medium and 0.035% sodium polyanetholesulfonate as an
anticoagulant.
(ii) Bottles and C02 sensor. A C02 sensor is bonded to the
bottom of each bottle and is separated from the broth
medium by a semipermeable membrane (Fig. 1). The sensor
is impregnated with water vapor when the bottles are autoclaved
during the manufacturing process. The membrane is
impermeable to most ions, including hydrogen ions, and to
components of media and whole as well as degraded blood.
It is nearly impermeable to water but is freely permeable to
C02. Carbon dioxide produced by growing organisms diffuses
across the membrane into the sensor and dissolves in
the water, thereby generating hydrogen ions according to the
following equation:
C02 + HO * H2CO3 *-> H++ HCO3-
Free hydrogen ions can interact with the sensor, which is
blue to dark green in the alkaline state. As C02 is produced
and dissolves in the water, the concentration of hydrogen
ions increases and the pH decreases. This causes the sensor
to become lighter green and eventually yellow, which results
in an increase of red light reflected by the sensor.
(iii) Colorimetric detector and instrument. The prototype
BacT/Alert system is a self-contained incubator (the temperature
can be adjusted between 35 and 37°C ± 0.5°C), shaker,
and detector. Within the instrument are 11 blocks, each of
which contains 48 wells. The blocks, suspended at either
end, rock continuously at a rate of 60 rpm. Each well
contains a colorimetric detector (Fig. 1). The detectors
consist of a red-light-emitting diode and a red-light-absorbing
photodiode. Light emitted from the light-emitting diode is
reflected off the sensor onto the photodiode, which produces
a voltage signal proportional to the intensity of the reflected
light and the concentration of C02 in the bottle. The instrument
scans each well once every 10 min. After amplification
and filtering, voltage signals are digitized and transmitted to
a microcomputer for analysis.
(iv) Computer analysis and detection algorithm. BacT/Alert
tests for C02 production in each bottle 144 times per day.
The data points are plotted as reflectance units versus time
and result in a growth curve. The algorithm for detection of
growth is based on an analysis of the rate of change of C02
concentration in each bottle. Thus, the concentration of C02
1608