{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,30]],"date-time":"2026-09-30T03:57:47Z","timestamp":1790740667263,"version":"4.1.0"},"reference-count":33,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2025,9,2]],"date-time":"2025-09-02T00:00:00Z","timestamp":1756771200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Pathogens"],"abstract":"<jats:p>Background: Mitis group non-pneumococcal streptococci (MGNPS), specifically Streptococcus mitis, Streptococcus infantis and Streptococcus oralis, have recently been shown to cause pneumonia and\/or bacteremia. These organisms often have capsular (cps) operons resembling those in pneumococci, and some express cps-generated polysaccharides that antigenically cross-react with pneumococcal capsular serotypes. But, to date, a series of MGNPS isolates has not been studied by electron microscopy (EM) for the presence of a capsule. Methods: We studied 21 MGNPS; 11 were isolated from sputum and determined to have caused pneumonia, 3 were isolated from blood, and 7 were commensal isolates cultured from the oral cavity of healthy adults. Two reacted with a pneumococcal anticapsular antibody. Isolates were fixed with two different protocols and examined by transmission EM. Results: EM of MGNPS after standard fixation and staining with uranyl acetate did not show capsules. In contrast, the 21 MGNPS isolates that we studied after fixation with ruthenium red and lysine acetate were all shown to be encapsulated. The thickness and density of capsules was related to their species: Streptococcus pneumoniae had the most prominent encapsulation and Streptococcus oralis had the least. However, within a species, there was no apparent difference in capsules between disease-causing and colonizing strains. Conclusions: EM with ruthenium red staining demonstrated capsules on 21 MGNPS, but within a species, there was no apparent difference between disease-causing and commensal isolates. It seems reasonable to conclude that the capsule, together with inoculum size, host\u2019s ability to clear aspirated organisms, and other as yet unidentified virulence factors, all contribute to the pathogenesis of MGNPS pneumonia.<\/jats:p>","DOI":"10.3390\/pathogens14090876","type":"journal-article","created":{"date-parts":[[2025,9,2]],"date-time":"2025-09-02T16:05:22Z","timestamp":1756829122000},"page":"876","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Encapsulation of Disease-Causing and Commensal Mitis Group Non-Pneumococcal Streptococci"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7571-066X","authenticated-orcid":false,"given":"Daniel M.","family":"Musher","sequence":"first","affiliation":[{"name":"Michael E. DeBakey Veterans Affairs Medical Center, Houston, TX 77030, USA"},{"name":"Baylor College of Medicine, Houston, TX 77030, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5624-1378","authenticated-orcid":false,"given":"Mathias","family":"M\u00fcsken","sequence":"additional","affiliation":[{"name":"Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"M. John","family":"Hicks","sequence":"additional","affiliation":[{"name":"Baylor College of Medicine, Houston, TX 77030, USA"},{"name":"Texas Children\u2019s Hospital, Houston, TX 77030, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lesley","family":"McGee","sequence":"additional","affiliation":[{"name":"Centers for Disease Control and Prevention, Atlanta, GA 30333, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5315-1070","authenticated-orcid":false,"given":"Bernard","family":"Beall","sequence":"additional","affiliation":[{"name":"Centers for Disease Control and Prevention, Atlanta, GA 30333, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2025,9,2]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"e01985-19","DOI":"10.1128\/mBio.01985-19","article-title":"Identification of Virulence-Associated Properties by Comparative Genome Analysis of Streptococcus pneumoniae, S. pseudopneumoniae, S. mitis, Three S. oralis Subspecies, and S. infantis","volume":"10","author":"Kilian","year":"2019","journal-title":"mBio"},{"key":"ref_2","unstructured":"Heffron, R. (1939). Pneumonia with Special Reference to Pneumococcus Lobar Pneumonia, The Commonwealth Fund."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"913","DOI":"10.1093\/clinids\/17.5.913","article-title":"A brief history of the pneumococcus in biomedical research: A panoply of scientific discovery","volume":"17","author":"Watson","year":"1993","journal-title":"Clin. Infect. Dis."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Denapaite, D., Bruckner, R., Nuhn, M., Reichmann, P., Henrich, B., Maurer, P., Schahle, Y., Selbmann, P., Zimmermann, W., and Wambutt, R. (2010). The genome of Streptococcus mitis B6\u2014What is a commensal?. PLoS ONE, 5.","DOI":"10.1371\/journal.pone.0009426"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"89","DOI":"10.1111\/j.2041-1014.2010.00601.x","article-title":"Streptococcus mitis: Walking the line between commensalism and pathogenesis","volume":"26","author":"Mitchell","year":"2011","journal-title":"Mol. Oral Microbiol."