© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.4.2.5 Mechanical Bacteriocide

Bacteria are unicellular microorganisms capable of independent metabolism, growth, and replication. Their shapes are generally spherical or ovoid (cocci), cylindrical or rodlike (bacilli), and curved-rod, spiral or comma-like (spirilla). Bacilli may remain associated after cell division and form colonies configured like strings of sausages. Bacteria range in size from 0.2-2 microns in width or diameter, and from 1-10 microns in length for the nonspherical species; the largest known bacterium is Thiomargarita namibiensis, with spheroidal diameters from 100-750 microns.³²²⁵ Spherical bacteria as small as 50-500 nm in diameter have been reported,²¹⁴⁹ but it has been theorized that the smallest possible cell size into which the minimum essential molecular machinery can be contained within a membrane is a diameter of ~40-50 nm.⁵²⁷ Many spherical bacteria are ~1 micron in diameter; an average rod or short spiral cell is ~1 micron wide and 3-5 microns long. Each bacterial cell consists of a mass of protoplasm enclosed within the usual thin lipid bilayer plasma membrane. Most Gram-positive bacteria are surrounded by a thick, mechanically strong but porous peptidoglycan cell wall. Gram-negative bacteria like E. coli surround themselves with an additional two-layer coat atop the peptidoglycan layer. This coat has an unsaturated inner lipid layer but a more rigid outer leaflet composed of an unusual lipid, called lipopolysaccharide (LPS), in which fatty acid chains are all saturated and 6-7 chains are covalently linked in a single LPS molecule.²¹³⁴

Bacterial cells have no internal membranous surfaces and no internal organelles, although some functional compartmentalization does exist.³⁶¹⁶ Bacteria possess small internal vacuoles, ribosomes, and granules of stored food, and usually one or more externally-attached flagella (Section 9.4.2.5.2). Some rod-shaped bacteria can form tiny spherical or oval endospores that survive when conditions become inhospitable for metabolism, such as extreme heat (>100°C) or desiccation (for up to 60 years).¹²²⁵ As prokaryotes, bacteria have no distinct nucleus. However, a single circular chromosome is organized into one or more compact aggregates, called nucleoids, that may occupy about one-third of cell volume.⁹⁹⁷ E. coli, a well-studied cylindrical bacterium measuring 0.65 microns wide and 1.7 microns long (cell volume ~ 0.6 micron³), has one double-stranded DNA chromosome ~1.3 mm (~4.2 megabases or ~10⁹ daltons) in length (strand volume ~ 0.002 micron³), organized in ~40 kilobase loops.⁹⁹⁷ Chromosomes spilled from lysed bacteria show ~80% DNA content, suggesting loop stabilization by proteins and at least a primitive membrane-associated cytoskeleton.

Table 10.6 gives the approximate mean composition of a 4 micron³ bacterium. There are at least three methods for eliminating an unwelcome microbe from the human body, described below. An additional serious problem in bacterial infections is that many microbes release toxins into the bloodstream that can wreak havoc even if all the bacteria are killed. Toxin cleanup (Chapter 19) is a necessary component of any comprehensive antibacterial treatment strategy.

Last updated on 24 February 2003