THE DEMONSTRATION BY RIBOTYPING OF THE STABILITY OF ORAL STREPTOCOCCUS-MUTANS INFECTION OVER 5 TO 7 YEARS IN CHILDREN

The distribution of serotypes and ribotypes of mutans streptococcal is olates obtained from seven unrelated children at 5 and at 10 or 12 yr of age was investigated. For ribotyping, chromosomal DNA from 5 to 13 isolates per subject was digested with restriction endonucleases EcoRI and HindIII. The DNA fragments were electrophoretically separated, bl otted on to nylon membrane and hybridized to the plasmid pKK3535, whic h contains the rRNA operon of the Escherichia coli chromosome. The rib otypes were unique for each child. In five children only one ribotype and serotype (c, e or f) was found. In one child two serotypes (c and f) were found at baseline and only one (serotype c) in the follow-up s ample. In one child the same serotype was not found in the baseline (s erotype e) and in the follow-up (serotype c) samples. Every child exce pt one had a ribotype that was identical to one found 5-7 yr later. Th e results suggest that, at the age of 5 yr, infection by Streptococcus mutans has already stabilized and the colonizing strain remains perma nent.