The Effect of Bacterial Products on Amoebic Growth

The type of problem is seen in the following inconclusive experiment where single amoebae (Hartmanella hyallna) from a pure culture with “YB”¹ bacteria were isolated into soil extract to which approximately equal numbers of bacteria of four kinds were added. The bacteria selected were “YB,” “SE,”¹B. prodigiosas and B. fluorescens liquifaciens. After 24 hours’ growth the reproductive rates of the amoebae were very different in the four cases (Table I). The reproductive rate for any time is calculated from the formula : where A is the number at the beginning and B that at the end of the period.

It is impossible to say whether the “YB” bacteria alone provided a really adequate food supply, or whether the other three types in varying degrees produced conditions inhibiting the reproduction of the amoebae. Since in all cases a certain number of “YB” bacteria are present, carried over with the single amoeba, it is probable that at least in the case of B. prodigiosas there was a definite check, other than lack of food, on the growth of the amoebae. On the other hand, further experiments have shown that by continuous subculturing it is possible to keep Hartmanella alive in pure culture with B. prodigiosas¹. If amoebae that have been grown with B. prodigiosas and “YB” respectively are isolated into soil extract containing on the one hand B. prodigiosas and on the other “YB,” the results suggest that continuous growth with the former has produced a tolerance to the otherwise unsuitable bacterium (Table II).

In the experiments so far quoted the bacterial numbers were not counted ; but in all parallel experiments the initial numbers were the same and the reproductive rates are therefore comparable.

If growth products are responsible for the lowering of the reproductive rate of the amoeba a similar effect should be obtained by using a filtrate made from a culture of the appropriate bacterium. Previous work with the bacterium “SE “and B. mycoides showed that this only obtained when old cultures were used; further work with B. prodigiosus has given the same result (Table III). In this experiment amoebae from two pedigree strains of Hartmanella, one with B. prodigiosus and one with “YB,” were isolated into soil extract containing 50 per cent, filtrate of each of the two species of bacteria prepared from young cultures. Each subculture so made was fed with the type of bacterium to which it was accustomed. From the results it would appear that the effect of the filtrate was negligible compared with that produced by the direct feeding.

There is, however, the further possibility that though a filtrate from a growing culture gave no effect, yet one obtained from disintegrated bacterial cells would give a positive result. B. mycoides was chosen to test this point, the cells being thoroughly ground in a glass bacterial mill with hay infusion and the resulting suspension filtered. At this time a culture of Hartmanella was not available, but another common soil amoeba (Naegleria gruberi) was used. The active forms of these two species are almost indistinguishable, although the cysts are different, and the latter reproduces more slowly than the former. For example, with similar bacterial feeding the average reproductive rate in twenty-four hours for eight single-cell cultures of Naegleria was 2·93, while that for Hartmanella was 6·63. On this occasion mass cultures instead of single cells were used, as the cultures were to be kept for some time so that daily observations could be made to determine the changes in the numbers of the population and the time of the onset of cyst formation.

In the control (Fig. 1) the end of the initial growth period is reached on the third day, and of the total population of 307,500 amoebae, 57,500 were encysted. The toxin culture (Fig. 2), however, was in comparison much less active and the initial peak was not only delayed by twenty-four hours but was much lower, the total number present being 140,000 of which all were encysted. After these periods the numbers in both cultures agree in falling, and in the toxin culture remain more or less constant to the end of the experiment, while with one exception 100 per cent, are in the form of cysts ; whereas in the control, although the percentage of cysts still remains high, there is definite evidence that reproduction is still taking place since the numbers are rising and falling.

As it was evident that the toxin was exercising an inhibiting influence this culture was centrifuged to collect the amoebae and the supernatant fluid was boiled for half an hour; to this material the amoebae were returned and the experiment continued (Fig. 3). Boiling had no appreciable effect.

The control culture was similarly treated and behaved in the same manner. The apparent discrepancy between the numbers at the end of Fig. 1 and at the beginning of Fig. 3 in this culture is due to the fact that the volume of the fluid was reduced and therefore the concentration of organisms per cubic centimetre was increased. After seven days the cultures were again centrifuged and boiled, and on this occasion the amoebae from each culture were divided and while half were returned to their own fluid (Fig. 3) the other half were put into sterilised hay infusion (Fig. 4). Again in the case of the original toxin solution boiling had no effect, although on the twenty-second day there is a drop in the numbers, implying that a small amount of excystation had occurred followed by death since on the previous day there were 100 per cent, cysts.

The original control, which had also been boiled, gave much the same result, though there was an initial excystment, accompanied by slight reproduction, after which it behaved like the toxin culture, though to a greater extent, there being excystation followed by death.

The cysts from both cultures which had been put into new medium showed an entirely different picture as regards activity, for not only did considerable excystation occur, but this was accompanied by a reproductive rate of 0·94 in one case (control) and 2·59 in the other (toxin).

Judging from the results obtained from the treated culture the effect of the B. mycoides toxin was twofold, firstly it undoubtedly depressed the whole level of activity, and, secondly, it appears to have encouraged cyst formation. The question of the causation of cyst formation is very much debated; an idea that adverse external conditions such as lack of food, drought and toxic conditions are the sole determining causes of encystation is often encountered, but various considerations make this untenable.

In certain Protozoa it is known that the formation of cysts plays a regular part in the life cycle of the animal. For example, in Oicomonas termo when two individuals conjugate the result is the formation of a cyst, from which a single active animal emerges ; while in Colpoda cucullus and many other ciliates true reproductive cysts occur as part of the life cycle, the encystation of one animal results in the production of four or more others. Another type of cyst also occurs as a regular episode in the life history of Amoeba diploidea where, instead of two individuals conjugating, fusion takes place between the two halves of an animal which has itself just divided within the cyst.

Indirect evidence that cyst formation is a regular occurrence in the life history of free-living Protozoa is afforded by their behaviour in soil, where there are a number of species of amoebae and flagellates continually altering in numbers and playing their part in the soil economy (1). That they are successful members of the population and largely independent of the changing soil conditions is evidenced by the large numbers which are present throughout the year (Tables IV and V). It is impossible to imagine that such an average percentage of cysts, built up as it is from records of daily observations, when the numbers of cysts may fluctuate from o to 100 per cent, of the total numbers, can be explained on any assumption but that encystment is a normal stage in the animal’s life history, though certain external conditions will undoubtedly help to determine the time at which the cysts are produced. The truth of this can be seen in the B. mycoides culture where the cysts were produced at an earlier date than in the control. Further, a consideration of Figs. 1,2,3 and 4 suggests that during an amoeba’s career it may at any moment be faced with conditions both internal and external which make further reproduction impossible and compel it to encyst or die ; such occasions are seen in Fig.1 on the third and fourth days, and in Fig. 4 on the fifth day of the toxin culture.

Encystment cannot be an easy process because if this were the case one would expect the whole of a culture to form cysts, without any appreciable drop in numbers; but since death nearly always accompanies encystment it seems more likely that it is a matter of delicate adjustment; and unless the animal’s internal condition, either physiological or morphological, coincides with the appropriate condition of the medium the cyst cannot be formed and death inevitably follows.

1. Filtrates prepared from young cultures of “YB” bacteria and of B. prodigiosas do not depress the rate of growth of Hartmanella hyalina appreciably.

2. A filtrate prepared from disintegrated cells of B. mycoides inhibits reproduction in Naegleria gruberi, and also hastens the onset of cyst formation.