Evaluation of culture conditions used for isolation of Chlamydia pneumoniae.

As only a few viable strains of the frequent respiratory pathogen Chlamydia pneumoniae have been isolated worldwide, improvement and standardization of isolation procedures is mandatory for adequate diagnosis and recovery of strains for further investigation. Growth rates of eight C pneumoniae strains were assessed for five host cell lines, for the growth-promoting effect of host cell cytostasis, for infection of adherent or suspended host cells, for optimal first passage incubation time, and for use of multiple blind passages. Cycloheximide-treated HEp-2 or NCI-H 292 monolayers appeared most sensitive for isolation of C pneumoniae. No significant improvement of sensitivity was achieved by extending first passage incubation periods from 3 to 7 days. In contrast, continuous propagation in 4 culture passages yielded on average a 7-fold enhanced chlamydial recovery. Additional comparative isolation of two wild-type strains showed highest inclusion counts in HEp-2 cells, but subculture was necessary for optimal sensitivity. A widely applicable protocol using tissue culture plates for isolation of C pneumoniae is suggested.