Hepatitis B virus genomic heterogeneity: variation between quasispecies may confound molecular epidemiological analyses of transmission incidents

Nucleotide sequence variability studies were conducted on a 263-base pair fragment of the core-coding genomic region of hepatitis B'virus (HBV), amplified by the polymerase chain reaction (PCR) from three surgeons with varying circulating levels of HBV, all of whom were thought to have transmitted HBV to their patients post-surgically, DNA sequencing was applied to amplicons obtained directly from serum and those cloned into plasmid vectors, and from single HBV molecules in serum separated by a limiting dilution procedure. In one surgeon, who had a titre of similar to 3 x 10(5) genome equivalents ml(-1), the direct sequence was identical to none of 29 other sequences and differed by one base substitution from the sequence amplified from the single patient he infected. In another surgeon, who had a titre of similar to 2 x 10(6) genome equivalents ml(-1), the direct sequence was identical to 17 of 36 (47%) sequences; however, the sequence common to all three infected patients was identical to a unique sequence in the surgeon that differed by three base substitutions from the direct sequence. By contrast, the direct sequence in the third surgeon, who had a titre of similar to 4 x 10(7) genome equivalents ml(-1), was identical to 25 of 38 (66%) sequences, and to the sequence common to all 11 infected patients. Assessment of HBV DNA sequences directly amplified from clinical specimens may not be appropriate to studies of transmission in which the source of infection harbours a relatively dilute,heterogenous mix of viral variants.