Principles of selection and use of antibacterial agents - In vitro activity and pharmacology

Appropriate selection and use of an antimicrobial agent is based on characteristics of the etiologic organism and its pattern of susceptibility to antimicrobial agents, the host, and the drug. Characteristics of the host that are relevant are immune status, the site of the infection, function of body systems responsible for absorption and elimination of the drug and history of drug allergies. Some of the important features of the drug itself are the pharmacokinetics (that is, absorption, distribution into body tissues, and elimination) and pharmacodynamics (that is, the mechanism of action, the cidal or static nature of the antimicrobial effect, and the rate at which it occurs) of the drug. Before definitive identification of the etiologic agent is made, initial choice of an empiric antimicrobial regimen depends primarily on knowledge of the spectrum of organisms likely to cause infection at the infected site. For example, a patient with a diverticular abscess may be treated with antimicrobial agents aimed at gram negative enteric bacilli and anaerobes based on the knowledge that these organisms constitute normal bowel flora and are therefore the most likely organisms to cause infection in the bowel wall. If the patient has been in the hospital or nursing home and is likely to be colonized with resistant organisms, a knowledge of local resistance patterns is important. Whenever possible, an attempt should be made to culture the infecting organism before "empiric" therapy is initiated. This is particularly important in patients with serious infections; however, even in patients with less severe infections, precise knowledge of the organism and its antimicrobial susceptibility pattern is valuable. In any situation, simpler, less costly, and more focused treatment may be possible once the organism is identified.