antibiotics

Maria Ellery Mendez, MD,

DPASMAP,FPAMS,FPAAAM

Department of Microbiology

Our Lady of Fatima University

ANTIMICROBIAL AGENT

• any chemical or drug used to treat an infectious disease, either by inhibiting or killing the pathogens in vivo

ANTIMICROBIAL AGENT

Ideal Qualities:

1. kill or inhibit the growth of pathogens

2. cause no damage to the host

3. cause no allergic reaction to the host

4. stable when stored in solid or liquid form

5. remain in specific tissues in the body long enough to be effective

6. kill the pathogens before they mutate and become resistant to it

ANTIBIOTICS

Substances derived from a microorganism or produced synthetically, that destroys or limits the growth of a living organism

ANTIBIOTICS – Sources

1. Natural

a.Fungi – penicillin, griseofulvin

b.Bacteria – Bacillus sp. (polymixin, bacitracin) ; Actinomycetes (tetracycline, chloramphenicol, streptomycin)

2. Synthetic

ANTIBIOTICS – Classification

I. Accdg to antimicrobial activity

1. Bactericidal

2. Bacteriostatic

II. Accdg to bacterial spectrum of activity

1. Narrow spectrum

2. Broad spectrum


III. Accdg to absorbability from the site of administration to attain significant concentration for the treatment of systemic infection

1. Locally acting

2. Systemic


IV.Accdg to mechanism of action

1. Inhibit bacterial cell wall synthesis

2. Alter the function and permeability of the cell membrane

3. Inhibit protein synthesis (translation and transcription)

4. Inhibit nucleic acid synthesis

Inhibition of cell wall synthesis

Target: block peptidoglycan (murein) synthesis

Peptidoglycan

Polysaccharide (repeating disaccharides of N-acetylglucosamine and N-acetylmuramic acid) + cross-linked pentapeptide

Pentapeptide with terminal D-alanyl-D-alanine unit required for cross-linking

Peptide cross-link formed between the free amine of the amino acid in the 3rd position of the peptide & the D-alanine in the 4th position of another chain


A. -lactam antibiotics

inhibit transpeptidation reaction (3rd stage) to block peptidoglycan synthesis involves loss of a D-alanine from the pentapeptide

Steps:

a. binding of drug to PBPs

b. activation of autolytic enzymes (murein hydrolases) in the cell wall

c. degradation of peptidoglycan

d. lysis of bacterial cell



Penicillin binding proteins (PBPs)

enzymes responsible for:

a. cross-linking (transpeptidase)

b. elongation (carboxypeptidase)

c. autolysis



Lysis of bacterial cell

o Isotonic environment cell swelling rupture of bacterial cell

o Hypertonic environment – microbes change to protoplasts (gram +) or spheroplasts (gram -) covered by cell membrane swell and rupture if placed in isotonic environment



o intact ring structure essential for antibacterial activity

o inhibition of transpeptidation enzyme due to structural similarity of drugs (penicillin and cephalosporin) to acyl-D-alanyl-D-alanine



PENICILLIN

Source: Penicillium spp (molds)

inhibit final cross-linking step

bind to active site of the transpeptidase & inhibit its activity

bactericidal but kills only when bacteria are actively growing

inactivated by -lactamases



CEPHALOSPORINS

similar structure and mechanism of action as penicillin

most are products of molds of the genus Cephalosporium


B. Other -lactam antibiotics

CARBAPENEMS

structurally different from penicillin and cephalosporin

Imipenem

with widest spectrum of activity of the -lactam drugs

Bactericidal vs. many gram (+), gram (-) and anaerobic bacteria

not inactivated by -lactamases


A. Other -lactam antibiotics

MONOBACTAMS (Aztreonam)

activity vs. gram negative rods

useful in patients hypersensitive to penicillin


C. Other Cell Wall Inhibitors

Inhibit precursor for bacterial cell wall synthesis

VANCOMYCIN

Source: Streptomyces orientalis

Inhibit 2nd stage of peptidoglycan synthesis by:

a. binding directly to D-alanyl-D-alanine block transpeptidase binding

b. inhibiting bacterial transglycosylase

S. aureus & S. epidermidis infection resistant to penicillinase-resistant PEN



CYCLOSERINE

Inhibit 2 enzymes D-alanine-D-alanine synthetase and alanine racemase catalyze cell wall synthesis

inhibit 1st stage of peptidoglycan synthesis

structural analogue of D-alanine inhibit synthesis of D-alanyl-D-alanine dipeptide

second line drug in the treatment of TB



ISONIAZID & ETHIONAMIDE

Isonicotinic acid hydrazine (INH)

