Morphology is small (0.5–0.6 × 1.0 µm), slightly curved bacillus. Not acid-fast. Faintly stains Gram negative (stains poorly or not at all with many aniline dyes) but satisfactorily with Romanowsky's or Giemsa's stain. Warthin-Starry silver staining of fixed tissue sections reveals bacilli in clusters. May be seen in stained blood films appearing as rounded or ellipsoidal forms or as slender, straight, curved, or bent rods, occurring singly or in groups. They characteristically occur in chains of seve ral segmenting organisms, sometimes swollen at one or both ends and frequently beaded. In the tissues, they are situated within the cytoplasm of endothelial cells as isolated elements or are grouped in rounded masses. Intraerythrocytic forms occur in the blood of felines, small rodents, birds, fish, and other animals. In cultures, the cells may be very autoadherent. Some species possess unipolar flagella. The presence of pili is associated with the marked adherence and may mediate specific interaction with host endothelial cells and erythrocytes leading to intracellular localization.Type species: Bartonella bacilliformis
