Nasopharyngeal pneumococcal isolate 307.14 is composed of encapsulated and nonencapsulated variants and the phenotype is transferred with the capsule operon. Strain 307.14 was cultured in CDM with 5.5 mM glucose and viewed by (A) fluorescence microscopy in the presence of FITC-dextran (100× objective). The black arrow indicates pneumococci expressing a large amount of capsule, the white arrow indicates pneumococci expressing no capsule. The two phenotypes were subcultured separately and viewed by transmission electron microscopy (EM). (B) 307.14 encapsulated pneumococci expressing a thick polysaccharide capsule (indicated by the black arrow), (C) 307.14 nonencapsulated pneumococci expressing no visible capsule. Capsule switch mutants were created in which the capsule operons of the two phenotypes were exchanged. (D) “307.14 nonencapsulated” in which the operon has been replaced with that of “307.14 encapsulated” (creating mutant 307.14 cap+), expressing a thin capsule (marked with a black arrow). (E) “307.14 encapsulated” in which the operon has been replaced with that of “307.14 nonencapsulated” (creating mutant 307.14 cap-), expressing no detectable capsule. EM pictures (B) to (E) were taken at a magnification of 53 000×.