Journal of Postgraduate Medicine
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Year : 1981  |  Volume : 27  |  Issue : 2  |  Page : 86-9  

The bacitracin sensitivity test for identifying beta-haemolytic streptococci of Lancefield group A.

AD Mondkar, SS Kelkar 

Correspondence Address:
A D Mondkar

How to cite this article:
Mondkar A D, Kelkar S S. The bacitracin sensitivity test for identifying beta-haemolytic streptococci of Lancefield group A. J Postgrad Med 1981;27:86-9

How to cite this URL:
Mondkar A D, Kelkar S S. The bacitracin sensitivity test for identifying beta-haemolytic streptococci of Lancefield group A. J Postgrad Med [serial online] 1981 [cited 2023 Mar 29 ];27:86-9
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Streptococcal diseases have shown a changing pattern. The serious morbidity of the pre-antibiotic era is now largely controlled and sometimes even forgotten. However, streptococci continue to be important in our environment and poststreptococcal diseases are a common cause of morbidity and mortality.[4], [9] Betahaemolytic streptococci are a diverse group of organisms. Lancefield's group A is the frequent human pathogen that causes classical acute suppurative spreading infections and sequelae (post-streptococcal diseases). The conventional Lancefield grouping is both laborious and slow and therefore need to evolve and evaluate simpler tests. Maxted[7] introduced sensitivity to the antibiotic bacitracin as a presumptive test for the identification of Lancefield group A streptococci. A few subsequent studies[2], [5] to have described refinements of this test-variations in concentration of bacitracin and the diameters of the zones of inhibition obtained-in an endeavour to make it both sensitive and specific, and there is some controversy. We describe here our attempts at evaluating the bacitracin sensitivity test in this context.



A total of 135 strains of beta haemolytic streptococci isolated from patients at the Sir J.J. Group of Hospitals, Bombay, were studied. The isolations were from diverse sites and sources as follows: throat, 95; sputum, 18; urine, 11; pus, seven; urethral scrapings, two and one each from the cerebrospinal fluid and the blood.

Lancefield-grouping of isolates

This was done by the counterelectrophoresis technique according to Mondkar and Kelkar.[8] The sera for grouping were obtained from Decruz Corporation and included those of Lancefield's groups A, B, C, D, F and G. This was taken as an accurate assessment of the actual group of an isolate.

Bacitracin sensitivity test

Discs containing three concentrations of Bacitracin, 0.025, 0.7 and 2.5 IU were prepared as suggested by Frankel et al.[3] Each strain was tested against each of these discs according to the method of Bauer et all and the resistance or sensitivity noted. The diameters of the zones of inhibition obtained were carefully recorded.


Serological grouping of the 135 strains by counterelectrophoresis showed that 54 belonged to group A, 36 to groups other than A, and that 45 strains were not groupable with the sera used. The 36 strains of groups other than A comprised of six strains of group B, eight of group C, one of group D, 15 of group F and six of group G. The results of Bacitracin sensitivity were read in two different fashions. Firstly, according to the standard Kirby Bauer method[1] a zone of inhibition with a diameter of 14 mm was taken as the critical level (Sl or RI depending, on whether it was more or less than this value). Secondly, any zone of inhibition at all around the disc was taken to indicate sensitivity (S2) and growth right upto the disc as indicative of resistance (R2). [Table 1] gives the details of the resistance of 54 group A strains tested against the three concentrations of Bacitracin and read as both Rl and R2. These figures indicate the false negatives obtained in this test with respect to group A strains. Similarly the table gives the sensitivity of non-group A strains of streptococci and the results indicate the false positives obtained. The total thus obtained is the total of false results (false positive + false negative) of the Bacitracin sensitivity test with each of the variations attempted.


