PP 6 - Evaluation of an In-House Gentamicin Blood Agar as a Selective Medium for Isolation of Streptococcus Species from Clinical Specimens in a Tertiary Hospital, Sri Lankarom clinical specimens in a tertiary care hospital, Sri Lanka

Keeragala KADCK¹, Dissanayake DMBT², Chandrasiri NS³ , ¹Post Graduate Institute of Medicine, University of Colombo, ²Department of Microbiology, Faculty of Medical Sciences, University of Sri Jayewardenepura,  ³Colombo South Teaching Hospital, Kalubowila

Abstract

Introduction

Streptococci are nutritionally fastidious, catalase-negative, Gram-positive bacteria of major clinical importance. Selective media facilitate their isolation and identification, enabling accurate diagnosis and treatment. However, routine use of commercially available selective media (Columbia CAN agar) is costly. Gentamicin blood agar (GBA) is an established selective medium globally, and its local preparation may provide a cost-effective alternative.

Objectives

To assess the effectiveness of an in-house GBA for isolation of streptococci and compare its performance with conventional blood agar.

Design, setting and methods

This descriptive cross-sectional study was conducted from November 2024 to March 2025 at a tertiary hospital. A total of 147 clinical specimens, including wound swabs, high vaginal swabs, tissues and pus aspirates, were processed. Samples were inoculated onto sheep blood agar, chocolate agar and in-house GBA and incubated under standard conditions. GBA was prepared using 10% sheep blood supplemented with gentamicin (5 µg/ml). The medium was validated using Streptococcus pneumoniae ATCC®️ 6305 and a clinical strain of Streptococcus pyogenes as positive controls, and Escherichia coli ATCC®️ 25922 and Staphylococcus aureus ATCC®️ 25923 as negative controls. Presumptive identification was based on colony morphology, biochemical tests and Lancefield grouping.

Results

Streptococci were recovered from all positive specimens on both BA and GBA, demonstrating a 100% recovery rate. Of the 147 specimens, 131 (89.1%) yielded Lancefield-groupable streptococci. Among the identified isolates, Group B Streptococcus predominated (102/131; 77.9%), followed by Group A Streptococcus (24/131; 18.3%), Groups G and D (2/131 each; 1.5%), and Group C Streptococcus (1/131; 0.8%). Conventional BA yielded 33 contaminant isolates, predominantly Staphylococcus spp. (12/33; 36.4%) and coliform organisms (12/33; 36.4%), followed by yeasts (6/33; 18.2%), diphtheroids (2/33; 6.1%), and enterococci (1/33; 3.0%). GBA effectively suppressed the growth of susceptible bacterial contaminants, particularly staphylococci, coliform organisms and diphtheroids, while yeast growth was not consistently inhibited, thereby improving the selectivity of streptococcal isolation.

Conclusion

The in-house gentamicin blood agar demonstrated excellent performance as a selective medium, maintaining complete recovery of streptococci while reducing bacterial contaminants commonly encountered on conventional blood agar. The predominance of Group B Streptococcus and the improved selectivity of GBA support its use as a simple medium for the routine isolation of clinically significant streptococci in diagnostic microbiology laboratories.