Spread and persistence of infection with a trachoma biovar strain of Chlamydia trachomatis in multiplying and nonmultiplying McCoy cells.

The trachoma biovar of Chlamydia trachomatis enters host cells in culture with difficulty, and cell-to-cell spread resulting in amplification of the initial growth usually does not occur. An experimental model was devised to demonstrate that, by using conditions that more closely approximate those encountered in man, the trachoma biovar of C. trachomatis can readily achieve cell-to-cell passage. Fresh McCoy cells were sequentially added to monolayers that had been inoculated with a trachoma biovar strain of C. trachomatis 3, 6, and 9 days earlier. Subsequent incubation resulted in intercellular propagation, with an increase in the number of inclusions from 500 to 80,000 per coverslip. A second experiment demonstrated the reproducibility of this phenomenon and showed that cell-to-cell spread can occur at a low rate in overcrowded, not overlaid, cell layers; it also showed that, in multiplying cell layers, the infection tends to become persistent.