Infection of HeLa cells with Salmonella typhimurium 395 MS and MR10 bacteria

After interaction with HeLa cells cultured in vitro, the fraction of adhering extracellular and that of internalized smooth Salmonella typhimurium 395 MS and rough 395 MR10 have been determined by two different techniques. (i) By using the indirect fluorescent-antibody technique on unfixed and acetone-fixed HeLa cell preparations, intracellular bacteria were considered to become stained only after acetone fixation. (ii) Based on the assumption that gentamicin affects only extracellular bacteria, disintegration of the infected HeLa cells and viable count allowed the determination of internalized bacteria. Both techniques showed that MS as well as MR10 bacteria gained intracellular access, the fraction of MR10 cells doing so being much greater. The net increase in the intracellular bacterial population was small within 3 h of incubation.