Introduction To Antibiotics

27.3.06

What?
• Compounds that act against bacteria (antimicrobials - against parasites, fungi).
• Kill (bacteriocidal) or inhibit (bacteriostatic) - not useful distinction nowadays.
• Administered orally, parenterally or topically.
• Resistance may rapidly develop.

Complex relationship
• Drug ⇔ [potency versus resistance] ⇔ Bug.
• Bug ⇔ [virulence versus immunity, attack versus defence] ⇔ Host.
• Host ⇔ [metabolism versus toxicity] ⇔ Drug.
• Balance makes outcome good or bad.

How are bacteria classified?
• 2 ways - colour on staining or shape.
• Gram staining - gram + (purple) or gram - (pink).
• Shape - coccus (round) or bacillus (rod).

Some medically important bacteria
Gram +
• Cocci (generally throat).
-Staphylococcus.
-Streptococcus.
• Rods.
-Clostridia - cause of antibiotic diarrhoea - antibiotics kill colonic flora and new ones recolonise.

Gram -
• Cocci.
-Neisseria - neisseria meningitidis causes meningitis, neisseria gonorrhoea.
-Haemophilis - also slightly bacillus with capsule - more virulent - meningitis in <4 years, swells without capsule, troat infection, epiglottis - HiB vaccine, haemophilius, influenza B vaccine - capsule.
• Rods - generally gut/abdomen.
-E. coli.
-Proteus.
-Klebsiella.
-Salmonella.

• Important: gram + cocci and gram - rods.
• Anaerobic bacteria - strep faecalis etc.
• Human bites dirtier than dog bites.
• Staphylococcus aureus - boils - druggies get most often.

Infections
Is there infections? Take culture. Fever? Pain, swelling, redness, tachycardia, sweats, pus, toxaemia.

Where is site? What are likely organisms?

What antibiotics are likely to be effective?
• β lactams.
-Penicillins (amoxycillin, flucoxacillin, benzylpenicillin)
-Cephalosporins (cephtriaxone).
-Others.
• Aminoglycosides.
-Gentamycin.
• Macrolides.
-Erythromycin, clarithromycin.
• Quinolones.
-Ciprofloxacin against gut rot/diarrhoea.
• Tetracycline.
• Glycopeptides.
• Metronidazole.

Is there likely to be resistance?

Will the antibiotic penetrate to the site?

Route of administration?

What is the toxicity and cost?

Likely resistance?
• Community acquired or hospital acquired?
• Previous antibiotics?
• Travel history?
• Nasocomial infections - hospital acquired - tend to be resistant eg. MRSA, clostridium difficile.
• Preventable - hygience, antibiotic control.
• Broader spectrum used.

Penetrating to site? Route? Consider
• Very sick?
• Serious infection?
• Barrier to drug absorption?
• Malabsorption - post-surgery?
• Vomiting or swallowing problems?
• Poor bioavailability.

Common mistakes
• Often not effective.
-Tonsillopharyngitis (viral).
-Gastroenteritis (self-limiting usually).
-Colonisation versus infection.
• No prior culture obtained.
• Inappropriate dose/route.
• Continued for too long.