Soil Organisms and Disease

by SELMAN A. WAKSMAN

FEW OF THE bacteria and other micro-organisms that cause human and animal diseases survive long in the soil. Once introduced, they are inhibited or killed by antagonistic organisms, which produce active chemical substances known as antibiotics.

This purifying effect of the soil has been recognized since the early days of modern bacteriology, that is, since about the latter part of the nineteenth century. But because antibiotics vary greatly in chemical nature, selective antibacterial properties, toxicity to animals, and activity, it is only in recent years that a systematic attempt has been made to use them to control infections in man and animals. The most promising antibiotics now known are penicillin, tyrothricin, and streptomycin.

Only comparatively few groups of soil organisms have so far been examined for their antibacterial potentialities. Nothing is known yet of the ability of soil organisms to combat viruses and other infectious agents for which no adequate methods of control are now available. The field for further investigation is broad, therefore, and offers promise of potential practical developments.

Pasteur was the first to observe, in 1877, that the presence of certain common bacteria" in the culture of an anthrax organism brought about considerable modification of the pathogenic properties of the latter. He suggested, therefore, the possibility of utilizing "common bacteria" for therapeutic purposes. When soon afterward, in 1881, the gelatin-plate method for counting and isolating bacteria was introduced by Koch, the attention of the medical bacteriologist was focused upon the soil as a habitat of disease-producing bacteria and as a possible source of infections and epidemics. The soil was analyzed for the total number of bacteria and for the presence of organisms capable of causing infections and epidemics. The results obtained were, however, entirely negative; they did not justify the fear that human pathogens may multiply or even remain in a viable state for any length of time in the soil. On the contrary, it soon became established that the great majority of bacteria causing the common infections, such as diphtheria, anthrax, typhoid, cholera, dysentery, undulant fever, various staphylococci, and even the tuberculosis organism rapidly disappears from the soil.

More detailed and more recent investigations of the problems of soil pollution by intestinal infections revealed that the rate of destruction of typhoid, dysentery, and colon bacteria in the soil depends upon a number of factors. Chief among them are the moisture content of the soil and its reaction, and the nature and abundance of its microbiological population.

In most soils, a large proportion of such disease-producing bacteria was found to die out within 10 days. It was also found that various other pathogenic organisms that cause some of the most deadly human and animal scourges, namely, leprosy, pneumonia, bubonic plague, influenza, cattle mastitis, cattle abortion, and many human and animal virus infections, that constantly find their way into the soil in large numbers, also disappear there sooner or later.

As a result of these and numerous other studies, no one now ever raises the question concerning the role of the soil as a carrier of the great majority of disease-producing organisms or as the cause of severe or even of minor epidemics and infections. One must, of course, exclude from consideration certain spore-forming bacteria, responsible for such infections as tetanus, gas gangrene, and anthrax, or capable of producing toxic substances when they find their way into improperly sterilized foods, such as botulinus. These organisms are of only minor importance and are readily subject to control.

It was quite logical, therefore, that the question should have been raised as to what becomes of all the bacteria excreted by infected individuals. This was expressed by the writer and H. B. Woodruff in 1940 as follows: "If one considers the period for which animals and plants have existed on this planet and the great numbers of disease-producing microbes that must have thus gained entrance into the soil, one can only wonder that the soil harbors so few bacteria capable of causing infectious diseases in man and in animals. One hardly thinks of the soil as a source of epidemics."

Several theories have been proposed in order to explain the rapid disappearance. from the soil of disease-producing bacteria and other micro-organisms. The theories can be grouped into six categories:

· The soil offers an unfavorable environment for the growth of disease-producing micro-organisms.

· The soil is lacking in a sufficient or in a proper food supply for the growth of such micro-organisms.

· The disease-producing bacteria are destroyed in the soil by various predaceous agents, such as protozoa and other animals.

· The bacteria are destroyed in the soil by specific bacteriophages.

· The soil-inhabiting micro-organisms acting as antagonists are responsible for the destruction of the pathogenic bacteria and other organisms that find their way into the soil.

· These antagonists form specific toxic or antibiotic substances which destroy the pathogenic bacteria, the viruses, and the other disease-producing agents.

It has gradually become recognized that the effectiveness of the soil as a purifier depends upon its nature, to a considerable extent. Actually, the soil exerts a double action upon the contaminating bacteria. In the first place, it removes them by physical adsorption. Heavy foams or clay soils, incidentally, are far more efficient in removing the bacteria from sewage or from contaminated soils than light, sandy, porous soils. Secondly, soil removes bacteria by biological destruction. Different soils show differences in abundance of antagonistic micro-organisms. As a result of early studies on the survival of the cholera organism, some soils are now recognized as "cholera-immune" or "cholera-destroying" soils.