Antimicrobial prophylaxis general principles and drug used

General principles
01.Anti microbials cannot   eliminate all micro organisms in the host and cannot prevent all types of infection. hence prophylaxis should be directed against a specific pathogen or used to prevent infection at a specific site

02.Shorter the duration of prophylaxis the larger the range of pathogens affected. For prevention of all infections in leucopoenias
  • Broad spectrum  anti microbials -> short term
  • Long term use-> resistance
  • Group “a” strep ->long term use  
03.Prophylaxis more effective against pathogens less likely to develop resistance 
       eg. penicillin for grp.”a” strep->effective not for gonococcus 

04.Drugs for therapy may not be good for prophylaxis

eg.penicillin in meningococcal meningitis not effective for prophylaxis

05.Prophylactic drugs should only be used when efficacy has been documented cost, toxicity, superinfection development of resistance

Surgical prophylaxis

  • Antimicrobials must be efffective against majority of organisms causing post.op. infections
  • Start therapy 1-2hours before and stop 12-48 hours after op.
  •  A single pre op.dose gives maximal benefit
  • Antimicrobials activity must be present in surgical wound before closure


  •  Cefazolin ->drug of choice  (first generation cephalosporin) 0.5gm -2gm i.v.        
  • Cefepime 4th generation cephalosporin more resistance to hydrolysis by beta lactamase

 eg. enterobacter that may    inactivate 3rd generation ceph.

Drug combinations        


  • Very ill patients with infections of unknown origin eg septicaemia anti staph (nafcillin-cloxacillin0 with gm –ve bacilli (gentamicin tobramycin amikacin)  immunocompromised
  • Mixed infections peritonitis anaerobes– Metronidazole, Gram +ve bacteria- clindamycin coliforms –Aminoglycosides
  • Prevent resistance eg : TB (isoniazid,ethambutol,rifampin)
  • Enhance effect of single drug eg. sulphonamide with trimethoprim penicillin with aminoglysoide -> enterococcus faecalis

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