Bacteria--Small and Mighty

A. Y. Riker, A. C. Hildebrandt.

Bacteria are an important part of the world we live in. They cause diseases in man such as tuberculosis, typhoid fever, and diphtheria, and in animals such as anthrax, brucellosis, and swine erysipelas.

Many bacteria are helpful to man. They produce useful food and chemicals, assist in the decomposition of wastes, and increase the fertility of soil. Most of us know about the nitrogen-accumulating root nodules of clover, alfalfa, and other legumes that are produced by bacteria growing in their roots.

Bacteria also cause disease in plants. The discovery of this role was made only 75 years ago. After Louis Pasteur proved that bacteria could produce animal disease, Professor T. J. Burrill. of Illinois, who studied in Europe, was filled with enthusiasm. He began working with a devastating disease of unknown cause that was sweeping through pear and apple orchards of the Midwest. Burrill soon proved that the disease, which we now call fire blight, was caused by bacteria. Burrill's brilliant pioneer discovery swept aside earlier speculation and ignorance. It pointed the way.

But Burrill's work did not go unchallenged. Many doubters spoke up. Dr. Erwin F. Smith and his associates of the United States Department of Agriculture carried the work forward meanwhile. They overcame the opposition, scorn, and derision of some distinguished scientists. They proved that bacteria caused many plant diseases. Such researches provided a solid background for later work on plant bacteria. In his book, Bacterial Diseases of Plants, in 1920 Dr. Smith summarized much of this work. Now we have knowledge of more than 170 different kinds of bacteria, which cause diseases in flowering plants belonging to 150 genera of 50 families.

Among living agents that cause plant disease, bacteria are perhaps the smallest (if we do not consider viruses as living). Such bacteria are so minute that about 25,000 laid side by side and 8,000 to 12,000 laid end to end would not measure more than an inch.

Each bacterial cell is an independent plant. If many cells adhere to one another, a mass of cells may be formed. But each cell acts independently; the numbers of bacterial cells increase when the cells cut themselves into two. They multiply by dividing, so to speak. Under favorable circumstances reproduction by cell division may happen as often as three times in an hour, and enormous numbers of bacteria may he produced within 2 or 3 days.

The large surface area of each bacterial cell and the myriad bacteria usually present in diseased plants give the invading bacteria a great advantage as they attack the cells of the affected plant. That helps to explain the rapid progress a bacterial disease makes under favorable conditions.

Their survival depends upon their ability to utilize the living or dead organic compounds they find in their host plants.

The bacteria that live only on dead animal or plant remains are termed saprophytes. Those that produce disease are parasites or pathogens. Many kinds of bacteria, especially those that cause plant disease, can live either as parasites or as saprophytes. Many disease-inciting bacteria are able to over-winter or maintain themselves between successive susceptible crops by living saprophytically on plant refuse.

Bacteria that induce plant disease are all more or less short and cylindrical. They are described as rodlike. None is spherical like the coccus forms which cause some animal or human diseases. Some species of plant parasitic bacteria have one to several filamentous, or hairlike, motile appendage, called flagella, which they can wave or vibrate and by them move for short distances in water or in plant juices Some kinds have flagella at one end or both ends (polar). Still others develop flagella at many places on the surface of the cell (peritrichous). Even those that have no flagella, however, can be carried rapidly from one place to another by flowing or splashing water, or by wind-blown droplets of moisture, by insects, and by various agricultural operations. Any agricultural practice that involves the transport of soil may serve to carry pathogenic bacteria from one place to another.

Bacteria gain entrance to plants through uninjured tissue, natural openings, and wounds. The potato scab organism usually enters through lenticels but can penetrate thin-skinned tubers directly. Root-nodule bacteria of legumes enter through the fragile root hairs. The small natural openings to the atmosphere the stomata in the leaves are the ports of entrance for the bacteria that cause angular leaf spot of cotton. The bean blight organism may also enter through stomata. The bacteria that cause blackleg of cabbage enter through hydathodes. Hydathodes, if present, usually are found at the edge or tip of the leaf. They are specialized gland cells that excrete fluids. Often under highly humid conditions small droplets of water may be seen adhering to the margins of such leaves as cabbage or wheat. As these droplets later may be resorbed by the leaf, any bacteria Present may also gain entrance and multiply. The bacteria that cause fire blight of pear, apple, and quince trees enter at blossom time through the specialized cells of the flower that Produce nectar. This type of environment is especially suited to the growth and multiplication of the parasites. Many kinds of bacteria enter their host plants through wounds. Those producing soft rots of fruits and vegetables and the species causing crown gall of several kinds of plants are good examples.

