

• Eg: β-lactamase destroys β-lactam ring present in certain antibiotics

Methods of transfer of Plasmid & Genetic material:

1. Transduction:Plasmid enclosed in bacteriophage → transferred from resistant to Susceptible Staphylococcus → Acquisition of penicillin resistance

2. Conjugation:R-factors transferred by conjugationCommon mode of spread of multiple drug resistance

3. Transformation:Transfer of naked DNA carrying drug resistance genes

4. Transposition:Certain DNA segments → ability to move around between chromosomal & extra-chromosomal DNA → Jumping genes

Host - Parasite Relationships

Bacteria are consistently associated with the body surfaces of animals.

Bacterial cells on the surface of a human body (including the gastrointestinal tract): More than human cells that make up the body (60-90 trillion).

Normal flora:

The bacteria and other microbes that are consistently associated with an animal

“Indigenous microbiota" of the animal.

These bacteria have a full range of symbiotic interactions with their animal hosts

Symbiosis: two organisms live in an association with one another.

Types of Symbiotic Associations:

1. Mutualism:

Both members of the association are benefited.

Eg: In humans, lactic acid bacteria that live on the vaginal epithelium of a woman.The bacteria are provided habitat with a constant temperature and supply of nutrients (glycogen) in exchange for the production of lactic acid, which protects the vagina from colonization and disease caused by yeast and other harmful microbes

2. Commensalism:

There is no apparent benefit or harm to either member of the association.

Commensals live in complete harmony with host without causing any harm

They constitute normal flora of body

Eg:Staphylococcus epidermidis of skinEscherichia coli of Gastrointestinal tract

3. Parasitism:

Parasite refers to an organism that grows, feeds on a different organism while contributing nothing to the survival of the host.

Parasite: capable of causing damage to the host & may become pathogenic if the damage to the host results in disease.

Some parasitic bacteria live as normal flora of humans while waiting for an opportunity to cause disease.

Other non-indigenous parasites generally always cause disease if they associate with a non-immune host

Pathogen is a microorganism that is able to produce disease.

Pathogenicity is the ability of a microorganism to cause disease in another organism.

In humans, some of the normal flora (Eg: Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae) are potential pathogens that live in a commensal or parasitic relationship without producing disease.

They don’t cause disease unless they have an opportunity (compromise or weakness in the host's anatomical barriers, tissue resistance or immunity.)

Bacteria which cause disease in a compromised host which typically would not occur in a healthy host are called as opportunistic pathogens.

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