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Summary: paramount for B. fragilis to maintain a long-term commensal
relationship in the human colon. As B. fragilis is the anaerobic
species most frequently isolated from clinical infections, and the
capsular polysaccharides are instrumental in the disease process,
phase variation may also contribute to the pathogenic potential of
this organism. M
Methods
Construction of insertion mutants
Sequences of primers used in this study are listed in Supplementary Information Table 2.
DNA used for homologous recombination for each of the four mutants was ampli®ed by
PCR from NCTC9343 using the following primers: PSE: U1-F, UpE-R; PSF: U2-F, UpF-R;
PSG: UY5-F, UpG-R; PSH: Y6F2, UpH-R. These 2.1±3.3-kb products were digested with
BamHI and cloned into pJST55 (ref. 9). Plasmids were introduced into B. fragilis
NCTC9343 by mobilization from E. coli and cointegrates were selected using erythro-
mycin.
Demonstration of inversion and PCR/digestion technique
Primers used to demonstrate inversionŠin the order of upstream primer, primer between
the inverted repeats and downstream primerŠare as follows: PSE: UpE-1, UpE-2, UpE-3;
PSF: UpF-1, UpF-2, UpF-3; PSG: UpG-1, UpG-2, UpG-3; PSH: UpH-1, UpH-2, UpH-3.
For the PCR/digestion technique in Fig. 4b, c, a PCR is performed, using chromosomal
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