The study, as designed, was meant to solve the problem of the clinical situation and define the accuracy of the Bacitracin sensitivity test in picking out group A streptococci. The results clearly indicate that the concentration of the antibiotic as well as the method of reading the results as sensitive or resistant make a lot of difference with respect to the results obtained. Also there is no accurate and clear cut-off point between the Bacitracin sensitivity and the status of the strain. Maxted[7] who devised the test was of the opinion that S2 and R2 should be used in interpreting it. Our results show that the Bacitracin sensitivity test is reasonably accurate in identifying clinical isolates of beta haemolytic streptococci. The best results were obtained with discs having a concentration of 0.025 IU of the drug, the results being interpreted as advocated by Maxted.[7] The total of false results obtained in this fashion were 16.3 per cent. However other combinations (see table) also gave almost similar results. In this context the WHO[6] had advocated a test in which bacitracin was used in a concentration of 0.7 IU and sensitivity or resistance was read with reference to a zone of inhibition of a diameter of 14 mm. Our results indicate that this method is slightly inferior (17 per cent false results).

The world literature gives a lot of conflicting information on the reliability of this test in delineating group A streptococci. Levinson and Frank[5] made use of bacitracin discs containing 1 IU per ml of bacitracin. With these discs, 807 of 819 strains of group A were sensitive. At the same time this concentration of bacitracin also inhibited 7 per cent of non-group A strains. Sharma and Bhatia[10] from Delhi used discs containing 5 IU of bacitracin. With this concentration, 94 per cent of the group A and 91.4 per cent of the nongroup A strains were resistant to bacitracin. With higher concentrations (5 IU per ml), 20 per cent of the non-group A strains were sensitive. Recently, Coleman and others[2] attempted to classify 125 group A and 122 non-group A streptococci using bacitracin discs containing 0.02, 0.04 and 0.1 IU of bacitracin. They observed that discs containing 0.04 IU of bacitracin were useful in routine screening of group A streptococci. With discs containing 0.1 IU of bacitracin, a specific zone size had to be determined for reading as sensitive or resistant.

Our results tally with those of Coleman and others.[2] We advocate use of bacitracin discs containing 0.025 IU of bacitracin with any zone of inhibition at all around the disc, for the presumptive identification of group A streptococci.


1Bauer, A. W., Kirby, W. M. M., Sherris, J. C. and Turck, M.: Antibiotic susceptibility testing by standardized single disc method. Amer. J. Clin. Path., 45: 493-501, 1966.
2Coleman, D. J., McGhie, D. and Tebutt, G. M.: Further studies on the reliability of the bacitracin inhibition test for the presumptive identification of Lancefiled group A streptococci. J. Clin. Path,, 30: 421-426, 1977.
3Frankel, S., Reitman, S. and Sonnenwirth, A. C.: In "Gradwohl's Clinical Laboratory Methods and Diagnosis." Vol. 2. Seventh edition. Published by The C. V. Mosby Company, 1970, pp. 1110-1111.
4Koshi, G. Jadhav, M. and Myers, R. M.: Streptococcal pharyngitis in children. Ind, J. Med. Res., 58: 161-167, 1970.
5Levinson, M. L. and Frank, P. F.: Differentiation of group A streptococci from other beta haemolytic streptococci, with bacitracin J. Bact. 69: 284-287, 1955.
6Manual WHO: "Manual of Reference Procedures in Streptococcal Bacteriology and Serology." Edited by Rotta, J. Published by WHO South-East Asia Regional Office (SEARO), New Delhi, 1976.
7Maxted, W. R.: The use of bacitracin for identifying group A streptococci. J. Clin. Path., 6: 224-226, 1953.
8Mondkar, A. D. and Kelkar, S. S.: Counterimmunoelectrophoresis for Lancefield grouping of streptococi. Ind. J. Med. Res., 68: 16-20, 1978.
9Prakash, K.; Chawda, S., Amma, B. P and Sharma, K. B.: Distribution of groups and types of beta haemolytic streptococci in cases of rheumatic heart disease and apparently healthy school children. Ind. J. Path. & Bact., 16: 5-14, 1973.
10Sharma, K. B. and Bhatia, S. L.: Studies on beta haemolytic streptococci isolated in Delhi. Ind. J. Med. Res., 54: 517-523, 1966.

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