Antimicrobial agents can act by - University of British ...med-fom-apt.sites.olt.ubc.ca/files/2016/02/Antimicrobials-2.pdf · • acting as “anti-metabolites” ... Antimicrobial

Mechanisms of action of antimicrobials

Antimicrobial agents can act by:

• interference with bacterial cell wall formation

• alteration of cell membrane integrity

• interference with nucleic acid metabolism/function

• interference with protein synthesis

• acting as “anti-metabolites”

Pharmacology of individual agents

Agents affecting cell membrane integrityAntifungal polyenes e.g., nystatin

• forms a complex with ergosterol(stabilizer of fungal cell membrane) → membrane disruption and cell death

• major uses: treatment of Candida albicans (yeast) infections in the mouth (“thrush”), skin, vagina, intestinealso: prophylaxis in patients at risk of developing fungal infections (e.g., individuals with AIDS, or who are on chemotherapy(nystatin has also been used to prevent growth of mould on works of art – e.g., wood panel paintings!)

Agents affecting cell membrane integrity

Antifungal imidazoles e.g., miconazole(other agents include ketoconazole, fluconazole)

• act by inhibiting synthesis of ergosterol→ membrane disruption and cell death

• use: treatment of yeast infections in mouth, stomach and intestine – also fungal infections of the skin, including tinea pedis (“athlete’s foot"), and tinea corporis (“ringworm“)

• preparations: miconazole (Monostat®, Daktarin® oral gel),ketoconazole (Nizoral®), fluconazole (Diflucan®)







• acting as “anti‐metabolites”

Agents interferening with nucleic acid metabolism and/or function

DNA gyrase inhibitors: Ciprofloxacin (bacericidal)

• Ciprofloxacin inhibits gyrase enzyme controlling DNA uncoiling/recoiling during DNA and mRNA synthesis

• good tissue penetration, and available in both oral and intravenous formulations

• broad antibacterial spectrum: Gm –ve

e.g., strains of E. coli, Haemophilus, Klebsiella, Proteus and Pseudomonas) and Gm +ve (e.g.,Staph aureus – exceptMRSA, Streptococci, Enterococci)

• uses: infections of urinary, respiratory and intestinal tracts, lungs, skin, bones, joints

Agents interferening with nucleic acid metabolism and/or function

Inhibition of RNA polymerase: Rifampin (bactericidal)

• Inhibits RNA polymerase andprevents transcription of RNA from DNA template → protein synthesis inhibited

• Rifampin – its use is largely restricted to the treatment of tuberculosis

Pharmacology of individual agentsAgents interferening with nucleic acid metabolism

and/or function

Inhibition of mitosis: Griseofulvin

• binds to tubulin in microtubules of fungal cells → inhibition of spindle formation →inhibition of mitosis

• used to treat fungal infections of skin and nails

• preferential binding of griseofulvin to microtubules of cancercells (relative to “healthy” cells) may have therapeutic potential (in combination with other antineoplastic agents)

Agents interferening with nucleic acid metabolism and/or function (cont’d)

Degradation of DNA: Metronidazole (bactericidal)

• preferentially taken up by anaerobicbacteria → converted to reactive metabolite →DNA strand breakage and destabilization of the DNA helix

• antibacterial spectrum: Gm –ve anaerobes (e.g., Bacteroides, Clostridium) - no effect on aerobic bacteria or human cells

• clinical uses: Tx of infections caused by anaerobic bacteria, Tx of pseudomembrane colitis (often antibiotic-induced) due to overgrowth of Cl. difficile, Helicobacter pylori eradication therapy, as part of a multi-drug regimen in peptic ulcer disease








Pharmacology of individual agents

Antimicrobials inhibiting protein synthesis

Summary of steps in protein synthesis

• binding of t-RNA containing triplet nucleic acid base codon to amino acid

• binding of t-RNA containing amino acid to corresponding codon on ribosome-associated m-RNA (for DNA-determined sequence of amino acids in chain)

• peptide bond formation to link amino acids (catalyzed by peptidyl transferase)

