Bacterial vaginosis. Illustration of Gardnerella vaginalis bacteria (rods) attached to epithelial cells, known as clue cells. G. vaginalis is one of the species of bacteria that cause vaginosis. Clue cells are a diagnostic feature of bacterial vaginosis found during light microscopy of a cervical smear. Bacterial vaginosis causes an abnormal vaginal discharge with an unpleasant fishy smell. It is caused by an imbalance of the normal vaginal flora. Treatment is with antibiotics.