The effect of x-rays on the rickettsiostatic activity of streptomycin, penicillin and aureomycin in the embryonate egg was investigated.

Only a slight reversal of the rickettsiostatic action of 5 mg. of streptomycin occurred in embryonate eggs given 100 r of x-rays at 4 days of incubation, whereas complete reversal occurred in those given 1000 r. Groups of eggs irradiated with intermediate doses of x-rays showed a graded response.

The rickettsiostatic activity of 5 mg. (1660 Oxford units/mg.) of penicillin or of 5 mg. of aureomycin was not reversed by doses of x-rays ranging from 100 to 1000 r.

500 r of x-rays reversed the rickettsiostatic activity of 2 mg., 4 mg., 8 mg., or 16 mg. of streptomycin per egg in proportion to the amount of streptomycin injected. The rickettsiostatic activity of 2 mg. of streptomycin was completely reversed and the activity of 16 mg. only partially reversed.

Reversal of the rickettsiostatic activity of streptomycin by x-rays was observed when radiation was given up to 6 days after the injection of the antibiotic into 7-day-old embryos.

Reversal of the rickettsiostatic activity of 4 mg. of streptomycin could be detected when the antibiotic was injected 3 days after 500 r of x-radiation.

Explanations of the observed phenomena are discussed in terms of biochemical and biophysical alterations of the cells of the host.

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