Manual of bacteriology / by Robert Muir and James Ritchie.

  • Muir, Robert, Sir, 1864-1959.
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: Manual of bacteriology / by Robert Muir and James Ritchie. Source: Wellcome Collection.

    39/564 (page 15)
    THE LOWER BACTERIA. I The Lower Bacteria.^—These, as we have seen, are minute unicellular masses of protoplasm surrounded by an envelope, the total vital capacities of a species bemg repre- sented in every cell. They present three distmct type forms, the coccus, the bacillus, and the spirillum, and endo- genous sporulation may occur. They may also be motile. I The Cocci.—In this group the cells range in different species from .9-8 /x in diameter, but mostly measure 1-1.5 /i. Before division they may increase in size in all directions. The species are usually classified according to the method of division. If the cells divide only in one axis, and through the consistency of their envelopes remain attached, then a chain of cocci will be formed. A species in which this occurs is known as a streptococcus. If division takes place irregularly the resultant mass may be compared to a bunch of grapes, and the species is often called a staphy- lococcus. Division may take place in two axes at right angles to one another, in which case cocci adherent to each other in packets of four (called tetrads) or sixteen, may be found, the former number being the frequent unit. To all these forms the word micrococcus is often generally applied. The individuals in a growth of micrococci often show a tendency to remain united in twos. These are spoken of as diplococci, but this is not a distinctive character, as every coccus as a result of division becomes a diplococcus, though in some species the tendency to remain in pairs is well marked. The adhesion of cocci to one another depends on the character of the capsule. Often this has a well-marked outer limit (micrococcus tetragenus), sometimes it is of great extent, its diameter being many times that of the coccus (streptococcus mesenteriodes). It is especially among the streptococci and staphylococci that the pheno- menon of the formation of arthrospores is said to occur. In none of the cocci have endogenous spores been certainly observed. The number of species of the streptococci and staphylococci probably exceeds 150. Besides those men- ^ For the illustration of this and the succeeding systematic paragraphs, vide Fig. i.