Evaluation of an optimal agar medium for detecting hypervirulent Klebsiella pneumoniae using string test

The string test is a screening method for detecting hypervirulent Klebsiella pneumoniae (hvKp). Agar media are used for string tests; however, the effect of the type of media on the test results remains unclear. We aimed to determine the optimal agar medium and cutoff value for the string test. We performed the string test on 99 Klebsiella strains using different agar media: sheep blood, chocolate, Drigalski's and MacConkey. The diagnostic accuracy was calculated in concordance with the rmpA, rmpA2 or iucA gene levels. The diagnostic accuracy rates for sheep blood, chocolate, Drigalski's and MacConkey agar were 0.79, 0.75, 0.73 and 0.64, respectively. When the cutoff was changed from 5 to 10 mm, the diagnostic accuracy rate for sheep blood agar decreased from 0.79 to 0.65. Our findings suggest that the type of agar medium impacts the string test results and sheep blood agar with a 5-mm cutoff is the optimal condition for detecting hvKp.