Antimicrobial agents may be classified according to the type of organism against which they are active and in this book follow the sequence:
• Antibacterial drugs
• Antiviral drugs
• Antifungal drugs
• Antiprotozoal drugs
• Anthelmintic drugs.
A few antimicrobials have useful activity across several of these groups. For example, metronidazole inhibits obligate anaerobic bacteria (such as Clostridium perfringens) as well as some protozoa that rely on anaerobic metabolic pathways (such as Trichomonas vaginalis).
Antimicrobial drugs have also been classified broadly into:
• bacteriostatic, i.e. those that act primarily by arresting bacterial multiplication, such as sulphonamides, tetracyclines and chloramphenicol
• bactericidal, i.e. those which act primarily by killing bacteria, such as penicillins, cephalosporins, aminoglycosides, isoniazid and rifampicin.
Less used in modern clinical practice, the classification is somewhat arbitrary because most bacteriostatic drugs can be shown to be bactericidal at high concentrations, under certain incubation conditions in vitro and against some bacteria.
Bactericidal drugs act most effectively on rapidly dividing organisms. Thus a bacteriostatic drug, by reducing multiplication, may protect the organism from the killing effect of a bactericidal drug. Such mutual antagonism of antimicrobials may be clinically important, but the matter is complex because of the multiple and changing factors that determine each drug's efficacy at the site of infection. In vitro tests of antibacterial synergy and
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