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"160","DOI":"10.1128\/JCM.44.1.160-165.2006","article-title":"Viridans streptococci isolated by culture from blood of cancer patients: Clinical and microbiologic analysis of 50 cases","volume":"44","author":"Han","year":"2006","journal-title":"J. Clin. Microbiol."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"256","DOI":"10.1016\/0002-9343(94)90009-4","article-title":"Bacteremia due to viridans streptococci in neutropenic patients: A review","volume":"97","author":"Bochud","year":"1994","journal-title":"Am. J. Med."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"762","DOI":"10.3201\/eid2005.130953","article-title":"Streptococcus mitis strains causing severe clinical disease in cancer patients","volume":"20","author":"Shelburne","year":"2014","journal-title":"Emerg. Infect. Dis."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"ofae063","DOI":"10.1093\/ofid\/ofae063","article-title":"Risk of Infective Endocarditis in Streptococcus mitis Bloodstream Infections Among Patients with Neutropenia from Hematologic Malignancies","volume":"11","author":"Monk","year":"2024","journal-title":"Open Forum Infect. Dis."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"ofaa307","DOI":"10.1093\/ofid\/ofaa307","article-title":"Normal Respiratory Flora as a Cause of Community-Acquired Pneumonia","volume":"7","author":"Musher","year":"2020","journal-title":"Open Forum Infect. Dis."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"3790","DOI":"10.1128\/IAI.01840-14","article-title":"Protective role of the capsule and impact of serotype 4 switching on Streptococcus mitis","volume":"82","author":"Rukke","year":"2014","journal-title":"Infect. Immun."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"e01844-16","DOI":"10.1128\/mBio.01844-16","article-title":"Capsular Polysaccharide Expression in Commensal Streptococcus Species: Genetic and Antigenic Similarities to Streptococcus pneumoniae","volume":"7","author":"Yao","year":"2016","journal-title":"mBio"},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Lessa, F.C., Milucky, J., Rouphael, N.G., Bennett, N.M., Talbot, H.K., Harrison, L.H., Farley, M.M., Walston, J., Pimenta, F., and Gertz, R.E. (2018). Streptococcus mitis Expressing Pneumococcal Serotype 1 Capsule. Sci. Rep., 8.","DOI":"10.1038\/s41598-018-35921-3"},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Pimenta, F., Gertz, R.E., Park, S.H., Kim, E., Moura, I., Milucky, J., Rouphael, N., Farley, M.M., Harrison, L.H., and Bennett, N.M. (2018). Streptococcus infantis, Streptococcus mitis, and Streptococcus oralis strains with highly similar cps5 loci and antigenic relatedness to serotype 5 pneumococci. Front. Microbiol., 9.","DOI":"10.3389\/fmicb.2018.03199"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"e01037-21","DOI":"10.1128\/mBio.01037-21","article-title":"Nonpneumococcal strains recently recovered from carriage specimens and expressing capsular serotypes highly related or Identical to pneumococcal serotypes 2, 4, 9A, 13, and 23A","volume":"12","author":"Gertz","year":"2021","journal-title":"mBio"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1720","DOI":"10.3201\/eid2808.212176","article-title":"Invasive Streptococcus oralis expressing serotype 3 pneumococcal capsule, Japan","volume":"28","author":"Chang","year":"2022","journal-title":"Emerg. Infect. Dis."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"95","DOI":"10.1111\/j.2041-1014.2011.00635.x","article-title":"Identification of a functional capsule locus in Streptococcus mitis","volume":"27","author":"Rukke","year":"2012","journal-title":"Mol. Oral Microbiol."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"1070","DOI":"10.1021\/acsnano.9b08631","article-title":"In Vivo Relationship between the Nano-Biomechanical Properties of Streptococcal Polysaccharide Capsules and Virulence Phenotype","volume":"14","author":"Marshall","year":"2020","journal-title":"ACS Nano"},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Ganaie, F.A., Beall, B.W., Yu, J., van der Linden, M., McGee, L., Satzke, C., Manna, S., Lo, S.W., Bentley, S.D., and Ravenscroft, N. (2025). Update on the evolving landscape of pneumococcal capsule types: New discoveries and way forward. Clin. Microbiol. Rev., 38.","DOI":"10.1128\/cmr.00175-24"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"13","DOI":"10.1007\/978-1-4939-9199-0_2","article-title":"Electron Microscopy to Study the Fine Structure of the Pneumococcal Cell","volume":"1968","author":"Hammerschmidt","year":"2019","journal-title":"Methods Mol. Biol."