Introduction Chlamydia trachomatis causes two different sexually transmitted diseases: genital discharge (GD) and lymphogranuloma venereum (LGV). Apart from differences in tissue tropisms, little is known about differences in pathogenicity to the cells they infect, especially keratinocytes, the primary target of infection for LGV chlamydia.

Methods Human keratinocytes (HaCaT cells), one LGV (serovar L2) and one GD (serovar E) isolate were used for all experiments with uninfected cells as the negative control. Experiments were performed at 37 and 33°C. For transmission electron microscopy (TEM) cells grown on Thermanox coverslips within 24-well plates were infected (MOI = 0.25) and incubated for various time-points over 48 h. The diameter of 15 mitochondria were measured in uninfected and infected cells at 1, 36 and 48 h post-infection using image analysis software. For the MTT assay cells grown in 96-well plates were infected (MOI = 0.25) and incubated for various time-points over 48 h. Median mitochondrial diameter was compared using Kruskal-Wallis nonparametric ANOVA with Dunn’s post-test. Percentage mitochondrial activity was compared using two-tailed paired T test. P ≤ 0.05 was considered significant.

Results The GD isolate alone caused mitochondrial swelling, cristae rearrangement and a pale mitochondrial matrix at 1, 36 and 48 h post-infection at 37°C but not 33°C. Median mitochondrial diameter was 430 nm in uninfected cells; 477 nm, 470 nm and 1221 nm after 1, 36 and 48 h post-infection respectively for serovar E; and 343 nm, 420 nm and 392 nm after 1, 36 and 48 h respectively for L2. Mitochondrial diameter was significantly larger in serovar E-infected cells 48 h post-infection compared to uninfected cells (P < 0.001) or L2-infected cells (P < 0.01). The MTT assay indicated a significant (P = 0.0373) reduction in mitochondrial activity upon infection with serovar E, but not serovar L2 at 37°C.

Conclusion C. trachomatis of the GD biovar but not the LGV biovar causes transient mitochondrial swelling and a decrease in mitochondrial activity in infected keratinocytes at 37°C.

Disclosure of interest statement This study is supported by the National Research Foundation of South Africa. The authors have no conflict of interest to declare.