Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 14

a flexible tail, giving mobility through the surrounding environment. Survival
requires cell growth, replication of the DNA and then division, usually sharing
the contents into two equal daughter cells. Under ideal conditions of environ-
ment and food supply, division of some bacteria may occur every 20 minutes,
however, 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 stor-
age 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, is of sufficient interest to warrant at
least one major symposium (Allison et al . 2000). 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, the overall tolerance to stress and metabolic diversity of individ-
ual 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 bac-
terial transformation. This is a procedure whereby a bacterium may absorb free
deoxyribonucleic acid (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 environ-
ment. 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 recently, there is laboratory evidence that lightning can impart compe-
tence to some bacteria (Demaneche et al . 2001). It is conceivable that conditions
allowing transformation prevail in biofilms considering the very high local con-
centration of microbes. Indeed there is evidence that such horizontal transfer of
DNA occurs between organisms in these communities (Ehlers 2000). In addition