• movement of growing peptide chain along ribosome unit

Antimicrobials inhibiting protein synthesisAminoglycosides (bactericidal)

• Streptomycin - first analogue of this class –introduced in 1940’s - newer agents followed (e.g., gentamicin, tobramycin, spectinomycin)

• resistance due to gradual induction of inactivating enzymes ∴ newest analogues most active

• bind to 30s ribosomal subunit and interfere with binding of t-RNA, causes “misreading” of the code

• bactericidal action (most protein synthesis inhibitors are only bacteriostatic) may involve additional actionto disrupt cell membrane

• aminoglycosides actively accumulated by aerobic organisms ∴ anaerobes resistant

Antimicrobials inhibiting protein synthesisAminoglycosides (cont’d)

• Aminoglycosides (AG) primarily used for infections involving aerobic, gram-negative bacteria (e.g., Pseudomonas, Acinetobacter, Enterobacter, and some Mycobacteria -including the bacteria that cause tuberculosis (AG were first drugs to effectively treat TB)

• AG sometimes used in combination with a penicillin for synergistic effect in serious (e.g., blood stream) infections

• major limiting toxicites of AG: renal damage and ototoxicity(damage to nerves to inner ear → vestibular and hearing impairment)


Tetracyclines (bacteriostatic)

• introduced as “miracle drugs” in 1948 –broad antibacterial spectrum – but widespread development of resistance

• mode of action: bind to 30s ribosome subunit and prevent binding of t-RNA to m-RNA

• clinical use: Tx of infections caused by Chlamydia, Rickettsia (e.g. typus fever), brucellosis and spirochetal infections (e.g., Lyme disease)

Antimicrobials inhibiting protein synthesisTetracyclines (cont’d)

• main medical uses: infections of respiratory tract, sinuses, middle ear, urinary tract - tetracyclines have has also been used (topically or systemically) to treat acne (mechanism of action may also involve non-antimicrobial actions – e.g., an antiinflammatory effect related to a reduction in neutrophil chemotaxis)

• dental applications: tetracyclines concentrate in the gingival fluid - used to treat gingivitis and gum disease ‐ tetracyclines reduce inflammation and block collagenase (destroys connective tissue and bone) - it may be these two actions which contribute most to periodontal protection

• tetracyclines chelate calcium and deposit in teeth and bones – use in pregnancy or in infants can → permanent staining of teeth

Antimicrobials inhibiting protein synthesisChloramphenicol (bacteriostatic)

• mode of action: binds to 50s ribosomal subunit and prevents linking of amino acids via peptide bond formation by inhibiting peptidyl transferase

• “broad” spectrum of activity – acts on Gm+ve and Gm-ve bacteria, including anaerobic organisms

• readily crosses the blood-brain barrier

• main clinical uses: treatment of brain abscesses caused by Staphylococci (or when causative organism unknown), treatment of meningitis(involving Neisseria meningitidis, Streptococcus Pneumoniae or Haemophilus influenzae) in individuals with penicillin allergy- also useful to treat infections caused by vancomycin-resistant enterococcus (“VRE”)


Chloramphenicol (cont’d)

Toxicities:• 1 in 24,000–40,000 risk of potentially fatal aplastic anemia

(unpredictable!) – risk highest with oral admin, lowest with eye drops

• Gray Baby Syndrome: immaturity of glucuronyl transferase in new-born → impaired inactivation of chloramphenicol (risk greatest with ivadministration)

symptoms include hypotension, cyanosis and cardiovascular collapse – usually preventable by adjusting drug dose

Antimicrobials inhibiting protein synthesisClindamycin (bacteriostatic)

• mode of action: binds to 50s ribosomal subunit and blocks translocation of growing peptide chain

• antibacterial spectrum: – aerobic Gm+ve

cocci (e.g, staphylococci, streptococci but NOT enterococci), and anaerobic Gm –ve rods (e.g., Bacteroides, Fusobacterium)

• main clinical uses: infections by above bacteria – especially in patients with penicillin allergy; used topically to treat acne

• major toxicity: pseudomembraneous colitis - caused by toxin produced by Cl. difficile (which is resistant to clindamycin)

