Division of single host cells after infection with chlamydiae

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Abstract
Mouse fibroblasts (L cells) were infected in suspension with Chlamydia psittaci (6BC) and then plated out on a solid substrate at a density of 80 cells/cm2 so that the effect of chlamydial infection on the division of single host cells and their progeny could be determined. Uninfected L cells multiplied with a mean generation time of 15 h. The generation time of single L cells infected with 1.5 50% infectious units (ID50) of C. psittaci was over twice as long. Half of the infected L cells divided once by day 4 after infection and the rest divided more than once. Division of infected cells frequently produced 1 infected and 1 uninfected daughter. About half of the L cells infected with 15 ID50 of C. psittaci divided at least once before most of them detached from their substrate before observation on day 3. Less than 10% of the L cells infected with 75 ID50 of C. psittaci divided before they were lost from their substrate by day 2. Comparable results were obtained with single L cells infected with a lymphogranuloma venereum (440L) strain of C. trachomatis and with single HeLa 229 cells infected with C. psittaci. High multiplicities of infection of host cells with chlamydiae may quickly bring cell division to a halt, whereas lower multiplicities slow but do not immediately stop the division of host cells. Indefinitely multiplying clones of chlamydia-infected host cells were not observed. The method used here should be applicable to other studies on the division of cells in culture.