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Updated: Jul 5, 2026
Capsular Serotyping of Streptococcus pneumoniae by Latex Agglutination
Published on: September 25, 2014
Accuracy of phenotypic methods for identification of Streptococcus pneumoniae isolates included in surveillance
Sandra S Richter1, Kristopher P Heilmann, Cassie L Dohrn
1Department of Pathology, University of Iowa Carver College of Medicine, Iowa City, Iowa 52242-1009, USA.
Abstract:
Similarities between Streptococcus pneumoniae and viridans group streptococci may result in misidentification of these organisms. In surveillance programs which assess antimicrobial resistance rates among respiratory tract pathogens, such identification errors could lead to overestimates of pneumococcal resistance rates. DNA probe analysis (Gen-Probe, San Diego, CA), the bile solubility test, optochin susceptibility, colony morphology, and the capsular swelling reaction with Omni serum (Staten Serum Institut, Copenhagen, Denmark) were used to characterize 1,733 organisms provisionally identified as S. pneumoniae in a 2004 to 2005 antimicrobial resistance surveillance program. These organisms were obtained in 41 U.S. medical centers. Among these, 1,647 (95%) were determined to be S. pneumoniae by DNA probe. Elimination of those isolates found not to be S. pneumoniae resulted in 1 to 2% decreases in resistance rate estimates with penicillin, erythromycin, tetracycline, and trimethoprim-sulfamethoxazole. With AccuProbe as a reference standard, the sensitivities and specificities of each phenotypic method for the identification of S. pneumoniae were, respectively, 98.8% and 82.6% for bile solubility, 99.3% and 74.4% for the capsular swelling reaction with Omni serum, and 87.9% and 59.3% for optochin susceptibility. Colony morphology was of limited value, as 391 (23.7%) isolates lacked the typical button or mucoid colony appearance of S. pneumoniae.
Insights
Misidentifying Streptococcus pneumoniae can inflate antimicrobial resistance rates. Accurate identification using DNA probes is crucial, leading to revised resistance estimates for key antibiotics.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Resistance
Background:
- Streptococcus pneumoniae shares similarities with viridans group streptococci, leading to potential misidentification.
- Misidentification in surveillance programs can overestimate antimicrobial resistance rates for pneumococci.
Purpose of the Study:
- To evaluate the accuracy of phenotypic identification methods for Streptococcus pneumoniae compared to DNA probe analysis.
- To assess the impact of identification errors on antimicrobial resistance rate estimations.
Main Methods:
- 1,733 isolates provisionally identified as S. pneumoniae were analyzed using DNA probe analysis (AccuProbe) and phenotypic tests.
- Phenotypic methods included bile solubility, optochin susceptibility, colony morphology, and capsular swelling reaction.
- Antimicrobial resistance rates were calculated before and after excluding misidentified isolates.
Main Results:
- DNA probe analysis confirmed 95% (1,647/1,733) of isolates as S. pneumoniae.
- Excluding misidentified isolates led to 1-2% decreases in resistance rates for penicillin, erythromycin, tetracycline, and trimethoprim-sulfamethoxazole.
- Bile solubility (98.8% sensitivity, 82.6% specificity) and capsular swelling (99.3% sensitivity, 74.4% specificity) showed higher accuracy than optochin susceptibility (87.9% sensitivity, 59.3% specificity).
- Colony morphology was unreliable, with 23.7% lacking typical features.
Conclusions:
- DNA probe analysis is a reliable method for accurate S. pneumoniae identification.
- Phenotypic methods like bile solubility and capsular swelling offer good, but not perfect, accuracy.
- Accurate identification is essential for reliable antimicrobial resistance surveillance data.
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