Bacterial physiology and metabolism

Enzymes

A list of bacterial enzymes (organic catalysts) includes many of the enzymes found in mammalian tissues, as well as many enzymes not found in higher forms of life. By combining with such enzymes, many antibiotics are able to exert a selective killing or inhibition of bacterial growth without causing toxic reactions in the mammalian host. The great capability of the bacterial cell to metabolize a wide variety of substances, as well as to control to some extent the environment in which the cell lives, is reflected in its ability to form inducible enzymes. The majority of bacterial enzymes require cofactors for activity. These cofactors may be inorganic cations of organic molecules called coenzymes. See Coenzyme, Enzyme

Bacterial enzymes may be classified in numerous ways, for example, on the basis of (1) whether they are inducible or constitutive (constitutive enzymes are defined as those enzymes formed by the bacterial cell under any or all conditions of growth, whereas inducible enzymes are formed by the bacterial cell only in response to an inducer); (2) whether they are degradative (catabolic; resulting in the release of energy) or synthetic (anabolic; using energy to catalyze the formation of macromolecules); or (3) whether they are exoenzymes (enzymes secreted from the cell to hydrolyze insoluble polymers—wood, starch, protein, and so on—into smaller, soluble compounds which can be taken into the cytoplasm of the bacterium).

In addition, bacterial enzymes are involved in the transport of substrates across the cell wall, in the oxidation of inorganic molecules to provide energy for the cell, and in the destruction of a large number of antibiotics.

Many pathogenic microorganisms excrete enzymes which may play an important role in pathogenesis in some cases (see table). The α-toxin (lecithinase) of Clostridium perfringens illustrates a highly active enzyme which is responsible for the necrotizing action associated with gas gangrene infections due to this microorganism. Streptococcus pyogenes excretes hyaluronidase which degrades ground substance (polymer of hyaluronic acid), and streptokinase, which activates plasmin resulting in a system that lyses fibrin. Other examples include coagulase of the Staphylococcus, which activates clotting of plasma, urease of Proteus vulgaris, which splits urea to ammonia and carbon dioxide, and collagenase of Clostridium, which hydrolyzes collagen. See Diphtheria, Staphylococcus

Many bacteria are able to synthesize enzymes which will hydrolyze or modify an antibiotic so that it is no longer effective. Essentially all of these enzymes are coded by DNA that exists in bacterial plasmids. As a result, the ability to produce enzymes which destroy antibiotics can be rapidly passed from one organism to another either by conjugation in gram-negative organisms or by transduction in both gram-negative and gram-positive bacteria.

Bacterial catabolism

Bacterial catabolism comprises the biochemical activities concerned with the net breakdown of complex substances to simpler substances by living cells. Substances with a high energy level are converted to substances of low energy content, and the organism utilizes a portion of the released energy for cellular processes. Endogenous catabolism relates to the slow breakdown of nonvital intracellular constituents to secure energy and replacement building blocks for the maintenance of the structural and functional integrity of the cell. This ordinarily occurs in the absence of an external supply of food. Exogenous catabolism refers to the degradation of externally available food. The principal reactions employed are dehydrogenation or oxygenation (either represents biological oxidation), hydrolysis, hydration, decarboxylation, and intermolecular transfer and substitution. The complete catabolism of organic substances results in the formation of carbon dioxide, water, and other inorganic compounds and is known as mineralization. Catabolic processes may degrade a substance only part way. The resulting intermediate compounds may be reutilized in biosynthetic processes, or they may accumulate intra- or extracellularly. Catabolism also implies a conversion of the chemical energy into a relatively few energy-rich compounds or “bonds,” in which form it is biologically useful; also, part of the chemical energy is lost as heat.

Bacterial intermediary metabolism relates to the chemical steps involved in metabolism between the starting substrates and the final product. Normally these intermediates, or precursors of subsequent products, do not accumulate inside or outside the bacterial cell in significant amounts, being transformed serially as rapidly as they are formed. The identification of such compounds, the establishment of the coenzymes and enzymes catalyzing the individual reaction steps, the identity of active forms of the intermediates, and other details of the reaction mechanisms are the objectives of a study of bacterial intermediary metabolism.

Many bacteria are able to decompose organic compounds and to grow in the absence of oxygen gas. Such anaerobic bacteria obtain energy and certain organic compounds needed for growth by a process of fermentation. This consists of an oxidation of a suitable organic compound, using another organic compound as an oxidizing agent in place of molecular oxygen. In most fermentations both the compounds oxidized and the compounds reduced (used as an oxidizing agent) are derived from a single fermentable substrate. In other fermentations, one substrate is oxidized and another is reduced. Different bacteria ferment different substrates. Many bacteria are able to ferment carbohydrates such as glucose and sucrose, polyalchohols such as mannitol, and salts of organic acids such as pyruvate and lactate. Other compounds, such as cellulose, amino acids, and purines, are fermented by some bacteria.

Bacterial anabolism

Bacterial anabolism comprises the physiological and biochemical activities concerned with the acquisition, synthesis, and organization of the numerous and varied chemical constituents of a bacterial cell. Clearly, when a cell grows and divides to form two cells, there exists twice the amount of cellular components that existed previously. These components are drawn, directly or indirectly, from the environment around the cell, and (usually) modified extensively in the growth processes when new cell material is formed (biosynthesis). This build-up, or synthesis, begins with a relatively small number of low-molecular-weight building blocks which are either assimilated directly from the environment or produced by catabolism. By sequential and interrelated reactions, they are fashioned into different molecules (mostly of high molecular weight, and hence called macromolecules), for example, lipids, polysaccharides, proteins, and nucleic acids, and many of these molecules are in turn arranged into more complex arrays such as ribosomes, membranes, cell walls, and flagella. Other typical anabolic products, of lower molecular weight, include pigments, vitamins, antibiotics, and coenzymes. The enzymes responsible for the sequential reactions in any one biosynthetic pathway or assembly sequence are often located on or in cellular structures and thus in physical proximity to the preceding and succeeding enzymes, and their products, and to the site(s) where cellular structures are to be formed. Anabolism also includes the transport of molecules into cells, of building blocks to reaction sites, energetic activations, and the transfer and incorporation of the finished products to their ultimate sites in or outside the cell.