INTERRELATIONS

Symbiosis

Certain species of bacteria grow well together, and the associated species accomplish harm¬ful or beneficial results that neither does alone. For instance, the staphylococci and influenza bacilli that can be seen using a compound binocular microscope, multiply more rapidly when grown together than either does when grown alone. This is known as synergism.

Symbiosis refers to the relation of mutual benefit existing between two organisms. For example, there is the beneficial relation between the leguminous plants and the nitrogen-fixing bacteria living in the root nodules of these plants. Commensalism, on the other hand, is the term applied when two organisms live together with benefit to one and no effect to the other. The organisms comprising the normal flora of the different body areas are generally considered to be commensals.

Antagonism

Sometimes the presence of certain species of bacteria inhibits the growth of others. For instance, growth of the gonococcus is inhibited by the presence of almost any other species of bacteria. This is known to be antagonism. Theories put forth to explain antagonism as one organism that secretes a substance toxic to the growth of the other. The one organism promotes a defense mechanism of the animal body against the other. The appearance of certain infections after the administration of antibiotics may be explained in terms of an antagonistic relationship between two organisms, only one of which succumbs to the action of the antibiotic. Released by the antibiotic, the other microorganism, which can be observed under a compound binocular microscope, then becomes aggressive.

Biologic activities

Bacteria engage in a complex of biologic activities of varying importance. Of prime consideration are those relating to the growth and integrity of the microorganism. Those resulting in the elaboration of toxins and substances harmful to other living cells have special significance in the production or aggravation of disease. Other events in the life of the bacterial cell that can be observed under a compound binocular microscope, although striking in their manifestations, are not vital but are of secondary importance.

MAJOR METABOLIC EVENTS

Bacteria first drew attention to their very existence by the dramatic changes resulting from their meta¬bolic activities. Today their biochemical capabilities are obvious when one considers the phenomena of fer¬mentation, putrefaction, decay, soil fer¬tility, and infectious disease.

Much of the general body of information concern¬ing metabolism in creatures more complex than bac¬teria has come from studies of comparable processes in bacteria and other microorganisms. Because bac¬teria multiply rapidly, we can learn much about their activities in 2 days or less with the help of a compound binocular microscope. To obtain comparable information about man would require 200 years or more.

The bacterial cell makes two biologic demands of its environment. First, it must obtain therein the chemical ingredients with which it can build and maintain it¬self. Second, at the same time it must derive there from the energy necessary to do its work. Metabolism encom¬passes all biochemical reactions occurring within the bacterial cell by which these two requirements are met.

Enzymes

Enzymes as organic catalysts are es¬sential, very efficient protein ingredients in the com¬plex maze of metabolic activity. Life is not possible without them of the multitude of proteins built by living cells, the majority function as enzymes, and over a thousand different enzymes have been identi¬fied among living cells.

Bacteria produce enzymes that are remarkably like those produced by the higher forms of life. A bac¬terium contains, on an average, about 2000 to 3000 enzymes. Elaboration of enzymes is under the direc¬tion of the DNA of the bacterial nuclear apparatus. For 2000 to 3000 enzymes there would be a correspond¬ing 2000 to 3000 control positions or genes on the bac¬terial chromosome. This is known as the one gene-one enzyme hy¬pothesis. Since the biochemical reactions occurring in bacteria are complicated and interrelated, bacterial enzymes may function in a group as an enzyme sys¬tem, with several enzymes being responsible for close¬ly related chemical changes.

Chemosynthesis

The plant cell, possessing chlorophyll, obtains energy from the environment in the form of light. This is what is commonly called photosynthesis. Microorganisms with no chlorophyll must gain their energy from the chemi¬cal alteration of the substances at hand, a process termed chemosynthesis. Because of the complexity of their metabolic enzymes, most bacteria can break down the proteins, fat, carbohydrates, and other or¬ganic compounds they contact. A series of enzymatic steps characterizes the chemical changes involved in chemosynthesis. The starting point is bacterial diges¬tion. The end point is biologic oxidation.

DIGESTION

The principal sources of energy for bac¬teria in their environment are mostly complex mole¬cules too large for the bacterial cell to deal with di¬rectly. They must be reduced to workable particle size. To accomplish this, bacteria rely on certain enzymes referred to as hydrolases that they release into the medium to split the large molecules chemically by a process that includes the addition of water. This is hydrolysis. The hydrolytic enzymes are examples of exoenzymes.

Briefly, digestion is the exoenzyme-catalyzed hydroly¬sis of a large molecule to secure fragments small enough for passage into the bacterial cell.

ABSORPTION

After the bacterial cell has reduced the large molecules into smaller particles, it must next absorb them. There are two ways of doing this. Be¬cause of the osmotic pressure gradient, the cell can passively allow the molecules to move in by diffusion, a slow’ and inefficient way at best. On the other hand, the cell can actively participate to speed the move¬ment. The manner in which this is brought about is referred to as active transport. As would be expected, the mechanisms of active transport, including the en¬zymes required, are located on the cell membrane, and their activity constitutes a “pump.”

OXIDATION

Some of the molecules that the cell pulls into itself from without are ready for the se¬quences of biologic oxidation. Most are not, and these must be prepared, that is, changed into a form that can be oxidized. If a phosphate group is attached to the molecule (phosphorylation), a very significant chemical bonding is made. When this phosphorylated compound is oxidized, the chemical bond of the phos¬phate group traps energy not only in a form suitable for later use but also in large amounts. Phosphoryla¬tion is the most important preliminary sequence to oxidation, but other chemical reactions may be en¬countered at this step.

Biologic oxidation is defined as the chemical setup whereby a cell is provided with energy that it can use biologically. The complicated reactions contributing to it, many of them chain reactions, are carried out notably by en¬zymes known as oxidases, dehydrogenases, or oxidoreduc¬tases, coenzymes,* and hydrogen acceptors or car¬riers. Oxidases act on a substrate to cause it to under¬go oxidation, that is, a reaction wherein electrons are removed from a substrate with accompanying hy¬drogen ions. Enzymes making possible the removal of hydrogen or electrons are called dehydrogenases. As the hydrogen ions are removed, there must be a suit¬able compound to receive them, a hydrogen acceptor or carrier, which in so accepting becomes reduced. Hydrogen acceptors or carriers, which can be oxidized or reduced, function in the transport of hydrogen or electrons from tissue metabolites to oxygen or some other end product. Transfer of electrons constitutes oxidation and reduction, and the end result must include oxidized product, a reduced one and energy liberated or trapped.

Storage of energy

There is no dissipation of heat in biologic oxidation. The energy released is held in the chemical bond between an organic molecule and a phosphate group. Bacteria store this energy-rich bond solely on the organic molecule adenosine diphosphate, ADP. When a phosphate group is transferred to ADP, there is formed adenosine triphosphate, ATP, from which energy-rich reserves are available.