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"60.e9","DOI":"10.1016\/j.cmi.2015.08.027","article-title":"Strain features and distributions in pneumococci from children with invasive disease before and after 13-valent conjugate vaccine implementation in the USA","volume":"22","author":"Metcalf","year":"2016","journal-title":"Clin. Microbiol. Infect."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"e3571","DOI":"10.7717\/peerj.3571","article-title":"Clinical streptococcal isolates, distinct from Streptococcus pneumoniae, but containing the beta-glucosyltransferase tts gene and expressing serotype 37 capsular polysaccharide","volume":"5","author":"Sheppard","year":"2017","journal-title":"PeerJ"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"563","DOI":"10.1038\/s41564-018-0151-5","article-title":"A high-frequency phenotypic switch links bacterial virulence and environmental survival in Acinetobacter baumannii","volume":"3","author":"Chin","year":"2018","journal-title":"Nat. Microbiol."},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Siber, G.R., Klugman, K.P., and Makela, P. (2008). The history of pneumococcal disease. Pneumococcal Vaccines: The Impact of Conjugate Vaccine, ASM Press.","DOI":"10.1128\/9781555815820"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"e97","DOI":"10.7717\/peerj.97","article-title":"Non-pneumococcal mitis-group streptococci confound detection of pneumococcal capsular serotype-specific loci in upper respiratory tract","volume":"1","author":"Pimenta","year":"2013","journal-title":"PeerJ"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"923","DOI":"10.1093\/infdis\/142.6.923","article-title":"Epidemiologic studies of Streptococcus pneumoniae in infants: Acquisition, carriage, and infection during the first 24 months of life","volume":"142","author":"Gray","year":"1980","journal-title":"J. Infect. Dis."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"441","DOI":"10.1093\/clinids\/24.3.441","article-title":"Emergence of antibody to capsular polysaaccharides of Streptococcus pneumoniae during outbreaks of pneumonia: Association with nasopharyngeal colonization","volume":"24","author":"Musher","year":"1997","journal-title":"Clin. Infect. Dis."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"e02235-18","DOI":"10.1128\/AEM.02235-18","article-title":"Intranasal Immunization with the Commensal Streptococcus mitis Confers Protective Immunity against Pneumococcal Lung Infection","volume":"85","author":"Shekhar","year":"2019","journal-title":"Appl. Environ. Microbiol."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"416","DOI":"10.1165\/ajrcmb\/5.5.416","article-title":"Pneumolysin induces the salient histologic features of pneumococcal infection in the rat lung in vivo","volume":"5","author":"Feldman","year":"1991","journal-title":"Am. J. Respir. Cell Mol. Biol."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"5683","DOI":"10.1128\/iai.62.12.5683-5688.1994","article-title":"Immunization of mice with pneumolysin toxoid confers a significant degree of protection against at least nine serotypes of Streptococcus pneumoniae","volume":"62","author":"Alexander","year":"1994","journal-title":"Infect. Immun."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"7812","DOI":"10.1038\/s41467-024-52120-z","article-title":"Population genomics of Streptococcus mitis in UK and Ireland bloodstream infection and infective endocarditis cases","volume":"15","author":"Richard","year":"2024","journal-title":"Nat. Commun."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"210","DOI":"10.1097\/00005792-200007000-00002","article-title":"Bacteremic and nonbacteremic pneumococcal pneumonia. A prospective study","volume":"79","author":"Musher","year":"2000","journal-title":"Medicine"},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"2076","DOI":"10.1128\/IAI.01554-08","article-title":"Protection against Pneumococcal colonization and fatal pneumonia by a trivalent conjugate of a fusion protein with the cell wall polysaccharide","volume":"77","author":"Lu","year":"2009","journal-title":"Infect. Immun."}],"container-title":["Pathogens"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2076-0817\/14\/9\/876\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,9]],"date-time":"2025-10-09T18:38:20Z","timestamp":1760035100000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2076-0817\/14\/9\/876"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2025,9,2]]},"references-count":33,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2025,9]]}},"alternative-id":["pathogens14090876"],"URL":"https:\/\/doi.org\/10.3390\/pathogens14090876","relation":{},"ISSN":["2076-0817"],"issn-type":[{"value":"2076-0817","type":"electronic"}],"subject":[],"published":{"date-parts":[[2025,9,2]]}}}