ABSTRACT
In the early stages of molecular biology, bacteria such as the Gramnegative Escherichia coli and the Gram-positive Bacillus subtilis were used as model organisms of molecular biology relying on the belief that cultures of single bacterial species are homogenous in cell populations. Recently, however, this concept has been challenged and our up-to-date understanding is that in natural environments, bacteria form organized communities, in which each cell plays a unique and specifi c role different from other cells. In exponential growth phase under laboratory culture conditions suitable for bacterial growth such as in rich media and under enough aeration, bacteria grow as planktonic single cell states. Most of the important knowledge in molecular biology such as the mechanisms and regulation of gene expression were established based on the experimental data obtained using model bacteria growing under laboratory culture conditions. Under stressful conditions in nature and even in controlled culture conditions such as in poor media and under anaerobic conditions, however, bacteria gather together into sessile communities, attached on solid surfaces, forming biofi lms. Nowadays, it is widely accepted that biofi lm is the major habit of microorganisms in nature.
