The recent bacillary dysentery in this country can be characterized by such a trend as becoming milder, while tending more often to occur in the form of massive outbreak due to antibiotic-resistant bacilli.
It can be presumable that some changes occur in the biological properties of bacteria as a result of the acquisition of drug-resistance, which should possively be attributable to the recent trend of bacillary dysentery. In this respect, the following experiments were carried out.
By means of the stepwise selection method previously reported, using the media containing various concentrations of chloramphenicol, chlortetracycline and dihydrostreptomycin sulfate, twelve strains of Shigella which were originally sensitive to above drugs were made resistant to that of increasing concentrations.
After their resistance reached to 100 γ/ml antibiotic concentration level, their biological properties were contrasted with that of the original sensitive ones.
1) Virulence test
Bacterial suspensions of the sensitive and resistant strains in the amount of 0.1mg, 0.5mg, 1.0mg, and 5.0mg respectively were injected into the abdominal cavities of 4 weeks old dd-strain mice weighing 14 gm, and the rate of mortality was compared.
The bacterial quantity for LD
50 was greater in the resistant group than in the sensitive one, and this would indicate that with the increase of antibiotic resistance the virulence tend to become weaker.
2) Vitality test
One drop of each bacterial suspension from capillary pipette was added to 50 ml of distilled water, physiological saline solution and well water respectively. These were then incubated at 30°C. On each successive day afterwards, the liquid was cultured on the solid media and bacterial survival was examined. In this manner, the surviving term of bacteria was measured.
The surviving term would be, the Author views, one of the most tangible manifestations of the vitality of bacteria.
The test showed the days of survival were longer, i. e. the vitality is greater, in the resistant strains than in the originally sensitive ones.
Considering above findings, it may be reasonably concluded that the dysentery bacilli tend to be weakened in its virulence by developing the resistance to antibiotics, and, at the same time, tend to enhance the surviving term, the vitality.
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