Treatment of Infection

Professor Mark Pallen

Treatment of Infection How Do Antimicrobials Work?

• Key concept: selective toxicity– the antimicrobial

agent blocks or inhibits a metabolic pathway in a micro-organism which is either absent or is radically different in the mammalian cells of the human host

Student

Bug

"magic bullet"

Principle of antibiotic spectrum

• Different antibiotics target different kinds of bacteria– i.e., different spectrum of activity

• Examples:– Penicillin G (= original pen.) mainly streptococci (narrow

spectrum)– Vancomycin only Gram-positive bacteria (intermediate

spectrum)– Carbapenems many different bacteria (very broad spectrum)

Treatment of Infection Anti-Microbial Drug Targets

Antimicrobials acting on the bacterial cell wall

• Interfere with synthesis of peptidoglycan layer in cell wall– eventually cause cell

lysis– bind to and inhibit

activity of enzymes responsible for peptidoglycan synthesis

• aka “penicillin-binding proteins”

Antimicrobials acting on the bacterial cell wall

• Beta-lactams: Penicillins– benzylpenicillin – flucloxacillin – ampicillin – piperacillin

Rβ

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beta-lactam ring

Antimicrobials acting on the bacterial cell wall

• Beta-lactams: Cephalosporins– Orally active

• cephradine• cephalexin

– Broad spectrum• cefuroxime • cefotaxme• ceftriaxone• ceftazidime

Cephalosporins

synthetic side chains change the spectrum

of action

β

a house with a garage & basement

Antimicrobials acting on the bacterial cell wall

• Unusual beta-lactams– Carbapenems

• Imipenem, meropenem– very wide spectrum

– Monobactams• Aztreonam

– only Gram-negatives

• Glycopeptides– only Gram-positives, but

broad spectrum– vancomycin– teicoplanin

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β

Antimicrobials acting on nucleic acid synthesis

• Inhibitors Of Precursor Synthesis– sulphonamides & trimethoprim are synthetic,

bacteriostatic agents• used in combination in co-trimoxazole

– Sulphonamides inhibit early stages of folate synthesis

• dapsone, an anti-leprosy drug, acts this way too– Trimethoprim inhibits final enzyme in pathway,

dihydrofolate synthetase.• pyramethamine, an anti-toxoplasma and anti-PCP drug

acts this way too

Antimicrobials acting on nucleic acid synthesis

• Inhibitors of DNA replication – Quinolones (e.g ciprofloacin) inhibit DNA-gyrase– Orally active, broad spectrum

• Damage to DNA– Metronidazole (anti-anaerobes), nitrofurantoin (UTI)

• Inhibitors of Transcription – rifampicin (key anti-TB drug) inhibits bacterial RNA

polymerase– flucytosine is incorporated into yeast mRNA

Antimicrobials acting on protein synthesis

• Binding to 30s Subunit– aminoglycosides

(bacteriocidal)• streptomycin, gentamicin,

amikacin.– tetracyclines

• Binding to the 50s subunit– chloramphenicol – fusidic acid – macrolides (erythromycin,

clarithromycin, azithromycin)

30s subunit

50s subunit

mRNA

protein

Antimicrobials acting on the cell membrane

• amphotericin binds to the sterol-containing membranes of fungi

• polymyxins act like detergents and disrupt the Gram negative outer membrane.– Not used parenterally because of toxicity to

mammalian cell membrane

• fluconazole and itraconazole interfere with the biosynthesis of sterol in fungi

Mechanisms of resistance

• Resistance can arise from chromosomal mutations, or from acquisition of resistance genes on mobile genetic elements– plasmids, transposons, integrons

• Resistance determinants can spread from one bacterial species to another, across large taxonomic distances

• Multiple resistance determinants can be carried by the same mobile element– Tend to stack up on plasmids

Impact of antibiotic resistance

• Infections that used to be treatable with standard antibiotics now need revised, complex regimens:– e.g., penicillin-resistant Strep. pneumoniae now requires

broad-spectrum cephalosporin

• In some instances, hardly any antibiotics left: – e.g., Multiresistant Pseudomonas aeruginosa– e.g., Vancomycin-resistant Staph. aureus