Antimicrobials inhibiting protein synthesisMacrolides (bacteriostatic)

(erythromycin ‐ clarithromycin is amore acid‐stable analogue)

• mode of action: binds to 50s ribosomal subunitof 70s complex, prevents binding of amino acid-linked to t-RNA to template, blocks peptide elongation

• antibacterial spectrum: similar to penicillin (mainly Gm+ve organisms) but Staph usually resistant

• clinical use: alternative to penicillin in penicillin allergy, respiratory infections (e.g., Mycoplasma pneumonia)

• major toxicity: gi disturbance – due to activation of motilin receptors in gut








Antimicrobials acting as antimetabolites

Acyclovir (antiviral)

• structural analogue (“antimetabolite”) of purine nucleoside guanosine

• inactive parent drug (a “pro-drug”) is converted to monophospate by viral thymidine kinase → further phosphorylation by host cell kinases to acyclovir triphosphate → inhibition of viral “reverse transcription”

• “reverse transcription” involves synthesis of double-stranded DNA from viral DNA(e.g. herpes) or RNA (e.g. retroviruses) templates

Antimicrobials acting as Antimetabolites

Acyclovir (cont’d)

Antimicrobials acting as antimetabolitesAcyclovir (cont’d)

• active against sensitive strains of herpes simplex virus (e.g., cold sores) – but has low activity against cytomegalovirus

• affinity of acylovir triphosphate for herpes viral DNA polymerase is orders of magnitude greater than that for the corresponding enzyme in non-viral cells

acyclovir has high selectivity and low toxicity

• resistance to acyclovir is rare – but can occur as a result of mutations leading to markedly reduced affinity of the target enzyme for acyclovir triphosphate

• commonly used oral preparations: acyclovir (Zovirax®), famciclovir (Famvir®) and valacyclovir (Valtrex®).

Antimicrobials acting as antimetabolites

Sulfonamides (bacteriostatic)

• sulfonamides were the first chemicalsever used to treat bacterial infections

• Mechanism of action: “sulfa” drugs arestructural analogues of p-amino-benzoic acid (PABA) – a component of folic acid(an essential B vitamin) – they block folate synthesis by acting as PABA antagonists

• folic acid is a essential co-factor for certain enzymes involved in production of the nucleic acid base thymidine and in the synthesis of the amino acid methionine

Antimicrobials acting as antimetabolitesSulfonamides (cont’d)

• two steps in the formation of tetrahydro-folate (the active form of folic acid) are targets of antimicrobial action:- the incorporation of PABA is blocked by sulfonamides, and trimethoprim inhibits the enzyme conversing dihydrofolate to active tetrahydrofolate (the active form)

• the synergistic action of a “sulfa” and trimethoprim is exploited in two widely used commercial preparations containing both sulphamethoxazole and trimethoprim:

Bactrim® and Septra®


• basis of sulfonamide selectivity: sensitive bacteria must synthesize their own folic acid - resistant bacteria and host cells can use pre-formed folate

• antibacterial spectrum: originally active against many Gm +ve and Gm –ve organisms (NOT Group A streptococci), but resistance now widespread

• bacteria resistant to a sulfonamide alone may respond to the combination of a “sulfa” and trimethoprim

• major clinical use: infections of urinary tract (“sulfas” concentrated in urine), upper respiratory tract and ear


Adverse effects• hypersensitivity (allergic) reactions common – may be

delayed – usually starts 2-8 weeks after initiating therapy• contraindicated in late pregnancy, in neonates and in

breast feeding mothers: - risk of kernicterusMechanism of kernicterus development: in neonate, levels of bilirubin high (breakdown of fetal hemoglobin and immaturity of glucuronyl transferase), binding of bilirubin to plasma albumin reduced by competitionwith sulfonamide → increased levels of unbound bilirubin – crosses blood-brain barrier incompletely developed in neonate) → → → brain damage

Thanks for your attention!!

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Antimicrobial agents can act by - University of British ...med-fom-apt.sites.olt.ubc.ca/files/2016/02/Antimicrobials-2.pdf · • acting as “anti-metabolites” ... Antimicrobial