Correlation of growth of aerobic blood cultures in hypertonic broth with antibiotic therapy

Enhanced detection of Candida in blood cultures with the BACTEC 460 system by use of the aerobic-hypertonic (8B) medium

The blood culture records during the 6-year period 1984-9 were reviewed to compare the performance of the BACTEC aerobic (6B) and aerobic-hypertonic (8B) media for the recovery of Candida spp. and Torulopsis glabrata from blood. There were 137 positive blood culture sets that contained both a 6B and an 8B bottle. Sixty-eight different yeasts were recovered from 65 patients including 35 Candida albicans, 19 C. tropicalis, 6 C. parapsilosis, 4 C. krusei, 1 C. pseudotropicalis and 3 Torulopsis glabrata. The 8B medium detected 120 of the positive cultures (87.6%) and was the only positive medium in 35 (25.6%) sets, while the 6B medium detected 102 positive cultures (74.4%) and was the only positive medium in 17 (12.4%) sets (P less than 0.04). For those sets in which both bottles were positive, radiometric detection occurred first in the 8B bottle in 39 sets and first in the 6B bottle in 11 sets (P less than 0.001). The superior performance of the 8B bottle was not related to the administration of amphotericin B. Cultures of stock strains of C. albicans, C. tropicalis, and C. parapsilosis in 6B and 8B media with and without added blood confirmed the finding that 8B was substantially superior to 6B for the detection of candidaemia. It is concluded that an 8B bottle should be included in the blood culture set whenever candidaemia is suspected.

Comparative evaluation of radiometric tryptic soy broth versus radiometric tryptic soy broth with 10% sucrose for detection of bacteremia and fungemia in pediatric patients

We compared BACTEC radiometric blood culture media with (8B) and without (6B) 10% sucrose for the detection of bacteremia and fungemia in pediatric patients at four university teaching hospitals that used identical methods for obtaining and processing specimens. Overall, the yields of microorganisms from 5,714 blood culture sets were no different in the two media, although a trend was noted favoring 6B for the detection of pneumococci. Speed of detection of positive results was faster in the 6B than in the 8B medium (P less than 0.05), largely due to the faster detection of Staphylococcus aureus in the 6B medium. We conclude that, overall, with pediatric patients the hypertonic 8B radiometric medium has no advantage and that it possibly has a modest disadvantage, compared with isotonic 6B radiometric medium.

Effect of antimicrobials on blood cultures in endocarditis

To study the effect of antimicrobials on bacterial growth in blood cultures, we used both simulated blood cultures and cultures obtained from rabbits with experimental endocarditis. Four strains of bacteria were incubated individually with six antimicrobials in nine blood culture media. Positivity rates varied with the ratio of the antimicrobial concentration to the MIC of the organism: 161 of 162 cultures (99%) were positive when the ratio was less than 1/10; 52 of 108 (48%) were positive when the ratio was between 1/10 and one; and none of 54 were positive when the ratio was greater than one. Endocarditis was produced in 28 rabbits with either E. coli, P. aeruginosa, S. aureus, or viridans streptococcus. Following a single dose of an antimicrobial, blood was taken for culture in eight media. Only for viridans streptococcus did recovery rates vary significantly in different media. Recovery rates for this organism in two supplemented peptone broths (78% and 89%) and in hypertonic supplemented peptone (78%) were each higher than in thioglycolate (22%), Columbia (22%), Bactec aerobic and anaerobic (11%), and trypticase soy broths (11%) (p less than 0.05 for each pair). Growth of bacteria in blood cultures containing antimicrobials depended on the ratio of the antimicrobial concentration to the MIC and, for viridans streptococcus, the blood culture medium.

Advantages of BACTEC hypertonic culture medium for detection of Haemophilus influenzae bacteremia in children

The hypertonic and aerobic culture media in the BACTEC system were compared for the detection of Haemophilus influenzae bacteremia in children. Of 1,611 blood cultures, 30 were positive for this pathogen. The aerobic and hypertonic media gave positive results in 28 and 29 cultures, respectively. Within the first 12 h, H. influenzae was detected in the hypertonic medium in 48.5% of the positive cultures as compared to 35% for the aerobic medium. Importantly, after the first 12 h, the hypertonic medium yielded positive results sooner than did the aerobic medium, the difference being statistically significant (P less than 0.01). The hypertonic medium yielded positive results earlier than the aerobic medium in nine cultures; the reverse was seen in only one culture. Furthermore, the aerobic medium gave negative growth index readings despite growth, as shown by microscopy and subculture, in 43% of the total cultures in contrast to only 13% for the hypertonic medium, a significant difference (P less than 0.05). Thus, the present study indicates a distinct advantage of hypertonic medium compared with aerobic medium in the automated BACTEC system for earlier detection of H. influenzae bacteremia and is recommended for those age groups in which this pathogen plays a predominant role.