Cell Composition

Nucleus

As special staining methods designed to visualize the chemical substances making up the nucleus in the higher forms of life were applied to the study of bacteria, it became evident that bacteria contained nuclear material. This was seen in electron micrographs as a distinct and relatively transparent structure of rounded proportions with no detectable nuclear membrane. The nuclear material in bacteria consists essentially of a circular molecule of deoxyribonucleic acid (DNA) present as a single chromosome, which if unfolded would stretch approximately 1 mm. It is hooked atone point to an infolding of the plasma membrane known as a mesosome. Many bacteria possess one chromosome, but two or more may be seen in young, actively dividing cells because their rapid rate of growth does not allow sufficient time for DNA to replicate, as seen under a microscope.

Structurally a chromosome consists of coiled threads, each made up of a large number of strands of thee giant molecule deoxyribonucleic acid, DNA. A chromosome is the unit of organization for DNA and represents a binding together of DNA, ribonucleic acid, RNA polymerase (enzyme), and other proteins, but no histone (low molecular weight basic protein). In DNA three smaller chemical molecules-four organic nucleotide bases (guanine, cytosine, thymine, and adenine), a pentose sugar (deoxyribose), and phosphoric acid-are fastened together in a characteristic spiral pattern. Two sugar-phosphate ladders with every so many paired nucleic rungs-that is, two identical, unbranched, rigid, intertwining, spiral chains-are thus formed. These are at least 1500 times longer than they are wide and are twisted in opposite directions about a central shaft. Every living organism must possess somewhere in its makeup a master plan that formulates all aspects of its appearance and behavior. For even simple organisms such a master plan encompasses vast amounts of biologic information that, to be stored efficiently must be converted to some sort of code. Because of its vantage point in the cell, deoxyribonucleic acid is just right to do this biochemically. Its building blocks are linked in such a way that the four nucleotide bases (adenine, thymine, cytosine, and guanine) can serve as letters of a four-character code alphabet. Words can be formed in a biochemical and genetic language. In the language of life the four characters are expressed in linkages of three, called triplet codes. There are 64 such triplet codes with so many possible arrangements that they can describe easily the 4 billion human beings populating the world. For a given organism the specifications for its every structure and life process are contained in specific chemical sequences in its DNA.

The chromosome then, is a biologic document. From it can be transcribed biologic messages in code, over and over again, even the same message. The chromosomes carry the particular hereditary pattern for the given organism.

An unusual property of the DNA molecule is its ability to reproduce itself, that is, to replicate itself into two exact copies. Because of’ this, the hereditary biochemical pattern or genetic code may be passed from one generation to the next.

As it is for all cells, the nuclear material is precisely the governing force for the bacterial cell, and other activities cannot be carried on in a bacterial cell without it.

Chemical composition

Even in a unit such as the bacterial cell, 500 times smaller than the average plant or animal cell, the chemical composition is exceedingly complex. To gain an idea as to the chemical makeup of any bacterial cell, let us look at one that, since it easily grown and manipulated in the laboratories– is undergoing almost as intensive study today as a human being. This is the colon bacillus, Escherichia coli.

Biochemically, this microbe contains perhaps X100 to 6000 different types of molecules Of these, specific proteins account for 2000 to 3000 kinds. The amount of DNA present is postulated to be that required to code for the necessary amino acid sequences in these proteins. About the DNA are 20,000 to 30 000 spherical ribosornes composed of protein (40%) and RNA (60%). Water, water-soluble enzymes, and a large number of various small and less complex molecules are associated with these nucleic acids.

Motility

Many bacilli and all spirilla are motile when suspended in a suitable liquid at the proper temperature. This has been observed keenly under the microscope. True motility is seldom observed in cocci. The organs of bacterial locomotion are fine hairlike appendages known as flagella (little whips) that spring from the bacterial cell and cause it to move along by their wavelike, rhythmic contractions. Some spirochetes aid the action bf their flagella with a sinuous motion of the entire cell body. A bacterium may have one flagellum (monotrichous), a few, or many flagella in a tuft (lopho-trichous), and the flagella may be attached to one end, both ends, or all around the organism (peritrichous). Flagella, which chemically are elastic proteins, do not take the ordinary bacteriologic dyes but have to he stained by special methods.

Bacteria may he motile when grown on one medium and nonmotile when grown on another. Also, “True motility, in which the organism changes its position in relation to its neighbors, should not be confused with brown ion motion, a peculiar dancing motion possessed by all finely divided particles suspended in a liquid. They may be motile at one temperature and nonmotile at another. Different organisms travel at different rates of speed. The rod-shaped organism that causes typhoid fever (Salmonella typhi) is able to progress at a rate of 2000 times its own length per hour.

Pili

Pili (Latin, hairs) are surface projections like flagella found in. gram-negative bacteria. You will appreciate how this microorganism looks with the aid of the microscope. However, they are shorter and finer and do not propel the cell. Also called fimbriae, they may be part of the attachment of cells in conjugation; but otherwise their purpose is unknown.