A NOTE ON THE USE OF DEHYDRATED MILK AGAR FOR BACTERIAL COUNTS ON RAW AND DRIED MILKS

SUMMARY: The bacterial colony count of raw milk and of milk powder using milk agar prepared from a proprietary brand of the dehydrated medium has been compared with that obtained using the fresh medium prepared in the laboratory.
No significant difference was found in 17 milks which were examined using the roll-tube technique and 8 replicates/medium but in a second series of 32 milks the plate count (3 replicates/medium) was significantly lower on the reconstituted than on the fresh medium.
Some milk powders gave a much lower plate count on the reconstituted medium, due to the inability of the latter to support adequate growth of Streptococcus thermophilus.     

SOME STUDIES OF THE COLONY COUNT TECHNIQUE FOR SOIL BACTERIA

SUMMARY: A series of co-operative experiments was conducted to compare the bacterial colony counts of soil obtained by workers in different laboratories, using soil extract agar and other media for the determinations.
In the earlier experiments it was not possible to obtain a reasonable degree of reproducibility of results between laboratories even when the plating technique was carefully prescribed. By modification and more rigid standardization of the technique closer agreement was obtained in subsequent experiments. It is suggested that when co-operative investigations are contemplated the participating laboratories should check their technique by the examination of 'control' soils.
No evidence could be obtained to support the suggestion that higher colony counts are obtained by the use of soil extract media containing extract prepared from the same soil as the sample tested. The source appears to be immaterial so long as the soil for extract preparation is not of extreme type and has been well manured and cultivated.     

THE EFFECT OF FREEZING AND STORAGE ON THE BACTERIAL FLORA OF PASTEURIZED MILK

SUMMARY: Experiments were designed to determine the effect of freezing and storage on the bacterial population of pasteurized milk. Samples taken on separate occasions sometimes reacted differently to freezing treatment. Significant changes in bacterial numbers occurred after the frozen milk had been stored for 4–5 months at −12°, and this coincided with definite physical changes in the milk. Known numbers of coliaerogenes organisms were added to the pasteurized product in order to study their behaviour when frozen and stored at low temperatures. The counts were significantly lower after storage for 4 months.     

STUDIES OF THE BACTERIAL CONTENT OF WASHED MILK CANS

SUMMARY: Colony counts at 30° were greater than 10 times those at 37° in a high proportion of rinses of washed milk cans, the difference being most marked in those containing milk scale, where 58% of the colony counts at 30° exceeded 10⁶/can. A high proportion of the microflora was composed of thermoduric bacteria. Of 895 cultures from the milk scale, 33% were micrococci, 28% corynebacteria, 22% streptococci, and 9% were Gram-negative rods. Though aerobic sporing rods constituted only 5% of the microflora of the milk scale, they were present in large numbers and unsatisfactorily washed cans probably constitute one of the main sources of these organisms in milk.     

KINETICS OF GROWTH OF HAEMOPOIETIC COLONY CELLS IN AGAR

Cell kinetic parameters of mouse granulocytic and mononuclear cells growing in colonies in agar cultures have been measured. Analysis of flash and continuous labelling studies with ³H-thymidine together with determinations of colony size, growth fraction and mitotic indices, gave the following values for the phases of the cell cycle: G₁= 6·3 1·6 hr, S = 5·8 ± 1·4 hr, G₂= 1·7 ± 0·1 hr and M = 0·7 ± 0·1 hr (42 ± 8 min). No difference in the cell cycle parameters of granulocytic and mononuclear cells were found in this study.
Colonies of different size from cultures of the same age group had similar labelling indices, indicating that the size of a colony is not a function of the rate of proliferation of cells in the colony. Rather, variation in colony size is probably representative of an initial delay in the onset of colony development.     

