What are bacteria?

The most interesting facts connected with the subject of bacteriology concern the powers and influence in Nature possessed by the bacteria.  The morphological side of the subject is interesting enough to the scientist, but to him alone.  Still, it is impossible to attempt to study the powers of bacteria without knowing something of the organisms themselves.  To understand how they come to play an important part in Nature’s processes, we must know first how they look and where they are found.  A short consideration of certain morphological facts will therefore be necessary at the start.

Form of bacteria.

In shape bacteria are the simplest conceivable structures.  Although there are hundreds of different species, they have only three general forms, which have been aptly compared to billiard balls, lead pencils, and corkscrews.  Spheres, rods, and spirals represent all shapes.  The spheres may be large or small, and may group themselves in various ways; the rods may be long or short, thick or slender; the spirals may be loosely or tightly coiled, and may have only one or two or may have many coils, and they may be flexible or stiff; but still rods, spheres, and spirals comprise all types.

In size there is some variation, though not very great.  All are extremely minute, and never visible to the naked eye.  The spheres vary from 0.25 u to 1.5 u (0.000012 to 0.00006 inches).  The rods may be no more than 0.3 u in diameter, or may be as wide as 1.5 u to 2.5 u, and in length vary all the way from a length scarcely longer than their diameter to long threads.  About the same may be said of the spiral forms.  They are decidedly the smallest living organisms which our microscopes have revealed.

In their method of growth we find one of the most characteristic features.  They universally have the power of multiplication by simple division or fission.  Each individual elongates and then divides in the middle into two similar halves, each of which then repeats the process.  This method of multiplication by simple division is the distinguishing mark which separates the bacteria from the yeasts, the latter plants multiplying by a process known as budding.  Fig. 2 shows these two methods of multiplication.

While all bacteria thus multiply by division, certain differences in the details produce rather striking differences in the results.  Considering first the spherical forms, we find that some species divide, as described, into two, which separate at once, and each of which in turn divides in the opposite direction, called Micrococcus, (Fig. 3).  Other species divide only in one direction.  Frequently they do not separate after dividing, but remain attached.  Each, however, again elongates and divides again, but all still remain attached.  There are thus formed long chains of spheres like strings of beads, called Streptococci (Fig. 4).  Other species divide first