A STUDY ON THE RADIOACTIVE LABELED COLON BACTERIA AND THEIR APPLICATION TO FOOD SANITATION FIELD

—ON THE BACTERIAL CONTAMINATION MECHANISM OF LIVING OYSTERS—

Abstract

Since oysters have been a cause of the outbreak of enteric infectious deseases in many countries because they are customarily relished as fresh dishes, the bacteriological sanitary control of oysters in the growing area is one of the most important problems in the food sanitation field.
This article is designed to investigate the mechanism of bacterial contamination of the living Japanese oysters (Ostrea gigas, THUNBERG, obtained at the Hiroshima Bay) by means of laboratory experiment.
However, these experiments are complicated and very difficult to carry out by the traditional bacteriological techniques, because, for example, ordinary oysters are heavily contaminated with various bacteria and then it is impossible to detect and measure the bacteria which have been newly added for tracing out the behavior of those accumulated in the oyster. In this study, therefore, introduction of the bacteria labeled with a suitable radioisotope into the experimental system was examined to solve the above difficulty.
Colon bacteria (E. coli), a useful bacterial indicator of fecal contamination of foods, were examined for labeling with two radioisotopes, i.e. β emitter ⁹⁰Y and γ emitter ²⁰³Hg. The special merits of the both nuclides are as follows ; ⁹⁰Y is the short living and safe daughter of ⁹⁰Sr, which can be readily separated from ⁹⁰Sr whenever necessary, and ²⁰³Hg has high affinity to bacterial cells and the activity in materials can be easily measured without special sample preparation.
As an experimental result, the most suitable labeling conditions were as follows ; mix the washed bacterial cells and the RI solution and stand for 30 minutes at room temperature, and then wash. the cells once or twice in a centrifuge.
In case 1 mCi/ml of RI solution was reacted by means of this method to the bacterial mass (corresponding to 5 ml of the broth cultivated for about 20 hours), the bacteria labeled with about 10^-3 cpm for ⁹⁰Y or 1 cpm for ²⁰³Hg per cell can be prepared respectively. It was confirmed that radioactivity labeled to bacteria was stable in sea water.
Experimental studies on the contamination mechanism of living oysters using the above radio-labeled colon bacteria were as follows ; the shellfish was reared in the artificial sea water in aqualium vessel to which had been added suspension of the labeled bacteria. The radioactivity of the oyster, i.e. the bacterial number accumulated in the shellfish, was measured with the samples which have been asked by the wet method for ⁹⁰Y or in a fresh intact form for ²⁰³Hg.
The obtained results are as follows ; the radioactivity of the oyster increased exponentially up to about 20 hours after, and then reached equilibrium. As for the effect of rearing water temperature (ranging 10°C-28°C), the bacterial accumulation of oyster increased according to the increase of the temperature, although within the temperature ranges under 10°C or above 28°C (up to 35°C) the increase was little. However, it was observed that the oyster was contaminated with colon bacteria even at the coldest temperature as low as 3°C.
As the salinity of the rearing water reduced, the bacterial up take rate of oysters decreased to a considerable degree, especially in fresh water no uptake was observed at all. This result distinctly differs with the traditional theory.
It was determined that the bacterial burden of oysters depended mainly on the contamination of the gill, however, the contamination of the digestive tract was not negligible.
When the contaminated rearnig water was continuously supplied by flowing, the uptake rate of the oyster was as about double as those in the case of rearing under a closed condition.