Identification of natural pneumococcal isolates expressing serotype 6D by genetic, biochemical and serological characterization

Abstract

The recently discovered pneumococcal serotype 6C was created when the original wciN gene in the 6A capsule gene locus was naturally replaced with a new gene. Since the capsule gene loci of 6A and 6B serotypes may differ by only one base pair in the wciP gene, it was speculated that a new serotype '6D' would be possible if the new wciN gene were inserted into the 6B capsule gene locus. Although pneumococci expressing serotype 6D could be produced in the laboratory, initial searches for natural pneumococcal isolates expressing serotype 6D were unsuccessful. However, we now report the discovery of two naturally occurring pneumococcal isolates from Korea that have the serological, genetic and biochemical features predicted for serotype 6D.

Fig. 1.

Flow cytometry profiles of various pneumococcal isolates stained with mAbs. The mAbs used were Hyp6AG1 (thin black line), Hyp6AM3 (dashed line), Hyp6BM8 (thick black line), and Hyp6BM7 (thin grey line). Studies with controls showed the staining threshold to be about 20 fluorescence units (data not shown).

Fig. 2.

Binding of mAbs (y-axis) to 6B PS-coated ELISA plates in the presence of varying amounts of bacterial lysates (x-axis). The mAbs used were Hyp6BM8 (a) and Hyp6BM7 (b). The bacterial isolates studied are listed in the figure key.

Fig. 3.

Amount of antigenicity (y-axis) remaining after alkali hydrolysis for different time periods (x-axis). The PSs studied are listed in the figure key.

Fig. 4.

GLC profile of serotype 6B PS (a) and the PS purified from a pneumococcal isolate MNZ21 (b). The GLC instrument used here identified two peaks for rhamnose, two peaks for galactose and two peaks for glucose (Dr D. Pritchard, personal communication); these peaks are labelled in (a) along with the ribitol peak. The elution times (in minutes) for the labelled peaks (from left to right) are: (a) 10.325 (for ribitol), 10.692 and 10.805 (for rhamnose), 12.219 and 12.414 (for galactose), and 12.585 and 12.585 (for glucose); (b) 10.323, 10.690, 10.803, 12.582 and 12.655.