BACTERIA AND OTHER MICROORGANISMS OF THE SOIL - The Soil - page 138

Fluctuations in bacterial populations are sometimes due to predation by other
organisms, for example, amoebae and other protozoa. In early experiments ( ca.
1909-1913) at Rothamsted Experimental Station on partial sterilization effects on
soil, Sir J. Russell and E.B.Hutchinson suggested that there was an inverse rela-
tionship between bacterial populations and those of protozoa but later other workers
found the situation was more complicated. For instance, D.W.Cutler and L.M.Crump,
in the nineteen twenties, showed that protozoa were not general bacterial feeders;
some amoebae had definite preferences for particular species, and fluctuations in bac-
terial populations could still occur in soils from which protozoa were absent.
Moreover, in soil, bacteria are liable to be consumed by other organisms such
as myxobacteria ( see here ) , which require living bacteria for their nutrition, or to be
attacked by bacteriophages, which may be regarded as bacterial viruses. Several or-
dinary bacterial species may be parasitized by the tiny Bdellovibrio species. Bdellovi-
brios are very small motile bacteria (0.1-0.3 microns), which attach themselves to
their bacterial prey, penetrate through the cell wall and then secrete enzymes into the
cell contents, causing the dissolution of the prey bacterial membranes. During this
process, the bdellovibrios themselves multiply within the attacked cell and then, on
rupture of the body of the prey, they are released in increased numbers into the sur-
roundings where they can continue their predatory activity.
Bacteriophages, like other viruses, are not capable of independent existence and
can only multiply within their particular (bacterial) host cell. The action of the bac-
teriophage is to cause the lysis or dissolution of the host bacterium, at the same time
liberating many more bacteriophage particles. Bacteriophages are quite specific in
their action and attack and lyse only particular individual species of bacteria. For
example, bacteriophages that are specific to a given species of Rhizobium root nod-
ule bacteria, can be used as a means of identifying particular species and strains of
Rhizobium. Many other genera and species of bacteria are susceptible to lysis by their
specific bacteriophages. Furthermore, there are also phages which attack and lyse dif-
ferent actinomycetes and fungi; these are known, respectively, as actinophages and
mycophages.
Under suitable laboratory conditions, the action of bacteriophage can be quite
dramatic; when bacteriophage is added to a turbid suspension of susceptible bacteria,
the organism becomes lysed or dissolved and the initially cloudy suspension soon
clears and becomes transparent within a matter of twenty or thirty minutes. The bac-
teria are killed and the amount of bacteriophage is greatly increased. In soil, the de-
structive lysis of such useful bacteria as the symbiotic nitrogen-fixing nodule bacteria
may sometimes present a serious problem. It is a possible cause of the failure of some
legumes, for example clover, to thrive. The phenomenon known as 'soil sickness',