What Are Bacteria?
( Originally Published 1897 )
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 (Fig. I).
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 p. to 1.5 N, (0.000012 to 0.00006 inches). The rods may be no more than 0.3 µ in diameter, or may be as wide as 1.5 p. to 2.5 µ, and in length rods may be no more than 0.3 µ in diameter, or may be as wide as 1.5 p. to 2.5 µ, 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 micro-scopes 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 in one direction, then at right angles to the first division, and a third division follows at right angles to the plane of the first two, thus producing solid groups of fours, eights, or sixteens (Fig. 5), called Sarcina. Each different species of bacteria is uniform in its method of division, and these differences are therefore indications in species, or, according to our present method of classification, the different methods of division represent different genera. All bacteria producing Streptococcus chains form a single genus Streptot-metes, and all which divide in three division planes form another genus, Sarcina, etc.
The rod-shaped bacteria also differ somewhat, but to a less extent. They almost always divide in a plane at right angles to their longest dimension. But here again we find some species separating immediately after division, and thus always appearing as short rods (Fig. 6), while others remain attached after division and form long chains. Sometimes they appear to continue to increase in length without showing any signs of divis ion, and in this way longthreads are formed (Fig. 7). These threads are, however, potentially at least, long chains of short rods, and under proper conditions they will break up into such short rods, as shown in Fig. 7 a. Occasion-ally a rod species may divide lengthwise, but this is rare. Exactly the same may be said of the spiral forms. Here, too, we find short rods and long chains, or long spiral filaments in which can be seen no division into shorter elements, but which, under certain conditions, break up into short sections (Fig. 8).
RAPIDITY OF MULTIPLICATION.
It is this power of multiplication by di-vision that makes bacteria agents of such significance. Their minute size would make them harmless enough if it were not for an extraordinary power of multiplication. This power of growth and division is almost incredible. Some of the species which have been care-fully watched under the microscope have been found under favourable conditions to grow so rapidly as to divide every half hour, or even less. The number of offspring that would result in the course of twenty-four hours at this rate is of course easily computed. In one day each bacterium would produce over 16,500,000 descendants, and in two days about 281,500,000,000. It has been further calculated that these 281,500,000,000 would form about a solid pint of bacteria and weigh about a pound. At the end of the third day the total descendants would amount to 47,000,000,000,000, and would weigh about 16,000,000 pounds. Of course these numbers have no significance, for they are never actual or even possible numbers. Long before the offspring reach even into the millions their rate of multiplication is checked either by lack of food or by the accumulation of their own excreted products, which are injurious to them. But the figures do have interest since they show faintly what an unlimited power of multiplication these organisms have, and thus show us that in dealing with bacteria we are dealing with forces of al-most infinite extent.
This wonderful power of growth is chiefly due to the fact that bacteria feed upon food which is highly organized and already in condition for ab-sorption. Most plants must manufacture their own foods out of simpler substances, like carbonic dioxide (CO2) and water, but bacteria, as a rule, feed upon complex organic material already pre-pared by the previous life of plants or animals. For this reason they can grow faster than other plants. Not being obliged to make their own foods like most plants, nor to search for it like animals, but living in its midst, their rapidity of growth and multiplication is limited only by their power to seize and assimilate this food. As they grow in such masses of food, they cause certain chemical changes to take place in it, changes doubtless directly connected with their use of the material as food. Recognising that they do cause chemical changes in food material, and remembering this marvellous power of growth, we are prepared to believe them capable of producing changes wherever they get a foothold and begin to grow. Their power of feeding upon complex organic food and producing chemical changes therein, together with their marvellous power of assimilating this material as food, make them agents in Nature of extreme importance.