BIOCHEMICAL BASIS OF ANTIMICROBIAL ACTIVITY OF

ANTIBIOTICS

Guided & Checked By :Shreyas Bhatt sir

Shreya M. ModiMSc Sem- IIIRoll no.- 11

Content

Introduction

Introduction

Antibiotics are chemical molecules or compounds that specifically targets and kill cells. Not only antibacterial, but also antifungal, antiviral and also antineoplastic compounds are also classified as antibiotics.

Antibacterial action generally follows some of the mechanisms such as inhibition or regulation of enzymes involved in the synthesis of cell wall, nucleic acid synthesis and repair, or protein biosynthesis. Antibiotics target the cell functioning of rapidly dividing cells.

1. The target of an antibiotic can be present only in bacteria but not in the

eukaryotic host.

2. The target in bacteria is different from the homologous target in the

eukaryotic host.

Bases of antibiotic selectivity

Modern genomics provide a great tool for identifying targets of new selective

antibiotics

Selectivity of antibiotics is not ‘natural’

Natural antibiotics are weapons that bacteria or fungi use to compete with other microorganisms.

Selectivity is not a ‘natural’ feature of antibiotics.

Most of clinically-useful antibiotics are fortuitously selective antibacterials.

Many antibiotics are omni-potent and inhibit growth of a wide variety of organisms. Such

antibiotics can be developed into selective drugs through modification of their chemical structures.

Antibiotics are classified as bacteriostatic or bactericidal.

Bacteriostatic drugs make bacteria dormant, but do

not kill them. Most bacterial cells resume growth after

removal of the antibiotic (e.g. chloramphenicol)

Bactericidal drugs kill bacteria (e.g. ciprofloxacin)

Conti….

Antibiotics with a bactericidal mode of action are preferred, especially for treatment of immunocompromised

patients. The mode (static vs. cidal) of antibiotic action may differ for different pathogens and may depend on the drug

concentration.

The basis of bactericidal versus bacteriostatic effects is poorly understood but maybe related to the accumulation of reactive oxygen radicals in the bacterial cells upon treatment with bactericidal

drugs.

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2000Golden era in antibiotic discoveryNo principally new antibiotics

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Growth phase

Conventional antibiotics

Penicillins Cephalosporins Carbapenems Quinolones Aminoglycosides Macrolides Tetracyclines

Nitrofurantoin, metronidazole, clindamycin, vancomycin, teicoplanin, cotrimoxazole, fusidic acid, etc

Isoniazid, pyrazinamide, ethambutol, rifampin, cycloserine, etc

Inhibition of cell wall synthesis

Some of the antibacterial compounds interfere with the cell wall synthesis by weakening the peptidoglycan structures in bacterial cell wall, by this integrity of bacterial cell wall structure weakens and eventually disrupts.

Mammalian cells only have plasma membrane so these antibiotics specifically target only bacterial cells. That is these antibiotics do not induce any negative effect on the host mammalian cells.

Cont …

Antibacterial compound β-lactam can be used against both Gram-positive and Gram-negative bacterial cells.

Vancomycin another antibacterial compound also prevents cell wall biosynthesis in bacterial cells by interfering with transglycosylases enzyme activity.

But this compound can be used effectively against Gram-positive bacteria, as it is unable to penetrate the outer cytoplasmic membrane of Gram-positive bacteria.

Antibiotics inhibiting cell wall synthesis

Name Producer organism

Chemical nature

Site of action

Penicillin P.NotatumP.Crysogenum

Β lactum Antibiotic

TranspeptidaseReaction

Cephalosporine Cephalosporium aeremonium

Β lactum Antibiotic

TranspeptidaseReaction

Cycloserine Streptomyces spp.

Analogue of alanine

Inhibit formation of Park’s nucleotide

Bacitracin B.Subtilis Peptide Phosphatase reaction in lipid cycle

Vancomycin Str.orientatis Glycopeptide Polymerization step

The first stage

The second stage

Bacitracin

The third stage

ß-lactam antibiotics.

Fosfomycin

Beta-Lactam Antibiotics

Beta-lactam antibiotics Penicilins Cephalosporins Carbapenems Monobactams

All β-lactam antibiotic agents contain a β-lactam nucleus in its molecular structure.

Core structure of penicillins (1) and cephalosporins (2). Beta-lactam ring in red.

Beta-Lactams. Common features

All beta-lactams: are bacteriocidal. have the same mechanism of antibacterial

action. have no activity against MRSA and

atypical bacteria (Legionella spp., Mycoplasma spp., Chlamidia spp.).

have the allergic cross-reaction. have the same modes of bacterial

resistance.

Beta-Lactams Mechanism of action

β-Lactam antibiotics are analogues of D-alanyl-D-alanine amino acid

residues

irreversible binding to the active site of penecillin-binding proteins

(PBPs)

Penicillin-binding proteins (PBPs), enzymes that catalyze the last steps

of peptidoglycan synthesis (cross-linking).

Conti….

Inhibition of the PBPs prevents the final crosslinking of the nascent peptidoglycan layer

disrupting bacterial cell (bactericidal effect)

Penicillins. Clinical use

Generation Example Clinical use

Natural penicillins Penicillin G Syphilis, rheumatic fever meningitis, tonsillitis, scarlet fever, endocarditis

Antistaphylococcal penicillins

Methicillin Mild and moderate staphylococcal infections

Extended-spectrum penicillins

AmpicillinAmoxicillin

Noncomplicated community-acquired infections (lower and upper respiratory tract infections, UTIs, skin and soft tissues)

Antipseudomonal penicillins

Carbenicillin P.aeruginosa infections

Inhibition of nucleic acid synthesis

Some antibiotics inhibit the action of enzyme RNA polymerase, hence interfere with RNA (ribonucleic acid) synthesis in the cells. Antibiotics such as asdoxorubicin andactinomycin D interfere with RNA biosynthesis in both bacterial cells as well as in mammalian cells. These compounds are used in treating rapidly growing tumor cells in cancer patients. Some of the examples are Doxorubicin hydrochloride, Levofloxacin, Irinotecan hydrochloride, Rifampcin

Penicillins Penicillin G

Still useful for a number of diseases (e.g. meningitis, syphilis)

Cloxacillin For MSSA infections

Ampicillin, amoxicillin Active vs. Gram-positive (not MSSA), Gram-

negative organisms Augmentin, Unasyn

Broad spectrum, covers Gram-positive, Gram-negative and anaerobes

Piperacillin, Tazocin, Timentin Are active vs. Pseudomonas

CephalosporinsGeneration Example Spectrum

First Generation

Cefazolin Most active against gram-positive bacteria (staphylococci). Have no activity against gram-negative bacteria.

Second Generation

Cefuroxim Enhanced activity against gram-positive and some gram-negative bacteria.

Third Generation

Cefotaxime Broad-spectrum (gram-positive and gram-negative). Resistant to most type of beta-lactamases.

Fourth Generation

Cefepime Most active against gram-negative bacteria. Very active against P.aeruginosa. Resistant to beta-lactamases. Have little gram-positive activity.

Carbapenems

Imipenem Broad spectrum, covers Gram-positive,

Gram-negative (including ESBL-producing strains), Pseudomonas and anaerobes

Meropenem Less seizure-inducing potential, can be

used to treat CNS infections Ertapenem

Lacks activity vs. Acinetobacter and Pseudomonas

Has limited activity against penicillin-resistant pneumococci

Glycopeptide antibiotics Vancomycine

Is not absorbed from the gut.

IV administration. Excreted unchanged

by the kidneys.

VancomycinMechanism of action

Forms a complex with the C-terminal D-alanine of peptidoglycan precursors

Prevents the following addition of new units to the peptidoglycan

Inhibition of peptidoglycan synthesis

Bactericidal effect

Vancomycin activity

Do not penetrates the membrane of gram-negative organisms.

Gram positive organisms only Staphylococcus spp. including

Methicillin-resistant Staphylococcus aureus (MRSA)

Streptococcus spp. Enterococcus faecalis and E. faecium

Clostridium difficile and other Clostridia (cause pseudomembranous colitis)

Vancomycin. Clinical use

Serious, life-threatening gram-positive infections

MRSA infections

Pseudomembranous colitis caused by Clostridium difficile (oral administration of vancomycin)

VancomycinAdverse reactions

Nephrotoxity: mostly in combinations with aminoglycosides

Ototoxicity Red man syndrome (or red

neck syndrome): within 4–10 minutes after

the start of infusion flushing and an

erythematous rash at the face, neck and upper body.

is due to non-specific mast cell degranulation. It is not allergic reaction.

Quinolones

Ciprofloxacin Active vs. MSSA, Gram-negative and

Pseudomonas Levofloxacin

Has activity vs. Streptococcus pneumoniae, but slightly less active towards Pseudomonas compared to ciprofloxacin

Moxifloxacin Has activity vs. anaerobes but less active

towards Pseudomonas

Aminoglycosides

Active vs. some Gram-positive and Gram-negative organisms

Gentamicin Active vs. Pseudomonas

Tobramycin More active vs. Pseudomonas than gentamicin Shows less activity against certain other Gram-negative

bacteria Amikacin

More stable to enzymes, used in severe infections by gentamicin-resistant organisms

Streptomycin Used for tuberculosis

Macrolides

Erythromycin Active vs. Gram-positive organisms, atypicals GI side effects

Clarithromycin Slightly greater activity than erythromycin

Azithromycin Slightly less active than erythromycin vs.

Gram-positive but enhanced activity vs. some Gram-negative organisms

Tetracyclines

Drug of choice in infections caused by Chlamydia, Rickettsia, Brucella and Lyme disease

Value has decreased due to increasing bacterial resistance

Tetracycline Role in Helicobacter pylori eradication (less

frequently used than other antibiotics) Doxycycline

Once daily Minocycline

Broader spectrum

Other antibiotics

Clindamycin Vs. Gram-positive cocci and anaerobes

Metronidazole Vs. anaerobes Preferred therapy in antibiotic associated

diarrhoea (Clostridium difficile) than oral vancomycin, although unlicenced

Vancomycin, teicoplanin For Gram-positive organisms (including MRSA)

Other antibiotics

Cotrimoxazole Role in uncomplicated UTI, UTI prophylaxis,

acute exacerbations of chronic bronchitis Pneumocystis carinii (now jiroveci) infections

Nitrofurantoin For UTI, prophylaxis vs. UTI

Fusidic acid, rifampin For penicillin-resistant staphylococci Not for monotherapy due to risk of emergence

of resistance

Mechanisms of Action of Antibacterial Drugs

Inhibition of protein synthesis Structure of prokaryotic ribosome acts as

target for many antimicrobials of this class Differences in prokaryotic and eukaryotic

ribosomes responsible for selective toxicity Drugs of this class include

Aminoglycosides Tetracyclins Macrolids Chloramphenicol Lincosamides Oxazolidinones Streptogramins

Inhibition of nucleic acid synthesis

RNA, which participate in the protein biosynthesis.

DNA, which carries the entire genetic information for the characters to be expressed by the organisms, by acting as hereditary material.

Inhibition of RNA synthesis

Certain antibiotics are able to bind with the key enzyme involved in RNA synthesis like-

RNA polymerese. Binding of antibiotics to this enzyme

interferes with the functioning of this enzyme and prevent RNA synthesis.

Certain other antibiotics bind with GC pair of DNA and prevent unfolding of DNA, required for transcription. thus, they inhibit RNA synthesis.

E.g.- Mitomycin C Actinomycin D

Antibiotic Mode of action

Actinomycin D

Binds to GC pair of DNA and interferes with transcription And replication process.

Mitomycin C

Binds to GC pair of DNA and interferes with transcription And replication process.

Rifampicin Binds with β- subunit of bacterial RNA polymerese andInterferes with transcriptional process.

Rifamycin Binds with β- subunit of bacterial RNA polymerese andInterferes with transcriptional process.

Griseofulvin Binds to DNA polymerese

Anthramycin group

Binds to DNA and damage its structure and function.

Inhibition of protein synthesis

Protein synthesis is a multi-step process. Majority of antibiotics inhibit the process s that occurs in the 30S 0r 50S subunit of 70S bacterial ribosome, this in turn inhibits the protein biosynthesis.

Most of the antibiotics inhibits the formation of 30S initiation complex or altogether inhibits the formation of 70S ribosome by the 30S and 50S ribosome subunits or they inhibit assembling of amino acids into a polypeptide chain.

Tetracyclines, includingdoxycycline, block protein synthesis by preventing the binding of aminoacyl- tRNA in 30S ribosome subunit. These compounds block protein synthesis in both prokaryotic and eukaryotic system.

Streptomycin interferes with the formation of 30S initiation complex hence inhibits the protein biosynthesis. Erythromycin interferes with the assembly of 50S subunit of ribosome hence inhibit the protein synthesis.

Antibiotics lincomycin and clindamycin inhibits enzyme peptidyl transferase, hence prevent the protein synthesis.

Conti….

Whereas antibiotic puramycin does not inhibits the enzymatic process, but they act as an analoge of 3'-terminal end of aminoacyl-tRNA, hence disrupts protein synthesis and causes premature polypeptide chain termination. In other words this antibiotic produces non functional proteins in the cell.

Some of the examples for this category of antibiotics are Doxocycline hyclate, Erythromycin, Hygromycin B, Kanamycin disulfate salt and much more.

Name Chemical nature Target site of action

Puromycin Structural analogue

Compete with binding of tRNA aminoacyl tRNA at a side on ribosome.

Streptomycin Aminoglycoside Binds to 30s ribosomal subunit and cause misreading of codons.

Tetracycline Naphthalene ring structure

Binds to 30s ribosomal subunit and prevent binding of aminoacyl tRNA to ribosome

Chloramphenicol Nitrobenzene Ring

Binds to 50s ribosomal subunit and interferes with peptide bond formation.

Erythromycin Macrolide ring Binds to 50s ribosomal subunit and interferes with peptide bond formation as well as block translocation step.

Good news vs. bad news Good news

A few novel antibiotics have shown promising results / are undergoing clinical studies

Bad news As immunosuppressive diseases and use of

immunosuppressive agents become more prevalent, opportunistic infections becomes more common, esp. by organisms rarely encountered previously Diseases: e.g. HIV, leukemia Drugs: e.g. in solid organ transplants, bone

marrow transplants, rheumatoid disorders Development of bacterial resistance to

antibiotics is much faster than research and development of new antibiotics

Conclusion

Antibiotics inhibits the growth of infectious agents such as bacteria, virus, fungus or other types of microorganisms by inhibiting cell wall formation or nucleic acid synthesis or protein synthesis.

References

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