Three cultural variants of the Gram-negative anaerobic bacillus used in the Shaffer-Frye (S-F) medium for propagation of Entamoeba histolytica were studied in the S-F medium and in the modified S-F medium of Reeves, et al. Culture S1, which is now used routinely in the S-F medium was found to produce round bodies, probably protoplasts, under the influence of the penicillin G in both media. These bodies were ingested by the amebae during their propagation. At no time were ingested bacterial cells seen with culture S1. Culture LSU, which is used routinely in the modified S-F medium, developed considerable cellular granulation under the influence of the penicillin G, but did not form round bodies in either medium. With this culture, bacterial cells were ingested by the amebae during their propagation. Culture S6, which does not support propagation of E. histolytica in either medium, also did not form round bodies in either medium. Nevertheless, the amebae ingested the bacterial cells of this culture. Darkfield microscopy was used for all direct observations in these studies. Permanent preparations were made by fixation of the cultures with 10% formalin followed by staining with the May-Grünwald stain.
Within the limits of these experiments, it appears that the observed changes induced by penicillin G were inherent characteristics of the bacterial cultures and not the result of variation between the S-F and the modified S-F media. The fact that the ameba ingested bacterial cells (LSU and S6) or round bodies (S1) regardless of whether propagation was supported or not, was interpreted as an indication that ingestion of a particle does not necessarily imply that the particle is useful to the ameba.