Comparison of the Du Pont Isolator 1.5 Microbial Tube and Trypticase soy broth for the recovery of Haemophilus influenzae type b in experimental bacteremia.

The isolator 1.5 Microbial Tube (E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.) is a new blood culture system for detection of bacteremia in children which involves lysis of blood cells and direct plating of blood lysate on agar media. Haemophilus influenzae type b is a major pediatric pathogen and may be associated with low-density bacteremia (less than or equal to 50 CFU per ml), yet the clinical data on the efficacy of this system for recovery of H. influenzae b are limited. We used a rat model of H. influenzae b bacteremia and fresh, whole human blood inoculated in vitro with H. influenzae b to compare the 1.5 Microbial Tube and conventional Trypticase soy broth (BBL Microbiology Systems, Cockeysville, Md.) for the detection of low-density H. influenzae b bacteremia. There was a significantly lower recovery of H. influenzae b from blood of bacteremic rats and from in vitro-inoculated human blood by using the 1.5 Microbial Tube than by using Trypticase soy broth. Studies on the mechanism of this lower recovery suggested that the contents of the 1.5 Microbial Tube are toxic to H. influenzae b in the presence of human blood. Additional clinical data are necessary before the 1.5 Microbial Tube can be recommended for detection of low-density H. influenzae b bacteremia.