Size

Bacteria are so small (no larger than 1/50,000 of an inch*) that the highest magnification of the ordinary compound microscope must be used to study them. Cocci range from 0.4 to 2 µtn. (/j,) in diameter. The smallest bacillus is about 0.5 µm. in length and 0.2 µmL in diameter. The largest pathogenic bacilli are seldom greater than 1 µm. in diameter and 3 µm. is length; the average diameter and length of pathogenic bacilli are 0.5 and 2 µm. respectively. Nonpathogenic bacilli may be larger, reaching a diameter of 4 µm. and a length of 20 µm. The spirilla are usually narrow organisms from 1 to 14 µm. in length. Different species of bacteria show great variation in size, and there is some variation within a species, but as a rule the size of each species is fairly constant. With the use of the microscope the different shapes of bacteria can be better compared.

Structure

Bacteria are minute. They are slightly refractile that unless stained with dyes, they are difficult to see even, with the compound light microscope. When stained, they appear homogeneous or slightly granule. With the electron microscope, however, microbiologists can visualize minute details of bacterial structure

Cell wall

The shape of the bacterial cell is maintained by a cell wall. This cell wall is rigid. The protoplasmic substance of bacteria exerts such a high osmotic pressure, equivalent to that of a 10% to 20% solution of sucrose, that in ordinary environments, were it not for the high tensile strength of the cell wall, the bacterial cell would burst. If the bacterial cell is placed in a suitable hypertonic medium and the cell wall dissolved, the remainder of the bacterium is converted into a spherical protoplast. In an isotonic environment prootoplasts remain viable and grow.

The stability of the cell wall is derived from its chemical makeup, residing in the layer composed of a single, giant, complex molecule of peptidoglycan (murein, mucopeptide). Peptidoglycan is composed of a backbone of alternating amino sugars, N-acetylglucosamine and N-acetylmuramic acid; a set of identical tetrapeptide side chains attached to the N-acetyllmuramic acid, and a set of identical peptide cross bridges. For all bacterial species, that backbone is the same, but the side chains and the cross bridges vary from species to species.

The chemical structure of the cell wall is different In gram-positive bacteria from what it is in gram negative ones, a difference expressed .in their diverse Pam-staining reactions. One variation lies in the peptoglycan layer. In gram-positive bacteria, this layer consists of concentric sheets cross-linked chemically in three dimensions. However, in gram-negative ones the peptidoglycan layer forms two-dimensional monolayers. Still another difference is that most gram-positive cell walls contain teichoic acids in large amounts, even as much as 10% of the dry weight of the total cell; gram-negative cell walls contain none. Teichoic acids probably lie on the outer surface of the peptidoglycan layer and participate in the supply of magnesium to the cell by binding magnesium ions.

In the gram-negative cell wall, outside the peptidoglycan layer, are three polymers: lipoprotein, outer membrane, and lipopolysaccharide. The lipoprotein molecules cross-link the outer membrane with the peptidoglycan layer. A typical phospholipids bilayer, the outer membrane is relatively permeable to small molecules but hinders the penetration of larger ones. This discriminatory capacity in the outer membrane is believed to explain the relative resistance of gram-negative bacteria to certain antibiotics. Tightly bound to the outer membrane and the cell surface, the lipopolysaccharide consists of a complex lipid (lipid A) to which is attached a polysaccharide. This polysaccharide as the 0 antigen represents a major surface antigen of the bacterial cell. It is made up of a constant chemical core and a terminal series of repeat units, a repeat unit being unique to each gram- negative species. Lipopolysaccharide is very toxic to animals, is released only with lysis of the bacterial cells, and is the endotoxin of gram-negative microorganisms.

The viability of bacteria is directly dependent on the integrity of the cell wall. The complex chemical sequence of events, for the biosynthesis of the cell wall provides several possibilities for interference by antimicrobial compounds. Any such compound that inhibits any step can cause the wall to be weakened and consequently the cell to lyse. The sites of action are well known for certain antibiotics that inhibit cell wall synthesis.

Plasma Membrane

The cell wall is so narrow that it cannot be seen with the ordinary compound light microscope. In ultrathin sections it is revealed by the electron microscope as a well-defined structure surrounding a distinct layer, the cell membrane or plasma membrane which separates it from the cytoplasm of the bacterial cell. Composed of phospholipids and proteins, the plasma membrane is the site of important enzyme systems, including the respiratory enzyme system (cytochrome enzymes). In fact, in bacteria it corresponds to the mitochondria of higher organisms. In regulating the passage of food materials and metabolic by-products between the interior of the cell (where metabolic activities are carried on) and the surroundings, it functions osmotically both as a barrier and as a link. It blocks the entry of certain substances and catalyzes the transport of others into the cell.

Capsule

Surrounding many bacteria is a mucilaginous envelope or capsule Indistinct in most bacteria, it is well developed in few Streptococcus pneumoniae, Clostridium perfringerts, and Klebsiella pneumoniae). The capsule is formed by an accumulation of slime excreted by the bacterium. This material is usually a complex polysaccharide. If it is present about the cell in only small amounts, a distinct capsule does not appear.
Capsule formation is most prominent in organisms taken directly from the animal body, for when grown on artificial media, the same organisms often lose their ability to form capsules. A capsule does not stain with the ordinary bacteriologic dyes but may appear as a clear halo around the bacterium, even two or three times broader than the bacterium. It is stained by special methods. The presence of a capsule appears to increase the virulence of an organism by protecting it against phagocytosis, and in some cases the capsule gives the organism its specific immunologic nature. For instance, relative to the nature of their capsules, pneumococci are divided into at least 82 types. The specific antigenic nature of a capsule depends on its carbohydrate content.

Granules

Within some bacteria (such as Corynebacterium diphtheriae) at the ordinary magnification of the light microscope is seen granules that stain more deeply than the remainder of the cell. Known as metachromatic granules and enzymatically active, they are reserves of inorganic phosphate stored as polymerized metaphosphate (volutin). Metachromatic granules may be arranged irregularly within the bacterial cells or located in one or both ends of the cell where they are known as polar bodies. Sulfur-oxidizing- bacteria convert excess hydrogen sulfide from the environment into intracellular granules of elemental sulfur. At times, carbon source materials on reserve, converted by some bacteria to osmotically inert, neutral polymers, are stored in their cytoplasm as insoluble granules, available if needed.

Electron microscopy reveals a dense packing of ribosomes in bacterial cytoplasm and the presence of spherules and various submicroscopic granules. Ribosomes are tiny, uniform, beadlike granules found free in the cytoplasm of a cell and are rich in ribonucleic acid. Ribonucleic acid (RNA) is one of the two nucleic acids of physiologic significance, the other being deoxyribonucleic acid (DNA). RNA is similar to DNA chemically except that its sugar is a different pentose-ribose, one of its nucleotide bases is uracil instead of thvmine, and its chemical pattern is laid out as a single coil, not a double one. The ribosomes play an important role in protein synthesis. The submicroscopic granules are known to be biochemically complex and active; they may be an integral basic unit of the bacterial cell with a definite role in cell metabolism.

No relation exists between these various granules and the ability of the bacteria containing them to produce disease.