Assessment of a novel bile solubility test and MALDI-TOF for the differentiation of Streptococcus pneumoniae from other mitis group streptococci

Abstract

This study assesses a novel bile solubility test and MALDI-TOF for the differentiation of Streptococcus pneumoniae from other mitis group streptococci, including differentiation of S. pneumoniae from Streptococcus pseudopneumoniae. Eighty-four species verified mitis group isolates were subjected to our bile solubility test (which measures and calculates the differences of absorbance in the test tube containing 10% sodium deoxycholate versus a blank control tube, after incubation for 10 minutes at 36 °C using a spectrophotometer) and MALDI-TOF MS (both the standard result output and by visual spectra evaluation). Applying a calculated optimal cut-off absorbance-value of 2.1, differentiated S. pneumoniae from all but one other mitis group streptococci (one S. mitis isolate generated an OD-value above 2.1). MALDI-TOF score value identification identified correctly 46 S. pneumoniae and 4 S. pseudopneumoniae but misidentified 16 other mitis group strains. Visual spectra evaluation correctly identified all S. pneumoniae and S. pseudopneumoniae strains but misidentified 13 other mitis group strains. The bile solubility test based on spectrophotometric reading described in this study can differentiate S. pneumoniae from other Streptococcus species. Combining the bile solubility test and the MALDI-TOF spectra results provide a correct identification of all S. pneumoniae and S. pseudopneumoniae isolates.

Identification of Streptococcus pneumoniae and other Mitis streptococci: importance of molecular methods

“…Nevertheless, determining optochin susceptibility and bile solubility remains the key test for phenotypic identification of S. pneumoniae [ 38 ]. In the case of bile solubility, the use of standardized bacterial cultures and measurements of changes in absorbance determined spectrophotometrically rather than by a visual assessment was shown to improve the test performance [ 39 ].…”

Pneumococcal carriage among children under five in Accra, Ghana, five years after the introduction of pneumococcal conjugate vaccine

“…All swab samples suspected to contain S. pneumoniae isolates were transported on dry ice to Statens Serum Institut (SSI), Copenhagen, Denmark for further characterization. At SSI, the organisms were isolated from the swab samples and verified as pneumococcal isolates using phenotypic methods as described in previous studies [2, 19]. Briefly, 10 μl of the swab samples were cultured in serum broth overnight, the following day 1 μl of each serum broth was cultured on 10% horse blood agar plates and incubated overnight at 37 °C, 5% CO2.…”

“…All serum broths were screened for multiple serotypes by using the Pneumotest-latex agglutination kit (SSIDiagnostica, Denmark) [2]. Serotyping/grouping of the isolates was performed using the Pneumotest-latex agglutination kit (SSIDiagnostica, Denmark) and the results were confirmed by the Quellung reaction test using the serotype specific antisera (SSIDiagnostica, Denmark) [19]. Non-typeable strains were defined as isolates presenting no phenotypic detectable capsule.…”

Loop-Mediated Isothermal Amplification Methods for Diagnosis of Bacterial Meningitis

“…The matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) technique has emerged as a clinical microbiology laboratories for the identification of a wide range of pathogens, however, the specificity of MALDI-TOF test for the differentiation of pneumococci from other mitis group streptococci is still low and often recognized false-positives ( 22 ). Moreover, its apparatus is too expensive and complex to use in developing countries.…”