Sodium amylosulfate (SAS) has been reported to be an effective substance to inactivate the anti-bacterial activity of blood in blood culture media. The advantage of the use of SAS over sodium polyanethol sulfonate (SPS) is that it does not inhibit the growth of some bacterial species which are known to be inhibited by SPS. As to S. typhi, SPS is reported to enhance the growth, however the effect-of SAS on this organism is not known as yet.
Using 43 strains of S. typhi, isolated from clinical materials, the authors tried to determine the effect of SAS on this organism. The methods used for this study were : the SPS and SAS paper disk sensitivity test, tests on the growth in trypticase soy broth (TSB) with SPS and with SAS, and experimental blood culture in SPS and SAS incorporated TSB. The following results were obtained.
1). S. typhi strains with the turbidity of No. 0.5 tube of McFarland nepherometer were inoculated onto Mueller-Hinton plate and 1 mg disk of SPS and SAS were applied. After 24-hour incubation, none of the 43 strains showed inhibition zone by SPS disk, but all of them showed zones by SAS disk with a mean zone diameter of 9.5 mm (Table 1).
2) Inocula consisting of one to 54 viable, counts of 37 strains were inoculated into three different media; TSB with 0.05% SPS, TSB with 0.05% SAS and TSB alone. After 24-hour incubation the .mean of the optical densities of each medium were 0. 483, 0.482 and 0. 459 respectively, showing that SAS does not inhibit the growth of S. typhi. Moreover it was shown that there was no correlation between the amount of inocula and growth (Table 2 and Fig. 1).
3). Each set of media in 5 ml amounts consisting of one tube of TSB with 0. 05% SPS, one tube of TSB with 0. 05% SAS and two tubes of TSB were inoculated with 8, 64. 640 and 6400 viable counts of bacteria. Then 0. 5 ml of fresh normal blood was added to all tubes except for one tube of TSB.
Macroscopic observation after 24 hour incubation showed a heavy growth in all tubes except for the tube of TSB plus blood, which showed only a light growth in the tube of the heaviest inoculums. This result clearly demonstrates that the growth of S. typhi is inhibited by some antibacterial activities of fresh blood, which are counter acted by SPS and SAS (Table 3). Between SPS and SAS, there was no significant difference found (Table 4 and Fig. 2).
With all these results it can be postulated that the addition of SAS into a rountine blood culture media may raise the positivity of S. typhi isolation and shorten the incubation period.
|