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When 24 different aerobic organisms were grown in a shaken culture, all were found to first absorb gas from the headspace. In a rudimentary medium, such as tryptic soy broth, 16 of the 24 organisms did not produce gas following the initial gas absorption. We have developed a simple, noninvasive method for detecting both gas absorption and production in multiple culture vials. The time to positivity was compared with that obtained by the BACTEC 460 blood culture system. For nearly all of these organisms, there was no difference. For some of those organisms that did not produce gas, e.g. Staphylococcus epidermidis, Moraxella osloensis, and Neisseria meningitidis, detection by gas absorption was a few hours faster. Gas absorption appears to be a promising technique for a new automated blood culture system because of its simplicity and because medium without special additives can be used to detect organisms that do not produce gas.

This clinical study was designed to evaluate the standard laboratory protocol that requires blood specimens be diluted with greater than or equal to 10 volumes of media. Blood was collected from hospitalized patients, and 1 ml was inoculated into each of three vials containing 2.3, 7.3, and 24 ml of BACTEC 6B aerobic medium resulting in dilutions of 1:4, 1:10, and 1:30, respectively. The three test vials were treated identically, and the study was carried out at four hospitals. Of the 2,550 sets of vials inoculated, 174 were positive with clinically significant isolates from 105 patients. There was no difference in the number of positive cultures recovered by 24 h (67%) or 48 h (90%) from any dilution. These percentages agreed with other reports from BACTEC users. The number of positive vials (139, 144, 147, respectively) at each dilution was not significantly different, indicating that all three dilutions showed equal recovery of pathogenic microorganisms. Despite this overall equality, two patients, one on antibiotic therapy, were found to have correlated cultures which failed to grow at the 1:4 dilution. This finding implies that a 1:4 dilution of blood cannot be recommended unequivocably despite the higher overall recovery rate of positive cultures.

Decimal serial dilutions of eight common bacterial species were prepared, and the detection times were determined by measuring the (14)CO(2) metabolized from the (14)C-labeled glucose substrate. The detection time was proportional to the logarithm of the initial inoculum, and high sensitivity, down to 1 colonyforming unit, was demonstrated.