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Journal of Clinical Microbiology, March 2007, p. 881-886, Vol. 45, No. 3
0095-1137/07/$08.00+0     doi:10.1128/JCM.02245-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Frequent In Vitro Recombination in Internal Transcribed Spacers 1 and 2 during Genotyping of Pneumocystis jirovecii

Department of Parasitology, Mycology and Environmental Microbiology, Swedish Institute for Infectious Disease Control, Solna, and Department of Microbiology, Tumor and Cell Biology, Karolinska Institute, Stockholm, Sweden

Received 2 November 2006/ Returned for modification 28 November 2006/ Accepted 21 December 2006

Pneumocystis jirovecii is the causative agent of Pneumocystis pneumonia (PCP) in immunocompromised persons. Knowledge of the transmission and epidemiology of PCP is still incipient, and investigations on these subjects are based exclusively on applications of molecular typing techniques. The polymorphic internal transcribed spacers ITS1 and ITS2 in the ribosomal DNA operon, which in the P. jirovecii genome exist as single-copy DNA, are commonly used as target loci for isolate typing. In the course of genotyping P. jirovecii in respiratory specimens from PCP patients by amplification and cloning of a large number of ITS sequences, we found mixed infections (two or more types) in 50% of the samples. In a majority of the specimens with mixed infections, we detected many ITS haplotypes (combinations of ITS1 and ITS2 types) that appeared to be products of recombination between globally common ITS haplotypes present in the same sample. Here we present results of a series of experiments showing that essentially all ITS recombinants are chimeras formed during the genotyping process. Under standard conditions, as many as 37% of the amplified sequences could be hybrid DNA artifacts. We show that by modifying PCR amplification conditions, ITS chimera formation could be largely abolished and the erroneous establishment of artifactual haplotypes avoided. The accurate assessment of genetic diversity is fundamental for a better understanding of the epidemiology and biology of P. jirovecii infections.

Published ahead of print on 3 January 2007.