• Resistance rates worldwide increasing

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Mechanisms of resistance

• Enzymes modify antibiotic– widespread, carried on mobile elements

• beta-lactamases• chloramphenicol-modifying enzymes• aminoglycoside-modifying enzymes

• Permeability– antibiotic cannot penetrate or is pumped

out• chromosomal mutations leads to changes in

porins• efflux pumps widespread and mobile

Mechanisms of resistance

• Modification or bypass of target – by mutation or acquisition of extrinsic DNA– S. aureus resistance to flucloxacillin

• acquires an extra PBP2 to become MRSA

– S. aureus resistance to mupirocin• Chromosomal mutations in low-level resistance• Plasmid-borne extra ILTS gene in high-level resistance

– Rifampicin resistance in M. tuberculosis• Point mutations in RNA polymerase gene

Antibiotic susceptibility testing in the laboratory

• Bacterial cultures tested on artificial media

• Tests the ability to grow (or: be killed) in the presence of defined antibiotics

• Provides guidance for ongoing therapy

• Provides resistance rates for empiric therapy

• Problems: not all results correspond with clinical success or failure

Determination of MIC and MBC

Mims C et al. Medical Microbiology. 1998.

Disk diffusion testing

Cohen & Powderly 2004; http://www.idreference.com/

Questions to ask before starting antibiotics

• Does this patient actually need antibiotics?• What is best treatment?

– What are the likely organisms?– Where is the infection? – How much, how often, what route, for how long? – How much does it cost? – Are there any problems in using antibiotics in this

patient?

• Have you taken bacteriology specimens first?!

Clinical use of antibiotics

Gillespie SH & Bamford KB. 2003. Medical microbiology & infection at a glance.

Does this patient need antibiotics?

• Is the patient even infected?– e.g. urethral syndrome vs UTI

• Is it a viral infection?– e.g. the common cold

• Is the infection trivial or self-limiting?– most diarrhoea

• Are there more appropriate treatments?– physiotherapy for bronchitis– treatment of pus is drainage– treatment of foreign body infection is removing the foreign

body

Best antibiotic(s) for these organisms …?

• For some organisms sensitivities are entirely predictable– e.g. Streptococcus pyogenes always penicillin-sensitive

• For most organisms, sensitivity tests contribute to rational therapy– e.g. coliforms in UTI

• Knowledge of local resistance problems contributes to choice of empirical therapy

Best antibiotic(s) for this site of infection …?

• Depends on penetration of antibiotic into tissues– e.g. gentamicin given iv does not enter CSF or

gut– E.g. azithromycin accumulates in cells even

though levels low in serum

• Depends on mode of excretion– e.g. amoxycillin excreted in massive amounts in

urine

Are there any problems with this regimen in this patient?

• Allergy– usually only a problem with penicillins, and,

less often, with cephalosporins (~10% cross sensitivity)

• Ampicillin Rash– develops if patient has glandular fever or

lymphoma– Not related to general penicillin allergy

Are there any problems with this regimen in this patient?

• Side Effects• some occur with almost any antibiotic

– Gastric upset– Antibiotic-associated diarrhoea

• C. difficile infection• pseudo-membranous colitis an be fatal

– Overgrowth of resistant organisms• “Thrush” in the community• VRE’s, MRSAs, Candida in ITU

Are there any problems with this regimen in this patient?

• Organ-specific side effects• damage to kidneys, ears, liver, bone marrow

– chloramphenicol produces rare aplastic anaemia– vancomycin can cause "red man syndrome"– rifampicin discolours tears, urine contact lenses, can

cause "flu-likesyndrome"– erythromycin causes gastric irritation– ethambutol can cause ocular damage– Aminoglycosides and vancomycin can cause ear and

kidney damage

Are there any problems with this regimen in this patient?

• Care needed in patients with metabolic problems– renal failure– liver failure– genetic diseases

• Drug interactions– e.g. gentamicin and frusamide

• Use in pregnancy, breast feeding, children• Check in the BNF!

Other Questions to Ask

• How much?• How long for?• How frequently?• What route?

– In general, you should avoid “overdoing it” Microbiologists spend as much time telling people when to stop antibiotics as when to start!

– Switch from i-v to oral therapy as soon as you can– Treat UTIs for just three days