Characterization of BCG Vaccine by coulter counter and scanning electron microscopic techniques

BCG vaccine, an important immune stimulant used in cancer therapy, consists of a suspension of living and dead cells, cellular fragments and aggregated cells. For this reason, nephelometric methods are unable to determine the total numbers of cells in a vaccine system. Scanning electron microscopy has enabled average cell dimensions to be obtained of length 2.36 μm, width 0.47 μm (n = 1,227), equivalent to a spherical particle of volume 0.3887 μm3 and diameter 0.9055 μm. Measurement of the particle size of a diluted vaccine with a Coulter Multisizer, fitted with a 50 μm diameter orifice, enabled the total number and average size of aggregated cells to be determined. Multiplying the total number by the number of cells in an average aggregate (obtained by dividing the volume of the average aggregate by the average cellular volume) provided a direct estimate of the total number of cells in the system. The total wet weight of cells present in a vaccine ampoule can be estimated by measuring the methylated esters of palmitic acid (PAME) in the system using gas chromatography. This weight, divided by the weight of an average cell, provided an independent confirmation of the total number of organisms present. In order to obtain the cellular density a Percoll gradient sedimentation was carried out. Surprisingly the organisms separated into two fractions of closely similar density, an average value of 1.070 g/mL being obtained. The two fractions had different average sizes with different distribution functions. When data for a number of lots of TiceTM substrain prepared over a twenty‐two year period were compared, the correlation between the Coulter and PAME methods was seen to be relatively good, bearing in mind the intrinsic variability of biological systems. Comparison with viability (colony‐forming unit) measurements demonstrated a loss in viability with time after preparation.