Once bacteria get into a plant, some may be carried along with or move in the sap stream. Others may move short distances in plant juices by swimming or be pulled about by the movement of fluids between or in the cells. Capillary attraction and changes in the pressures and tensions on fluids that sometimes flood the spaces between the cells move the bacteria from place to place inside the plant. Flooding of plant tissues often increases as certain bacteria withdraw liquid from plant cells bordering the invaded tissue. In early stages of disease, bacteria commonly develop in the spaces between the cells. But as cell walls are injured and the cells of the plant are killed, they become perforated by bacterial action. Then the bacteria may penetrate inside the cells and continue the disintegration.

PLANTS RESPOND in many ways to invasion by bacteria. Their response often is so specific that the disease they cause can be identified by the symptoms. Among the symptoms of bacterial infections are galls, wilts, slow growth, dwarfing, imperfect fruits or ears, rots, color changes of various plant parts, retarded ripening, distortion of leaves, cankers, brooming, fasciation, and leaf spots. Rots may be either localized at one place or in one tissue or may involve the whole plant. Wilts generally affect the whole plant. Galls often affect only a part of the plant.

Among the many well-known bacterial galls are olive knot, cane gall, beet pocket rot, sweet pea fasciation, hairy root, and crown gall. All contain large swollen cells and small, rapidly dividing cells along with vascular cells in a relatively disorganized arrangement. Eventually these gall structures may interfere with the normal transmission of water and food supplies, and the plants may wilt and die. Of these, crown gall, which has a very wide host range, has been studied extensively because of the opportunity it provides for clarifying basic principles of diseased growth as a biological phenomenon. The legume root nodules are usually beneficial through their ability to fix nitrogen. They have great economic importance.

Bacterial wilts may be quite destructive for example, in sweet corn, cucumber, tobacco, and related plants. Such bacteria may produce a slime, which helps to plug the water-conducting tissue of the invaded plant. Closely related are such diseases as black rot of cabbage, ring rot of potatoes, and tomato canker, which may start in the water-conducting tissue but subsequently result in disintegration of surrounding tissue.

Cankers develop from the extensive tissue destruction, for example, by the fire blight bacteria, or from the lesions of the tomato canker organism. This latter bacterium may produce only local spots on the tomato fruits. Thus the symptoms induced by one organism may be quite different, depending on the plant tissue infected and other variables.

Local spots occur most commonly on the leaves, but sometimes appear elsewhere, as, for example, on many fruits. Symptoms of black arm of cotton show when the angular leaf spot bacteria enter the stem and girdle it. Bacterial blight of beans, halo blight of oats, potato scab, and many others appear primarily as local spots. The bacteria causing halo blight of oats and wildfire of tobacco produce toxic substances that are responsible for the yellowish areas immediately around the dead spots where the bacteria have invaded the tissue.

Soft rots develop in relatively fleshy tissues when certain bacteria invade them extensively. Such bacteria produce an enzyme that dissolves the pectic substance that cements plant cell walls together. The result is a slimy, often foul-smelling mass. The cell-wall-dissolving enzymes and toxins often destroy cells and tissues a short distance ahead of the bacteria that produce them. The soft rots often follow and extend invasion and damage by some other pathogen. For example, black rot of cabbage and late blight of potatoes would be much less serious except for the subsequent soft rot.

Symptoms of disease appear at varying lengths of time after bacteria attack and grow in a plant. Soft rots are sometimes evident within a day or so, angular leaf spot of cotton within 10 days, corn wilt within 1 to 2 months. Crown gall of orange may take 2 years. The time between first establishment of bacteria or fungi in the plant and the appearance of symptoms of disease is called the incubation period.

In some diseases bacterial ooze comes to the surface of affected plant parts. The exudate is often slimy and sticky and contains numberless bacteria. The ooze may come out of stomata or other natural openings, or may form on the surface of cankers or other lesions.

WE RECOGNIZE three major variables that influence the eventual severity of an outbreak of disease the host plant, the pathogen, and environmental conditions. They form an eternal triangle, each affecting the other within certain limits.

The variables among the host plants are substantial and important. They make possible selection and breeding for disease resistance. The origin of disease-resistant breeding material, plant structures that affect resistance, and control of disease through the use of disease-resistant plants are discussed in detail in other articles in the Yearbook. All ages of plants are affected from the seedling stage through to maturity. Fruits and seeds may be attacked. Juiciness of tissue, however, may predispose a plant to severe attacks by bacteria.