Poor performance of BACTEC NR 730 blood culture system in early detection of Neisseria meningitidis

Factors affecting detection of Brucella melitensis by BACTEC NR730, a nonradiometric system for hemocultures

The detection of Brucella bacteremia by subculture does not always correlate with a positive signal in the BACTEC NR730 nonradiometric system (Becton Dickinson Diagnostic Instrument Systems, Sparks, Md.). The effect of the inoculum size, pH, sodium polyanetholesulfonate, carbon sources (i-erythritol, sodium pyruvate, monosodium glutamate, D-glucose, and L-alanine), and urea in the release of CO2 was evaluated by using the reference strain Brucella melitensis 16M. In standard NR6 vials with or without blood, inocula 5 to 10 times larger (at least 265 CFU per vial) than those usually found in the blood of patients with brucellosis were necessary to produce a positive growth value (GV) in 4 days or less, and similar results were obtained with vials supplemented with the substrates listed above. GVs were consistently lower in vials with sodium polyanetholesulfonate than in vials without this agent. Vials with no blood inoculated with 265 CFU per vial showed turbidity 1 day before GVs became positive, proving that the major limiting detection factor was the low level of release of CO2 and not an inadequate growth medium. In NR6 vials buffered to pH 6.2, GVs became positive faster and were higher than those in standard vials. NR6 vials at pH 6.2 with 0.3% sodium pyruvate yielded a positive GV in the first day of bacterial turbidity.

Assessment of gelatin-supplemented BACTEC blood culture medium in a pediatric hospital

Sodium polyanetheolesulfonate (SPS), an anticoagulant used in blood culture media, adversely affects the isolation of Neisseria meningitidis. The addition of gelatin appears to counteract this effect. Studies using the radiometric BACTEC system, however, have noted a lower isolation rate of other bacteria from gelatin-supplemented media. We wished to evaluate the effect of the addition of gelatin (1.2%) to a nonradiometric BACTEC aerobic medium (NR6A) on the recovery of N. meningitidis and other pathogens. The NR6A medium with gelatin (NR6A analogue) also contained a lower concentration of SPS (0.025% vs 0.035%). We did 6045 paired comparisons of blood cultured in routine NR6A medium and the NR6A analogue. Eight isolates of N. meningitidis were recovered, five only from the gelatin-supplemented medium and three from both bottles. There was no statistically significant difference in total recovery of aerobic and facultative bacteria or Candida species from both bottles. Haemophilus influenzae was detected earlier in the nonsupplemented NR6A medium. We conclude that the use of the NR6A analogue medium appeared to increase the yield of N. meningitidis without adversely affecting the recovery of other common pathogens, although the recovery of H. influenzae was slightly delayed.