Chlamydia infected culture, stained, darkground illumination HFW = 300um. Chlamydia is a very common sexually-transmitted pathogen which can cause PID in women (but is 'silent' in the majority of patients of both sexes) and blindness and pneumonia in the newborn. Related to bacteria but the only genus in the Phylum. The photomicrograph is of a monolayer culture of rhesus monkey kidney cells in which the Chlamydia organisms have been cultured, stained with Giemsa. The cell nuclei appear red and the infective 'elementary bodies' of the Chlamydia which develop in 'blisters' in the cells, fluoresce green.