82 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”a process called conjugation, the equivalent of mating in higher organ-
isms. Sometimes a fragment of the chromosomal DNA can also be
transferred between distantly related organisms. Both types of horizon-
tal transfer of genetic material usually benefit the recipient and play a
role in evolution.^3 However, bacteria can reject an acquired DNA if it is
harmful. Some bacteria produce a protein that can distinguish self- from
non-self-DNA by its chemical composition, such as the ratio of the GC/
AT (guanine-cytosine/adenine-thymine) nucleotides.^4 Other bacteria can
eliminate unwanted foreign plasmids by cleaving them with an enzyme.^5
4.1.4 Social recognition in a community
Some bacteria form swarming colonies in culture. Colonies of the same
strain coalesce to form a single large colony, but those of different strains
remain separated, an example of distinguishing self from non-self. A gene
responsible for this recognition has been identified.^6
4.1.5 Quorum sensing for mass action
Quorum sensing is the way bacteria communicate with one another using
chemical messages. It allows the bacteria to estimate the population
density, to determine whether they have reached a critical mass, and to
carry out processes that requires collective action to be effective, such
as the massive secretion of a toxin to overpower the immune system of
the host. Other collective actions include biofilm formation and sporu-
lation. Biofilms are community-level stress response in which a network
of channels is formed that enhances transport of nutrients.^7 Bacteria use
multiple chemical signals to “talk” to others of the same species, but they
also use them for across-species interaction.^8 Some bacteria can engage
in electron transfer on their outer membrane from one cell to another,
forming long chains of information flow. In this manner thousands of
microbes can act in concert as a multicellular unit. This phenomenon
results in sharing of an energy source and aids in the effectiveness of
quorum sensing.^9