The Fundamentals of Bacteriology

CHAPTER VII.
CHEMICAL ENVIRONMENT.

REACTION OF MEDIUM.

Most bacteria are very susceptible to changes in the degree of acidity or alkalinity of the medium in which they grow. Some kinds prefer a slightly acid reaction, some a slightly alkaline, and some a neutral (with reference to litmus as indicator). The organism which is the commonest cause of the souring of milk thrives so well in the acid medium it produces that it crowds out practically all other kinds, though its own growth is eventually stopped by too much acid. Acid soils are usually low in numbers of bacteria and as a consequence produce poor crops. The disease-producing bacteria as a class grow best in a medium which is slightly alkaline.

Accurate determination of limits have been made on but few organisms. The reaction is a most important factor in growing bacteria on artificial media (see Making of Media, Chapter XVI).

INJURIOUS CHEMICAL SUBSTANCES.

CHEMICAL COMPOSITION.

The chemical composition is subject to wide variation chiefly for two reasons: First, the cell wall in most instances seems to exert only a slight selective action in the absorption of mineral salts so that their concentration within the cell is very nearly that of the surrounding medium. Second, the chief organic constitutents vary remarkably with the kind and amount of food material available—a rich protein pabulum increases the protein, a plentiful supply of carbohydrates or of fat results in the storing of more fat, especially and vice versa. These facts must be borne in mind in considering the chemistry of bacteria.

Of the chemical elements known, only the following seem to be essential in the structure of bacteria: carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potassium, calcium, magnesium, iron, manganese. Other elements, as sodium, iodine, silicon, aluminum, lithium, copper, etc., have been reported by different analysts, but none of them can be regarded as essential, except possibly in isolated instances.

These elements exist in the bacterial cell in a great variety of combinations of which the most abundant is water. The amount of water varies in different species from 75 to 90 per cent. of the total weight in growing cells, and is less in spores. The amount of ash has been shown by different observers to vary from less than 2 per cent. to as much as 30 per cent. of the dry weight. The following table compiled from various sources will give an idea of the relative abundance of the different elements in the ash.

As to the form in which the last six elements in the table exist in the cell, little is known. The sulphur and phosphorus are essential constituents of various proteins. The high percentage of phosphorus points to nuclein compounds as its probable source.

The carbon and nitrogen, together with most of the hydrogen and oxygen not united as water, make up the great variety of organic compounds which compose the main substances in the bacterial cell.

It has already been stated that the essential structures in the bacterial cell are cell wall and protoplasm, including the nuclein. These differ markedly in chemical composition. It is well known that the cell walls of green plants consist largely of cellulose and closely related substances.7 True cellulose has been recognized in but very few bacteria. (Sarcina ventriculi, Migula; Mycobacterium tuberculosis, Hammerschlag, Dreyfuss, Nishimura; Bacillus subtilis, Dreyfuss; Acetobacter xylinum, Brown; Acetobacter acidi oxalici, Banning; and a few others.) It is certainly not an important constituent of the cell wall in many. On the other hand, hemicellulose and gum-like substances have been identified in numerous organisms of this class as important constituents of the cell wall and of the capsule which is probably an outgrowth from the latter. Practically always associated with these substances are compounds containing nitrogen. One of these has been certainly identified as chitin or a closely similar substance. Chitin is the nitrogenous substance which enters largely into the composition of the hard parts of insects, spiders and crustaceans. It is an interesting fact to find this substance characteristic of these animals in bacteria, as well as other fungi.

Though it is extremely difficult to separate the cell wall of bacteria from the cell contents, in the light of our present knowledge it can be stated that the cell walls are composed of a carbohydrate body closely related to cellulose, though not true cellulose, probably in close combination with chitin.

Of the organic constituents of the cell contents the most abundant are various proteins which ordinarily make up about one-half of the dry weight of the entire cell. The “Mycoproteid” of Nencki, 1879, and other earlier workers is deserving of little more than historical interest, since these substances were certainly very impure and probably consisted of mixtures of several “proteins” in the more recent sense.

From later studies it seems probable that substances resembling the albumin of higher forms do not occur in bacteria, at least in appreciable quantities. Globulin has been reported by Hellmich in an undetermined bacterium, but is certainly not commonly found. The larger portion of the protein is of a comparatively simple type, in fact, consists of protamins most of which are in combination with nucleic acid as nucleoprotamins. Practically all recent workers find a high percentage of nuclein, both actually isolated and as indicated by the amounts of purin bases—xanthin, guanin, adenin—obtained, as well as by the abundance of phosphorus in the ash, already mentioned. Some of these nucleins have been shown to have poisonous properties.

Closely related to but not identical with the proteins are the enzymes and toxins which are formed in the cell and exist there as endo-enzymes or endo-toxins respectively. These substances will be discussed later under the heading “Physiological Activities of Bacteria” (Chapter XII).

Carbohydrates are not commonly present in the cell contents, though glycogen has been observed in a few and a substance staining blue with iodine in one or two others. This latter substance was at first considered to be starch “granulose,” but is probably more closely related to glycogen.

Fats seem to be very generally present. The commoner fats—tri-olein, tri-palmitin, tri-stearin have been found by many analysts. The “acid-fast bacteria” are particularly rich in fatty substances, especially the higher wax-like fats. Lecithins (phosphorized fats) and cholesterins (not fats but alcohols) have been repeatedly observed and probably occur in all bacteria as products of katabolism.

Organic acids and esters occur as cell constituents but will be discussed in connection with their more characteristic occurrences as products of bacterial activity, as will also pigments which may likewise be intracellular in some instances.

The following analysis of tubercle bacilli, from de Schweinitz and Dorset, while not intended as typical for all bacteria, still illustrates the high percentage of protein compounds which undoubtedly occurs in most, as well as showing the large amount of fatty substance in a typical “acid-fast” organism: