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CHAPTER 12Antibiotic Mechanisms of

Action and ResistanceDiagnostic Microbiology

Medical LaboratoryFaculty of Medicine & health sciences

Dr. Hafez Alsumairy

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Objectives1. Describe the mechanism of action of the different classes of

antimicrobials.2. Describe the targets of the different antibiotic classes.3. Describe and distinguish differences between intrinsic and

acquired mechanisms of antibiotic resistance.4. Discuss the mechanisms used by microorganisms to disseminate

resistant determinants.

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Introduction • Antimicrobial agents include antiseptics, antibiotics, preservatives,

sterilants, and disinfectants; all have the capacity to kill or suppress the growth of microorganisms.• Intrinsic resistance• Passed vertically to progeny

• Acquired resistance• Result of chromosomal mutations, or by the horizontal transfer

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inhibition of bacterial Cell wall biosynthesisPeptidoglycan biosynthesis

(1)synthesis of precursors in the cytoplasm (2)transport of lipid bound precursors across

the cytoplasmic membrane(3)insertion of glycan units into the cell wall(4)transpeptidation linking and maturation.

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Antibiotic targets and mechanisms of action

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inhibition of bacterial Cell wall biosynthesis• β-lactam antibiotics• functions as a structural analogue of

the normal substrate acyl-D alanyl-D-alanine

• penicillin-binding proteins (PBPs)• PBPs transpeptidase/transglycosylase

• Glycopeptides• Vancomycin

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Inhibition of bacterial Cell wall biosynthesis

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Inhibition of folate synthesis• Completely synthetic molecules • Sulfamethoxazole (SMZ)• Structural analogue para

aminobenzoic acid PABA

• Trimethoprim (TMP)• Binds to dihydrofolate reductase

• Urinary tract infections

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Interference of DNA Replication• Quinolones• Targeting topoisomerases II (DNA

gyrase) in gram-negative bacteria and IV in gram-positive bacteria• Enterobacteriaceae,

pseudomonads, and other non-Enterobacteriaceae, staphylococci, enterococci, neisseria, and streptococci species other than Streptococcus pneumoniae

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Interference of DNA transcriptionRifampin• A synthetic derivative of rifamycin

B• Mycobacterium tuberculosis• The target of rifampin is the RNA

polymerase β subunit at an allosteric site• staphylococci, enterococci,

Haemophilus spp., and S.pneumoniae

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Interference of mRNA translation• Formylmethionyl tRNA, which

binds to the P site of the 30S ribosomal subunit• Aminoglycosides are cationic

CHO-containing molecules provides the basis for their interaction with a specific region of the 16S rRNA in the 30S.• A-binding site

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Interference of mRNA translation• Polyketide class of antibiotics• Tetracycline, doxycycline, and

minocycline.

• Tetracyclines reversibly inhibit protein synthesis by binding to 16S ribosomal RNA near the amino-acyl tRNA acceptor (A) site

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Interference of mRNA translation• Macrolides such as

erythromycin, clarithromycin, and azithromycin target the 50S subunit specifially by binding tothe peptidyltransferase cavity in the proximity of the A and P loops, near adenine 2058 of 23S rRNA

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Interference of mRNA translation• Linezolid • Inhibitors by blocking the initiation

step while macrolides and tetracyclines block peptide chain elongation.• Streptogramin

• Glycylcycline• Binding to the 30S ribosomal

subunit and blocking entry of aminoacyl tRNA molecules into the A site of the ribosome

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Mechanisms of antibiotic resistance

• Intrinsic• Innate (chromosomal DNA)• Transmitted to progeny vertically

• Acquired• DNA by transformation• Recombination• Extrachromosomal DNA

transmitted horizontally• Plasmids• Transposons

Intrinsic (Chromosomal) Mechanisms of Resistance1. Impermeability2. Biofilms3. Efflux4. Enzymatic Inactivation

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Intrinsic (Chromosomal) Mechanisms of Resistance1. Impermeability

A. Lipopolysaccharide (LPS)composition• Strongly negatively charged core• O-antigen side chains

B. Outer membrane proteins (Omps) called porins.• E. coli are OmpF, OmpC, and OmpE• Decreases or loss of porin synthesis

• Glycopeptide

2. Biofilms• Biofilms are sessile bacterial

communities that are irreversibly attached to a substrate and embedded in an exopolysaccharide matrix. • Single or multiple bacterial

species• Chemical and physical

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Intrinsic (Chromosomal) Mechanisms of Resistance2. Biofilms• Innate resistance factors

• Structure and physiology• Persister cells

• Induced resistance factors

3. Efflux• Efflux pumps are naturally occurring • Five major efflux pump protein1. The major facilitator subfamily (MFS), 2. The resistance nodulation cell division

subfamily (RND), • Pseudomonas aeruginosa (MexAB)

3. The small multidrug regulator subfamily (SMR),

4. The adenosine triphosphate (ATP)-binding cassette (ABC) family

5. The multidrug and toxic effects (MATE) family.

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Intrinsic (Chromosomal) Mechanisms of Resistance

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4. Enzymatic Inactivation• β-Lactamases

1. A metallo-based mechanism• (class B β-lactamases) require zinc

2. A serine-based mechanism• (class A, C, and D β-lactamases)

• β-lactamases compete with PBPs for the antibiotic

• Class A on plasmid and Class C is chromosomal

• In gram-negative bacteria periplasmic space

• In gram-positive bacteria are secreted

Intrinsic (Chromosomal) Mechanisms of Resistance

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Intrinsic (Chromosomal) Mechanisms of Resistance

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Acquired Mechanisms of Resistance• Efflux• Can be acquired by horizontal gene

• Target Site Modification• Chromosomal Mutation• Enzymatic Target Site Alteration

• Acquisition of New Targets• MRSA with staphylococcal cassette chromosome mec (SCCmec)

• Enzymatic Inactivation of Antibiotics• Enzymatic modifiation of aminoglycosides results from N-acetylation, O-

phosphorylation, and O-adenylation.

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Dissemination

• Plasmids• Conjugation • Transformation • Transduction • Transposons • Integrons

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Thank you

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