What is it about?
Hypervirulent Klebsiella pneumoniae (hvKp) is a particularly dangerous variant of a common bacterium that can cause severe infections including liver abscesses, sepsis, and eye infections — even in otherwise healthy individuals. The string test is a simple bedside screening method to identify hvKp: a bacterial colony is stretched with a plastic loop, and a viscous string formation of 5 mm or more is considered a positive result. Despite its widespread use, it remained unclear whether the choice of agar medium affects the accuracy of the test. We performed string tests on 99 Klebsiella pneumoniae strains — 57 classified as hvKp and 42 as classical K. pneumoniae — using four agar media: sheep blood agar (TSA-SB), chocolate agar (CHO), Drigalski's medium (BTB), and MacConkey agar (MAC). hvKp was defined by the presence of virulence genes rmpA, rmpA2, and iucA. Diagnostic accuracy, sensitivity, and specificity were calculated for each medium, and two cutoff values (5 mm and 10 mm) were compared for TSA-SB. TSA-SB with a 5-mm cutoff showed the highest diagnostic accuracy (0.79), sensitivity (83%), and a specificity of 74%. CHO showed higher specificity (76%) but lower sensitivity (74%). BTB matched TSA-SB in sensitivity but had lower specificity (60%), risking overdiagnosis. MAC showed the lowest accuracy (0.64). Changing the TSA-SB cutoff from 5 mm to 10 mm reduced diagnostic accuracy from 0.79 to 0.65.
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Why is it important?
The string test is widely used in clinical microbiology laboratories because it requires no special reagents or equipment. However, previous studies have used different agar media and cutoff values, making results difficult to compare. Our study is the first to systematically evaluate the effect of agar medium type and cutoff on string test accuracy. The finding that TSA-SB — already commonly used in clinical laboratories — with a 5-mm cutoff is the optimal condition provides practical, evidence-based guidance for laboratories performing hvKp screening.
Perspectives
Clinicians and laboratory scientists should be aware that the choice of agar medium meaningfully affects string test performance. BTB, which is used in some laboratories, showed the lowest specificity, potentially leading to overdiagnosis of hvKp. Standardizing the string test using TSA-SB with a 5-mm cutoff would improve consistency across institutions. It is also important to note that hypervirulence and hypermucoviscosity are distinct phenomena — the string test detects the latter, not the former directly — and should be interpreted alongside virulence gene testing when resources allow. Further studies across different regions and institutions are needed to validate these findings in broader clinical settings.
Dr Naoki Watanabe
Hirosaki University
Read the Original
This page is a summary of: Evaluation of an optimal agar medium for detecting hypervirulent Klebsiella pneumoniae using string test, Access Microbiology, September 2024, Society for General Microbiology,
DOI: 10.1099/acmi.0.000834.v3.
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