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A comparison of the performance of bacitracin-incorporated chocolate blood agar with chocolate blood agar plus a bacitracin disk in the isolation of Haemophilus influenzae from sputum

Public Health Laboratory Service (Midlands), Group Headquarters, Royal Shrewsbury Hospital, Mytton Oak Road, Shrewsbury SY3 8XQ and *PHLS Statistics Unit, 61 Colindale Avenue, London NW9 5EQ

Corresponding author: Dr K. J. Nye.

Received 6 Dec. 1999; accepted 8 May 2000.

Abstract

The lack of selectivity of chocolated blood agar (CBA), routinely used for the isolation of Haemophilus influenzae, may lead to masking of the growth of H. influenzae due to overgrowth of competing flora. Bacitracin can be used as a selective agent, either incorporated into the medium or applied to the medium in a filter paper. However, neither method has been evaluated or compared in a large study. Sputum samples (1990) were examined in four laboratories and the isolation rates of H. influenzae on chocolated blood agar with bacitracin added to the medium (BCA) and chocolated blood agar (CBA) with a bacitracin disk were compared. A plain blood agar plate was also inoculated to facilitate the isolation of Streptococcus pneumoniae so that its effects on the isolation of H. influenzae could be assessed. No significant difference was found between the isolation rates of H. influenzae on BCA and CBA with a bacitracin disk, although competing flora was greatly reduced and quantity of growth of H. influenzae increased on BCA. The presence of S. pneumoniae did not affect the isolation of H. influenzae in this study.