Inhibit mycolic acid synthesis

ETHAMBUTOL

Interferes with synthesis of arabinogalactan in the cell wall



BACITRACIN

Source: Bacillus licheniformis

Prevent dephosphorylation of the phospholipid that carries the peptidoglycan subunit across the membrane block regeneration of the lipid carrier & inhibit cell wall synthesis

Too toxic for systemic use treatment of superficial skin infections

Inhibition of cell membrane function

A. POLYMYXINS

Source: Bacillus polymyxa

With positively charged free amino group act like a cationic detergent interact with lipopolysaccharides & phospholipid in outer membrane increased cell permeability

Activity: gram negative rods, especially Pseudomonas aeruginosa


B. POLYENES (Anti-fungal)

Require binding to a sterol (ergosterol) change permeability of fungal cell membrane

AMPHOTERICIN B

Preferentially binds to ergosterol

With series of 7 unsaturated double bonds in macrolide ring structure

Activity: disseminated mycoses


B. POLYENES (Anti-fungal)

NYSTATIN

Structural analogue of amphotericin B

Topical vs. Candida

C. AZOLES (Anti-fungal)

Block cytP450-dependent demethylation of lanosterol inhibit ergosterol synthesis

Ketoconazole, Fluconazole, Itraconazole, Miconazole, Clotrimazole

Inhibition of protein synthesis

Binds the ribosomes result in:

1. Failure to initiate protein synthesis

2. No elongation of protein

3. Misreading of tRNA-deformed protein


A. Drugs that act on the 30S subunit

AMINOGLYCOSIDES (Streptomycin)

Mechanism of bacterial killing involves the ff. steps:

1. Attachment to a specific receptor protein (e.g. P 12 for Streptomycin)

2. Blockage of activity of initiation complex of peptide formation (mRNA + formylmethionine + tRNA)

3. Misreading of mRNA on recognition region wrong amino acid inserted into the peptide



TETRACYCLINES

Source: Streptomyces rimosus

Bacteriostatic vs. gram (+) and gram (-) bacteria, mycoplasmas, Chlamydiae & Rickettsiae

Block the aminoacyl transfer RNA from entering the acceptor site prevent introduction of new amino acid to nascent peptide chain



OXAZOLIDINONES (LINEZOLID)

interfere with formation of initiation complex block initiation of protein synthesis

Activity: Vancomycin-resistant Enterococci, Methicillin-resistant S. aureus

(MRSA) & S. epidermidis and Penicillin-resistant Pneumococci


B. Drugs that act on the 50S subunit

CHLORAMPHENICOL

Inhibit peptidyltransferase prevent synthesis of new peptide bonds

Mainly bacteriostatic; DOC for treatment of typhoid fever



MACROLIDES (Erythromycin, Azithromycin & Clarithromycin)

Binding site: 23S rRNA

Mechanism:

1. Interfere with formation of initiation complexes for peptide chain synthesis

2. Interfere with aminoacyl translocation reactions prevent release of uncharged tRNA from donor site after peptide bond is formed (Erytnromycin)



LINCOSAMIDES (Clindamycin)

Source: Streptomyces lincolnensis

resembles macrolides in binding site, anti-bacterial activity and mode of action

Bacteriostatic vs. anaerobes, gram + bacteria (C. perfringens) and gram – bacteria (Bacteroides fragilis)


C. Drugs that act on both the 30S and 50S subunit

GENTAMICIN, TOBRAMYCIN, NETILMICIN

Treatment of systemic infections by susceptible gram (-) bacteria including Enterobacteriaceae & Pseudomonas

AMIKACIN

Treatment of infection by gram (-) bacteria resistant to other aminoglycosides

KANAMYCIN

Broad activity vs. gram (-) bacteria except Pseudomonas

Inhibition of nucleic acid synthesis

A. Inhibition of precursor synthesis

Inhibit synthesis of essential metabolites for synthesis of nucleic acid

SULFONAMIDES

Structure analogue of PABA (precursor of tetrahydrofolate) inhibit tetrahydrofolate methyl donor in synthesis of A, G and T

Bacteriostatic vs. bacterial diseases (UTI, otitis media 20 to S. pneumoniae or H. influenzae, Shigellosis, etc.)

DOC for Toxoplasmosis & Pneumocystis pneumonia


A. Inhibition of precursor synthesis

TRIMETHOPRIM

Inhibit dihydrofolate reductase (reduce dihydrofolic to tetrahydrofolic acid) inhibit purine synthesis

TRIMETHOPRIM + SULFAMETHOXAZOLE

Produce sequential blocking marked synergism of activity

Bacterial mutants resistant to one drug will be inhibited by the other


B. Inhibition of DNA synthesis

QUINOLONES

Inhibit subunit of DNA gyrase (+) supercoiling (-) DNA synthesis

Bactericidal; not recommended for children & pregnant women since damages growing cartilage

Fluoroquinolones (Ciprofloxacin), Norfloxacin, Ofloxacin, etc.



