Lactobacillus vaginalis. Light micrograph of a vaginal smear showing the rod shaped bacteria Lactobacillus vaginalis (black threads). Normal epithelial cells are also seen, stained pink & blue. These bacteria form the predominant fauna in the vagina of the sexually mature human female, particularly in the second half of the menstrual cycle and during pregnancy. They ferment lactic acid from the glycogen in the epithelial lining of the vagina. The acidic environment that they produce, and live in, inhibits the growth of many other bacterial forms and as such protects the vaginal passageway from infection. Papanicolaou stain. Magnification: x400 at 35mm size.