Friday, March 12, 2010

The Microbiology of Antibiotic Resistance, Part 1: Bacterial Cell Structure

“Last winter while being sickly and unable to taste, I examined the appearance of my tongue, which was very furred”.

- Anton van Leeuwenhoeck, 19 October 1674 Letter to the Royal Society, London. Royal Society. (Bardell D, Microbiological Reviews 1982)

From this examination of taste and tongue arose the first observation of “animalcules” by Anton van Leeuwenhoeck in 1674, which not only unraveled the mystery behind tongue furriness but also opened up a whole new world of life, invisible to the naked eye. Van Leeuwenhoeck’s discovery, eventually renamed bacterium by Christian Gottfried Ehrenberg in 1838, was revealed to be single-celled, prokaryotic (no membrane-bound organelles) microorganisms.

Improvements in visualization techniques have allowed scientists to study the structure of bacteria, which usually range from 0.5 – 5.0 micrometers in length and can come in a variety of shapes, from spheres to cylinders, spirals and rods.

At the most basic level, bacterial cells are encased by a cell envelope, which includes the most interior plasma (or cytoplasmic) membrane, the cell wall and, in some species of bacteria, an outer membrane. The cell wall filters, protects from internal pressure, and acts as an important classification characteristic based upon wall thickness and composition.

Gram-positive bacteria (which become purple when stained with Crystal Violet) possess a thick cell wall containing many layers of peptidoglycan, while Gram-negative bacteria (which retain no color when stained with Crystal Violet) have a double cell wall, containing only a few layers of peptidoglycan surrounded by an outer wall of carbohydrates, proteins and lipids.

Some bacteria have long, tail-like structure(s), projecting from the cell wall to the outside environment, called flagella. Through rotation, flagella provide a means of locomotion for bacteria. Short, hair-like projections called pili, usually covering the entire bacterial surface in high numbers, assist the bacteria in attaching to other cells or surfaces.

With no membrane-bound organelles (the equivalent of human organs), bacterial DNA, usually a singular, circular chromosome, is located in the nucleoid region of the bacterium’s inner gel-like matrix, cytoplasm. From DNA, ribosomes assemble amino acid chains, which eventually reorganize into functional proteins. Some bacteria also contain plasmids or extra DNA scattered throughout the cytoplasm that are not involved in reproduction.

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