NOVOBIOCIN

Inhibit subunit of DNA gyrase

FLUCYTOSINE (Anti-fungal)

Nucleoside analogue inhibit thymidylate synthetase limit supply of thymidine



METRONIDAZOLE

Anti-protozoal; anaerobic infections

Antimicrobial property due to reduction of its nitro group by bacterial nitroreductase (+) production of cytotoxic compounds disrupt host DNA


C. Inhibit RNA synthesis

RIFAMPICIN

Semisynthetic derivative of rifamycin B (produced by Streptomyces mediterranei)

Binds to DNA-dependent RNA polymerase block initiation of bacterial RNA synthesis

Bactericidal vs. M. tuberculosis and aerobic gram (+) cocci

RESISTANCE

ACQUISITION OF BACTERIAL RESISTANCE

INTRINSIC RESISTANCE

Stable genetic property encoded in the chromosome and shared by all strains of the species

Usually related to structural features (e.g. permeability of the cell wall) e.g. Pseudomonas cell wall limits penetration of antibiotics

RESISTANCE

ACQUISITION OF BACTERIAL RESISTANCE

ACQUIRED RESISTANCE

Species develop ability to resist an antimicrobial drug to which it is as a whole naturally susceptible

Two mechanisms:

1. Mutational – chromosomal

2. Genetic exchange – transformation, transduction, conjugation

RESISTANCE

INTRINSIC RESISTANCE – EXAMPLES:

1. Mutation affecting specific binding protein of the 30S subunit Streptomycin-resistant M. tuberculosis & S. faecalis

2. Mutation in porin proteins impaired antibiotic transport into the cell lead to multiple resistance P. aeruginosa

3. Mutation in PBPs Strep pneumoniae

4. Altered DNA gyrase quinolone-resistant E. coli

RESISTANCE

ACQUIRED RESISTANCE – EXAMPLES:

1. Resistance (R) plasmids

Transmitted by conjugation

2. mecA gene

Codes for a PBP with low affinity for -lactam antibiotics

Methicillin-resistant S. aureus

RESISTANCE

ORIGIN OF DRUG RESISTANCE

NON-GENETIC

1. Metabolically inactive organisms may be phenotypically resistant to drugs – M. tuberculosis

2. Loss of specific target structure for a drug for several generations

3. Organism infects host at sites where antimicrobials are excluded or are not active – aminoglycosides (e.g. Gentamicin) vs. Salmonella enteric fevers (intracellular)

RESISTANCE

GENETIC

1. Chromosomal

Occurs at a frequency of 10-12 to 10-7

20 to spontaneous mutation in a locus that controls susceptibility to a given drug due to mutation in gene that codes for either:

a. drug target

b. transport system in the membrane that controls drug uptake

RESISTANCE

GENETIC

2. Extrachromosomal

a. Plasmid-mediated

Occurs in many different species, esp. gram (-) rods

Mediate resistance to multiple drugs

Can replicate independently of bacterial chromosome many copies

Can be transferred not only to cells of the same species but also to other species and genera

RESISTANCE

MECHANISMS THAT MEDIATE BACTERIAL RESISTANCE TO DRUGS

1. Production of enzymes that inactivate the drug

. -lactamase

S. aureus, Enterobacteriaceae, Pseudomonas, H. influenzae

b. Chloramphenicol acetyltransferase

S. aureus, Enterobacteriaceae

c. Adenylating, phosphorylating or acetylating enzymes (aminoglycosides)

S. aureus, Strep, Enterobacteriaceae, Pseudomonas

RESISTANCE


2. Altered permeability to the drug result to decreased effective intracellular concentration

Tetracycline, Penicillin, Polymixins, Aminoglycosides, Sulfonamides

RESISTANCE


3. Synthesis of altered structural targets for the drug

a. Streptomycin resistance – mutant protein in 30S ribosomal subunit delete binding site Enterobacteriaceae

b. Erythromycin resistance – altered receptor on 50S subunit due to methylation of a 23S rRNA S. aureus

RESISTANCE


4. Altered metabolic pathway that bypasses the reaction inhibited by the drug

Sulfonamide resistance – utilize preformed folic acid instead of extracellular PABA S. aureus, Enterobacteriaceae

RESISTANCE


5. Multi-drug resistance pump

Bacteria actively export substances including drugs in exchange for protons

Quinolone resistance

RESISTANCE

LIMITATION OF DRUG RESISTANCE

1. Maintain sufficiently high levels of the drug in the tissues inhibit original population and first-step mutants.

2. Simultaneous administration of two drugs that do not give cross-resistance delay emergence of mutants resistant to the drug (e.g. INH + Rifampicin)

3. Limit the use of a valuable drug avoid exposure of the organism to the drug

THE END

antibiotics

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