Figure 1.

Infectivity and growth rates of Orientia tsutsugamushi strains in ECV304 cells.

The number of immunofluorescence (IF)-positive foci observed in ECV304 cells was quantified at 4, 24, 48, and 72 hr after inoculation (hpi) as an indication of the number of O. tsutsugamushi bacteria present. The amount of bacteria present at 4 hpi represents the infectivity of each O. tsutsugamushi strain or isolate. The increments of IF-positive numbers from 4 to 72 hpi represent the growth rate of each strain or isolate. The AFSC-4 strain demonstrates higher infectivity and larger bacterial numbers than the other 11 strains throughout the experiment.