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Selection of Distinctive Colony Morphologies for Detection of Multiple Carriage of Streptococcus pneumoniae

2013; Lippincott Williams & Wilkins; Volume: 32; Issue: 6 Linguagem: Inglês

10.1097/inf.0b013e31828692be

1532-0987

Carina Valente, Hermı́nia de Lencastre, Raquel Sá‐Leão,

Bacterial Infections and Vaccines

To the Editors: The introduction of multivalent pneumococcal conjugate vaccines has led to changes in serotype distribution both in carriage and disease.1 To understand the mechanisms leading to such changes, the study of pneumococcal multiple carriage is essential to distinguish between true serotype replacement and unmasking of nonvaccine types.1 Serotyping of multiple colonies is the most straightforward approach to detect multiple carriage but is of little value due to the high cost and effort needed to achieve a satisfactory sensitivity.2 Several initiatives aimed to develop, refine and validate alternative methodologies to detect multiple carriage are ongoing.3 We have recently studied multiple carriage among children in Portugal and how it has been affected by the 7-valent pneumococcal conjugate vaccine.4 We used a highly sensitive DNA capsular microarray that is able to detect and quantify over 90 serotypes. We observed that the distribution of the most abundant serotypes found in multiple carriage was similar to the one observed in single carriage or when only a single colony was studied. Under the scope of a study of pneumococcal competence pherotypes,5 we have retrospectively identified 190 nasopharyngeal samples from which 2 pneumococcal types had been detected based on colony morphology. These samples were extracted from a collection of 3406 samples obtained through cross-sectional studies carried out between 2001 and 2010 among young children. In those studies, 1 colony of each morphology was routinely picked from the agar plate. We have now reviewed the serotype distribution of the isolates present in the samples of the 190 cocolonized children and found that close to one-third of the isolates were either of serotype 3 (13.4%) or nontypeable (18.2%). The third most abundant type, 19F, accounted for 6.1% of the isolates. By contrast, the serotype distribution of the entire collection of 3406 samples revealed that, overall, serotype 3 and nontypeable strains accounted for significantly fewer isolates: 4.9% and 5.3%, respectively (both P values <0.001, χ2 test). Indeed, the 3 most abundant serotypes among the 3406 samples were 19A (9.0%), 19F (8.2%) and 6A (6.6%). Clearly, nontypeable and serotype 3 strains were overrepresented in the collection of cocarried strains selected on the basis of colony morphology. Experienced laboratory researchers know that serotype 3 and nontypeable isolates are easily distinguished on an agar plate from other pneumococci: the former for yielding colonies highly mucous, the latter for displaying minute (rough) colonies. Our observations may not be surprising but they clearly show and provide a quantitative measure on how biased the analysis of serotype distributions can be if multiple carriage is detected based on colony morphology. Taken together, these observations stress that detection of multiple carriage based on colony morphology does not reflect its real epidemiology, not only because it underestimates the true prevalence of these events4 but also because it is prone to detect types that display highly distinctive colony morphologies resulting in their significant overrepresentation. Carina Valente, BSc Raquel Sá-Leão, PhD Instituto de Tecnologia Química e Biológica Universidade Nova de Lisboa Oeiras, Portugal Hermínia de Lencastre, PhD Instituto de Tecnologia Química e Biológica Universidade Nova de Lisboa Oeiras, Portugal The Rockefeller University New York, NY