Antibiotics and ERCP: In vitro activity of aminoglycosides when added to iodinated contrast agents

In an in vitro study, the antibacterial activity of aminoglycoside antibiotics was evaluated after they were added to iodinated contrast agents and bile under conditions simulating their use in endoscopic retrograde cholangiopancreatography. The study was done in two parts: First, tobramycin was mixed with sterile bile and iodotholamate meglumine. Minimal bactericidal concentrations for tobramycin in this solution were determined against three representative bacteria. To test for interference by iodotholamate with tobramycin activity, the minimum bactericidal concentration of tobramycin in the mixture was compared with minimum inhibitory concentrations for tobramycin when used alone in broth cultures of the same organisms. The stability of tobramycin in bile-contrast mixture was determined at 1 and 24 hr by radioimmunoassay. In the second part of the study the bactericidal activity of another aminoglycoside, gentamicin, was determined. Gentamicin was mixed with two other contrast agents, meglumine diatrozoate and diatrozoate sodium, and diluted with broth to simulate concentrations expected after injection of the pancreaticobiliary tree. Cultures were then performed. After mixture with contrast agents, both tobramycin and gentamicin prevented growth of all test organisms. There was no evidence of tobramycin inactivation by iodotholamate, and no evidence of biochemical deterioration of tobramycin over 24 hr as determined by radioimmunoassay. The theoretical advantages of adding antibiotics to contrast agents are reviewed. The results of these experiments confirm that the antibacterial activity of aminoglycosides is not altered in an environment which simulates endoscopic retrograde pancreatography. Although they do not establish clinical efficacy, our results provide a logical experimental basis for the empirical clinical practice of adding aminoglycosides to contrast agents to be used for endoscopic retrograde cholangiopancreatography.