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Diagnostic Discs

BACITRACIN DISCS

Code: DD0002

For the differentiation of Lancefield Group A from other beta-haemolytic streptococci.

Contents
1 cartridge. containing 50 discs.

Description
Sterile paper discs each containing 0.04 units of bacitracin, for the differentiation of Lancefield Group A from other beta- haemolytic streptococci. When used as a screening test prior to serological grouping, Bacitracin Discs effect considerable economies in time, labour and materials.
Maxted1 showed that Group A streptococci were more sensitive to bacitracin than beta-haemolytic strains of other groups, and that bacitracin might therefore be used as a rapid diagnostic agent for Group A streptoccoci. He found that only 1.7% of 2,386 bacitracin sensitive strains proved not to be Group A, and only 2.5% of 851 resistant strains were Group A. Levinson and Frank2 who employed impregnated paper discs for the same purpose, observed that 93.2% of 866 sensitive beta-haemolytic streptoccoci belonged to Group A. Streamer et al.3 compared bacitracin disc, fluorescent antibody and Lancefield precipitin techniques for the identification of Group A streptococci. The bacitracin disc technique was considered to be the simplest and most practical for the routine clinical laboratory.
The use of bacitracin sensitivity testing is not restricted to the differentiation of beta-haemolytic streptococci. Guthof4 found sensitivity to optochin, bacitracin, and furacin to be a useful test for distinguishing Aerococcus viridans and Streptococcus milleri from enterococci and mitis streptococci.

Technique
Pure Cultures
1
Evenly inoculate the surface of a Blood Agar plate (Blood Agar Base No.2 CM0271 with 7% Sterile Oxalated Horse Blood SR0050) with a pure culture of the beta-haemolytic streptocccus to be tested.
2 Aseptically place a Bacitracin Disc on the inoculated surface.
3 Incubate for 18 to 24 hours at 35° C.
4 Examine for the presence of a zone of inhibition around the Bacitracin Disc. A zone indicates that the streptococcus is presumptively Group A - if desired, further confirmation is obtained by serological grouping.

Clinical Material
1
Inoculate a Blood Agar plate with the throat swab or other material. Streak the inoculum so as to ensure that discrete colonies occur on some portion of the plate - so that the species in a mixed growth may be determined.
2 Aseptically place a Bacitracin Disc on the secondary area of inoculation.
3 Incubate for 18 to 24 hours at 35° C.
4 Examine for zones of inhibition. Organisms other than beta-haemolytic streptococci are sensitive to bacitracin, so that the presence of a zone of inhibition does not necessarily indicate Lancefield Group A streptococci. It is possible to select presumptive Group A streptococci if attention is given to colonial morphology. Further confirmation is obtained by serological grouping.
Bacitracin Discs CT0005 are not recommended for routine testing of the sensitivity of organisms to Bacitracin. For this purpose it is advisable to employ Bacitracin Sensitivity Discs at a level of 10 units.

Quality Control
Use known Group A and non-Group A streptococci to monitor the accuracy of the discs and inoculum.

References
1 Maxted W. R. (1953) J. Clin. Path. 6. 224-226.
2 Levinson M. L. and Frank P. F. (1955) J. Bact. 69. 284-287.
3 Streamer C. W. et al. (1962) Amer. J. Dis. Children 104. 157-160.
4 Guthof O. (1960) Ztschr. f. Hyg. u. Infektionskr. 146. 425-432.

 
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