THYMIDINE SUICIDE IN VIVO AND IN VITRO OF SPLEEN COLONY FORMING AND AGAR COLONY FORMING CELLS OF MOUSE BONE MARROW

The ‘thymidine suicide’technique for indicating differences in the proliferation rate of early haemopoietic progenitor cells (spleen colony forming and agar colony forming cells) in C57BL mice has been evaluated. Special care was taken to use the same bone marrow cell suspension for the two progenitor cell assays. Both the in vivo and the in vitro techniques were employed. Following ³H-TdR in vivo, about 20% of both types of progenitor cell are killed in normal mice; however, after incubation in vitro with ³H-TdR, 35% of agar colony forming cells but only 4% of spleen colony forming cells are killed. Reasons for the difference between the in vivo and the in vitro results are discussed. With bone marrow from continuously irradiated animals, the thymidine suicide for both agar colony forming and spleen colony forming cells is in the range 42–50%, and there is no difference between in vivo and in vitro suicide. The in vivo results support the conclusion, based on the effect of proliferation dependent cytotoxic agents, that in C57BL mice agar colony forming and spleen colony forming cells are proliferating at the same rate in normal animals, and are speeded up to the same extent by continuous γ-irradiation. It is considered that in normal C57BL mice the in vitro method does not give a correct estimate of the proliferation rate of these progenitor cells. It would seem that the similarity in the proliferation rate of agar colony forming and spleen colony forming cells in C57BL mice is not true for other strains of mice: indeed using normal CBA and in vivo suicide, we have shown a significantly greater thymidine suicide for agar colony forming cells compared to spleen colony forming cells.     

THE EFFECT OF CYTOSINE ARABINOSIDE IN VITRO ON AGAR COLONY FORMING CELLS AND SPLEEN COLONY FORMING CELLS OF C57BL MOUSE BONE MARROW

This study reports the effect of cytosine arabinoside in culture on two classes of bone marrow progenitor cells in C57BL mice, agar colony forming cells (ACU) and spleen colony forming cells (CFU). Both normal cells and rapidly proliferating cells were studied. The results show that in normal mice, 23 % of ACU but only 7 % of CFU are killed following 1 hr incubation with the drug. With longer periods of incubation, the survival of ACU in the controls is poor, and the results for the drug-treated cultures suggest that the cells are held up in cycle. In continuously irradiated mice, the proportion of ACU and CFU killed after 1 hr incubation with drug is increased to 43–54%, confirming previous results that these cells are proliferating more rapidly than in normal mice. In mice treated with myerlan, 54 % of ACU are killed by 1 hr in vitro exposure to cytosine arabinoside, again confirming that ACU are rapidly proliferating. However, the proportion of CFU killed is lower (23 %). These results are compared with other studies of the effect of cytosine arabinoside in vivo and also with thymidine suicide in the same strain of mice. The results show that cytosine arabinoside has the same effect as tritiated thymidine, and also that the proportion of CFU killed by these agents in vitro is lower than when the agents are injected in vivo. It is suggested that the conditions in culture have an adverse effect on CFU, which cease DNA synthesis, and are protected from the killing effect of cytosine arabinoside and tritiated thymidine. Since cytosine arabinoside in vitro has an effect similar to tritiated thymidine in vitro on bone marrow progenitor cells in C57BL mice, in vitro incubation with cytosine arabinoside could be an alternative method to thymidine suicide for measuring differences in cell proliferation rate.     

寡聚糖生物农药对棉花体内细菌数量和作物生长的作用

通过常规分离方法,测定寡聚糖生物农药对中棉12品种的棉茎体内细菌群落数量的影响。2批实验结果表明喷寡聚糖后棉茎体内细菌群落数量均低于对照,喷寡聚糖10d后棉茎体内细菌群落数量显著下降(8.0×10     

A COMPARISON OF ROLL-TUBE AND PETRI DISH COLONY COUNTS ON RAW MILK

SUMMARY: A co-ordinated experiment was carried out at two centres, in which colony counts from raw milk samples determined by the roll-tube and Petri dish methods were compared. A bulk medium was standardized for both laboratories, as also were details of technique. Statistical analyses showed that roll-tube counts were generally lower than the corresponding Petri dish counts, but the difference varied considerably from milk to milk. The variation between replicate sub-samples was about the same for both methods. Experience and the results both indicate that roll-tubes were slightly more difficult to count than Petri dishes.