It has never been easy to find satisfactory criteria for defining the group of microorganisms which is regarded as “bacteria”. Most bacteria are very tiny, far smaller than the individual cells in the body of a multi-cellular plant or animal. In fact, some bacteria are as small as some of the largest viruses. However, even the smallest bacteria are fundamentally different from viruses in that they are cellular and they always contain both RNA and DNA.
There are many types of true bacteria or eubacteria. Most of which are small, unicellular organisms of various shapes equipped with rigid cell walls and often capable of locomotion in a liquid medium. Many unicellular true bacteria are from 0.5 to 1.5 microns in diameter and not more than 10 microns in length. A few organisms have considerably larger dimensions, but are also considered since they can be linked to the smaller bacteria.
Three basic forms of bacteria occur:
(1) spheres (cocci),
(2) cylinders with squared or rounded ends sometimes-assuming an almost elipsoidal shape (bacilli), and
(3) curved rods consisting of a single half turn (vibrios) or one to several complete helices (spirilla).
Motility is probably universal in the curved eubacteria, common in the cylindrical forms, and extremely rare in the spherical ones. Its occurrence is associated with the presence of locomotor orginals known as flagella which are very fine hair-like structures.
Multiplication of bacteria is accomplished by binary transverse fission. Sometimes cells remain attached after division forming small and characteristically arranged aggregates. Thus, spherical eubacteria may occur as chains (streptococcus type), as tetrads, or as cubical packets (sarcina type). Chain formation also commonly occurs in certain rod shaped eubacteria, notably some of those that form endospores.
A highly characteristic resting structure known as an endospore is produced by many true bacteria. One endospore arises in each vegetative cell by a process of free cell formation during which part of the cytoplasm is surrounded and cut off from the rest of the highly refractal and impermeable wall.
The endospore may then be liberated by the disintegration of the remaining cytoplasm and the surrounding vegetative cell wall. The formation of endospores is fairly common in rod shaped eubacteria.
Some true bacteria are capable of secreting extracellular structures known as capsules, stalks and sheaths. Capsule formation is quite common and consists in the deposition about the cell of a slime layer which may extend for a distance several times the diameter of the cell itself. The extent or physical sharpness and permanence of this slime layer are exceedingly variable being conditioned by such factors as the properties of the capsular arterial (in other words, its solubility in water) and the kind and amount of nutrients available.
Stalk formation is also probably not uncommon, particularly in habitats that are low in dissolved nutrients. Most simple stalked eubacteria (caulobacter type) consists of single cells attached at one end to the substrate by a hold fast or short stalk and equipped at the other end with one or more polar flagella. There is some evidence to suggest that organisms of the caulobacter type are free swimming in environments rich in nutrients and become cessal only when the supply of nutrients falls to a low level.
The majority of unicellular, true bacteria are non-photosynthetic, but there are a few photosynthetic forms. These green and purple eubacteria are the only organisms commonly regarded as bacteria that are characterized by a photosynthetic type of metabolism.
Like the blue-green algae, they show no microscopically observable localization of photosynthetic pigments within the cell (chloroplasts are absent).
The two major assemblages for that are used for the general designations of “swimming bacteria” and “gliding bacteria” include the bulk of the bacteria. Outside these designations, there remain a few small groups, which for one reason or another, cannot be fitted satisfactorily into either major category, yet have properties that allow them to be regarded as bacteria. Of these, the most important are the spirochaetes.
Spirochaetes are unicellular organisms with highly flexible spiral cells. The spirochaetal cell is extremely slender (sometimes no more than 0.3 microns in diameter). They can be divided into two subgroups which are distinguished on the basis of the length of the cell.
Cells of the smallforms (leptospiro type) rarely exceed 10 microns in length, whereas cells of large forms (spirochaeta type) may attain lengths of several hundred microns.
Spirochaetes are motile in a liquid medium and there is good evidence for the occurrence of flagella on some of the small cell forms. In spite of this, their slenderness and above all, their flexibility can easily serve to distinguish them from spiral eubacteria.