Effect of chemotherapeutic agents upon microorganisms isolated from cancer patients

Attempting to explain the predominance of Pseudomonas in leukemic patients, five of the most common gram-negative organisms isolated from sites of infection in cancer patients were exposed to several of the chemotherapeutic agents used in the treatment of this disease (methotrexate, cytosine arabinoside, cyclophosphamide, and 6-mercaptopurine). At concentrations of 125 mug/ml or higher, methotrexate inhibited all organisms except Pseudomonas. Cytosine arabinoside inhibited Escherichia and Klebsiella but appeared to stimulate the growth of Pseudomonas slightly at the higher concentrations. Thus, significant differences existed in individual susceptibilities to these agents. Clinical isolates were more resistant than the corresponding laboratory strains not previously exposed to these compounds. The resistance of Escherichia coli to cyclophosphamide was decreased 26% when it was grown in mixed culture with Pseudomonas. Only Pseudomonas was resistant to all of these compounds whether in pure or mixed culture. These observations may help to explain, in part, the predominant role that Pseudomonas plays as an infectious agent in leukemic patients.