Figure 1.

Residing within erythrocytes enhances the colonization of ticks by F. tularensis by protecting them from low pH. (A,B) Erythrocytes incubated with F. tularensis LVS bacteria to allow for invasion were either left intact or lysed to liberate intracellular bacteria. Each group was fed to A. americanum (A) or I. scapularis (B) ticks using a glass capillary tube. After feeding, the ticks were incubated for 24 h, and then homogenized and plated on media selective for Francisella. Increased colonization occurred with intact erythrocytes in comparison to lysed erythrocytes for both I. scapularis and A. americanum ticks (^**p < 0.01; unpaired Student's t-test). (C) F. tularensis LVS bacteria were incubated with human erythrocytes for invasion to occur. Erythrocytes were either left intact or lysed to liberate intracellular bacteria. Cells were then incubated in media containing 10% serum at the pH indicated. At designated time points, viable F. tularensis bacteria were enumerated and statistically significant differences were determined by two-way ANOVA followed by Sidak's multiple comparisons test (^*p < 0.05). Data shown are mean ± SD. For all panels, Data are representative of at least three independent experiments in which each individual iteration showed significant differences between the same groups. Each experiment contained at least four separate wells per group.