Sequences of Pneumocystis carinii f. sp. hominis strains associated with recurrent pneumonia vary at multiple loci.

The sequences of the internal transcribed spacer (ITS) of Pneumocystis carinii f. sp. hominis strains from 7 of 15 AIDS patients were found to vary during discrete episodes of P. carinii pneumonia. Changes in the ITS sequence correlated with changes in the mitochondrial large-subunit rRNA sequence. The coincidence of changes in the sequences of the ITS, which is located in the nucleus, with changes in a mitochondrial gene excludes mutation as the cause of the genetic differences between P. carinii f. sp. hominis strains isolated during different episodes of P. carinii pneumonia and supports the hypothesis that recurrent P. carinii pneumonia is caused by reinfection rather than by reactivation of latent organisms. Thus, limiting the exposure of immunocompromised patients to P. carinii f. sp. hominis should help prevent P. carinii pneumonia.