Overview of synergy with reference to double beta-lactam combinations.

Combination antimicrobial therapy is used to expand the bacterial coverage over a single agent, to prevent the emergence of resistant organisms, to decrease toxicity by allowing lower doses of both agents, or for synergy. Synergy is one of the most common of these reasons, especially in serious infections. The introduction of new broad-spectrum beta-lactam antimicrobials has led to their combination in the treatment of seriously ill patients. Whereas a combination of an aminoglycoside and a beta-lactam antimicrobial is frequently synergistic, much less is known about synergy between combinations of beta-lactams. In vitro testing shows most combinations of two beta-lactams to be indifferent or additive in their effects; rarely does synergy occur. Antagonism can sometimes be seen, particularly with combinations involving cefoxitin or imipenem, especially if the treated organism is Enterobacter or Pseudomonas. Results of clinical trials comparing double beta-lactam (DBL) therapy with aminoglycoside/beta-lactam combinations show no difference in clinical response rates. Highly active DBL combinations may substitute for standard aminoglycoside-containing regimens in certain situations, even though they are not reliably synergistic. However, in the treatment of seriously ill patients such combinations may be less desirable.