During a ten-month period from September 1981 to July 1982 three episodes of pseudobacteremia due to Bacillus species occurred at this 550-bed institution. The first involved eight isolates, the second 11, and the third seven isolates of the organism, all with the same antibiogram.

The patients involved did not exhibit clinical signs of septicemia, and in only one case was more than one specimen per patient positive when multiple blood samples were obtained. Occasional blood cultures of Bacillus species identified in between clusters revealed a different antibiogram.

Extensive epidemiologic investigation of patient locations, phlebotomists, and time of cultures yielded no common source. Components involved in the transport and processing of blood cultures, including the radiometric blood culture processor, were also sampled but without recovery of the organism. After the last episode, a layer of dust was noted inside the machine, and culture of this dust grew Bacillus spp. with the same antibiogram as those found in the blood cultures. The filter from an air conditioning unit in close proximity to the machine grew several species of Bacillus.

It is presumed that Bacillus spores in the dust were introduced into the blood culture bottles following the heat sterilization of the gas sampling (inoculation/removal) needles.

Modification of the cover of the machine was undertaken to prevent access of dust bearing microbes to the inside of the machine. In addition, maintenance now includes regular disinfection/cleaning of the “floor” of the machine, and more frequent changes of the air conditioner filter.