Isolation of Different-Sized Substrains from Three Stock Cultures of Entamoeba histolytica, with Observations on Spontaneous Size Changes Affecting Whole Populations

SYNOPSIS. Three stock cultures of E. histolytica , strains 200, K9, and 22, have each been shown by microisolation methods to consist of at least two substrains or variants differing in size. Cultures were grown with Trypanosoma cruzi in Phillips medium, and on egg slants with mixed bacteria and a single species of bacteria. For each strain, the variant that was bigger in one medium was also bigger in the other two media, in spite of the fact that absolute sizes changed greatly depending upon the medium. This shows that size patterns were primarily determined by endogenous factors, although absolute size was strongly influenced by the culture medium. In all cases the small variants remained larger than E. hartmanni.
In the initial weeks after microisolation, the variants showed consistent size patterns. After a few months, the big substrains of E.h. K9 and 22 showed a spontaneous reduction in size, eventually becoming smaller than the corresponding small variants. The change in E.h. K9 was shown to be due to a shift in the entire population, rather than to selection of individual mutants. In the E.h. 200 cultures, big and small variants retained their distinctive sizes throughout the 17 months during which they were observed.     

Evaluations of permethrin-impregnated clothing and three topical repellent formulations of deet against tsetse flies in Zambia

Permethrin-impregnated clothing and three topical repellent formulations of deet (diethyltoluamide) were field tested against natural populations of tsetse flies, mostly Glossina morsitans centralis Machado, in central Zambia. Volunteers wore different combinations of clothing impregnated with permethrin 0.125 mg ai/cm2 and repellents while riding in a vehicle that was driven slowly (4-6 km/h), with the windows and rear door open, through fly-infested areas. The mean rate of tsetse bites was about twenty per 75 min for unprotected people. The treatment combination of permethrin-impregnated clothing (blue cotton coveralls) and either of two controlled-release deet formulations on exposed skin of face and arms provided 91% mean protection, but this was not significantly better (P greater than 0.05) than wearing deet repellent alone (76-87% protection). No significant differences of protection were observed between the three repellent treatments, although the two controlled-release formulations (intended to be more persistent) were applied at approximately half the dosage of the standard 75% deet. Wearing permethrin-impregnated coveralls alone provided relatively poor protection (34%) for the untreated and exposed skin of head and hands. However, olive drab mesh jackets treated with permethrin reduced the tsetse biting rate by 75%.