Editors’ Note: This is Part 2 of Dr. Aswin Sai Narain Seshasayee’s series ‘Bacterial Expressions‘
Very naively speaking, bacteria are a bag of molecules of varying colours and shades and sizes that is capable of asexual reproduction. However, that does not quite do justice to the structures that make up a bacterium. Let us use the great model organism Escherichia coli, popularly referred to as E. coli, as a case study to describe the principal molecules that are its building blocks, while shying away from the complexities and pedagogy that are best left to textbooks and research papers.
Let us assume a small particle, a millionth of a millmetre in size, but with the ability to see the world around it. If this particle were to encounter an E. coli cell, it will countenance a rod-shaped object one thousand times its size along its long axis. If our curious particle were to get closer, it would see the E. coli cell covered by polymers of molecules, whose building units are puckered rings – these are sugar molecules, which are connected to the rest of the cell body by long chains of carbons, called lipids. This continuous structure of lipids and sugars, called lipopolysaccharides, make the cell boundary. Our particle sees large pores built into the lipopolysaccharide layer through which nutrients flow inwards, and our particle joins these nutrient molecules and somehow gets past this barrier. Inside, it sees a space within which is a second barrier, also comprising sugars linked with a new type of molecular linkage, which the particle will later realise is also part of proteins. This cell wall is closely associated with a bi-layered inner membrane, which is composed of what are basically detergents. The particle is astounded by the levels of activity associated with this inner membrane – many proteins doing their job, sensing signals including the presence of this foreign particle (which somehow is considered benign and let through unharmed), transporting molecules in and out, synthesising molecules that make up life’s energy currency.
Somehow, our particle bungles past the wall and the membrane and comes face to face with the inner belly of the cell, a water-filled space crowded with a variety of molecules, including proteins, sugars and nucleic acids, which make up DNA and RNA. It is not simply a static collection of large and small objects, but a dynamic space, involving processes – interconversions, construction and destruction among these. One molecule entering a protein machine, and coming out as another; large DNA molecules passing through larger proteinaceous structures, releasing more DNA or RNA; and RNA snaking through humungous protein architectures, producing more protein; amd lots more. All these seemingly coordinated beautifully such that the E. coli cell looks like an impossibly complex but an exceptionally well-organised machine.