Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 15

division, usually sharing the contents into two equal daughter cells. Under ideal
conditions of environment and food supply, division of some bacteria may occur
every 20 minutes, but most take rather longer. However, the result of many
rounds of the binary division just described, is a colony of identical cells. This
may be several millimetres across and can be seen clearly as a contamination on
a solid surface, or if in a liquid, it will give the solution a cloudy appearance.
Other forms of replication include budding off, as in some forms of yeast, or the
formation of spores as in other forms of yeast and some bacteria. This is a type of
DNA storage particularly resistant to environmental excesses of heat and pH, for
example. When the environment becomes more hospitable, the spore can develop
into a bacterium or yeast, according to its origins, and the life cycle continues.
Micro organisms may live as free individuals or as communities, either as a
clone of one organism, or as a mixed group. Biofilms are examples of microbial
communities, the components of which may number several hundred species.
This is a fairly loose term used to describe any aggregation of microbes which
coats a surface, consequently, biofilms are ubiquitous. They are of particular
interest in environmental biotechnology since they represent the structure of
microbial activity in many relevant technologies such as trickling filters. Models
for their organisation have been proposed (Kreft et al ., 2001). Their structure, and
interaction between their members, was of sufficient interest to warrant a major
Symposium ten years ago (Allison et al ., 2000), and several since. Commonly,
biofilms occur at a solid/liquid interphase. Here, a mixed population of microbes
live in close proximity which may be mutually beneficial. Such consortia can
increase the habitat range, and the overall tolerance to stress and metabolic diver-
sity of individual members of the group. It is often thanks to such communities,
rather than isolated bacterial species, that recalcitrant pollutants are eventually
degraded due to combined contributions of several of its members.
Another consequence of this close proximity is the increased likelihood of
bacterial transformation. This is a procedure whereby a bacterium may absorb free
DNA, the macromolecule which stores genetic material, from its surroundings
released by other organisms, as a result of cell death, for example. The process
is dependent on the ability, or competence, of a cell to take up DNA, and upon
the concentration of DNA in the surrounding environment. This is commonly
referred to as horizontal transfer as opposed to vertical transfer which refers to
inherited genetic material, either by sexual or asexual reproduction. Some bacteria
are naturally competent, others exude competence factors and there is laboratory
evidence that lightning can impart competence to some bacteria (Demaneche
et al ., 2001). It is conceivable that conditions allowing transformation, prevail
in biofilms considering the very high local concentration of microbes. Indeed
there is evidence that such horizontal transfer of DNA occurs between organisms
in these communities (Ehlers, 2000). In addition to transformation, genes are
readily transferred on plasmids as described later in this chapter. It is now well
established that, by one method or another, there is so much exchange of genetic
material between bacteria in soil or in aquatic environments, that rather than
discrete units, they represent a massive gene pool (Whittam, 1992).