Streptomycin, sodium azide, and merthiolate as bacteriostatic agents for use with the Treponema pallidum immobilization test.

The problems of bacterial contamination associated with the shipment or preservation of serum specimens to be tested with the conventional serologic tests for syphilis are increased when such specimens are to be tested with the Treponema pallidum Immobilization (TPI) test. Bacterial contamination interferes with the ease and accuracy of reading the test and the metabolic or somatic products of certain contaminants produce non-specific immobilization. This report confirms the value of streptomycin as a bacteriostat, as reported by Ledbetter and Cumming (1954), and also shows that sodium azide and merthiolate are satisfactory bacteriostats for use with the TPI test. Ledbetter and Cumming employed streptomycin at a concentration of 100 ug./ml. to prevent further growth in bacterially contaminated specimens submitted for testing. They concluded that a concentration of 100,ug./ml. didnot have a harmful effect on the viability of Treponema pallidum or the residual complement reaction. They did not report on the effect of 100 ,tg./ml. concentration on the sensitivity of the test. Studies in this laboratory indicate that 100 ,g./ml. does not lower test sensitivity significantly. Eleven quantitative tests with sera containing 100 jLg./ml. gave a geometric mean 50 per cent. immobilization titre of 1: 53, while without streptomycin the geometric mean titre was 1: 74. This small series of comparative tests indicates that streptomycin, in the concentration employed by Ledbetter and Cumming, does not reduce significantly the sensitivity of the test. In addition to the report by Ledbetter and Cumming, it has been reported from the Royal Free Hospital, London (WHO, 1957), that a final test concentration of 200 ,ug./ml. is also satisfactory. Streptomycin is added to the complement and treponeme suspension so that when the mixture is