Variability Of Pathogenic Powers
( Originally Published 1897 )
As has already been stated, our ideas of the relation of bacteria to disease have undergone quite a change since they were first formulated, and we recognise other factors influencing disease besides the actual presence of the bacterium. These we may briefly consider under two heads, viz., variation in the bacterium, and variation in the susceptibility of the individual. The first will require only a brief consideration.
That the same species of pathogenic bacteria at different times varies in its powers to produce disease has long been known. Various conditions are known to affect thus the virulence of bacteria. The bacillus which is supposed to give, rise to pneumonia loses its power to produce the disease after having been cultivated for a short time in ordinary culture media in the laboratory. This is easily understood upon the suggestion that it is a parasitic bacillus and does not thrive except under parasitic conditions. Its pathogenic powers can sometimes be restored by passing it again through some susceptible animal. One of the most violent pathogenic bacteria is that which produces anthrax, but this loses its pathogenic powers if it is cultivated for a considerable period at a high temperature. The micrococcus which causes fowl cholera loses its power if it be cultivated in common culture media, care being taken to allow several days to elapse between the successive inoculations into new culture flasks. Most pathogenic bacteria can in some way be so treated as to suffer a diminution or complete loss of their powers of producirig a fatal disease. On the other hand, other conditions will cause an increase in the virulence of a pathogenic germ. The virus which produces hydrophobia is increased in violence if it is inoculated into a rabbit and subsequently taken from the rabbit for further inoculation. The fowl cholera micrococcus, which has been weakened as just mentioned, may be restored to its original violence by inoculating it into a small bird, like a sparrow, and inoculating a second bird from this. A few such inoculations will make it as active as ever. These variations doubtless exist among the species in Nature as well as in artificial cultures. The bacteria which produce the various wound infections and abscesses, etc., appear to vary under normal conditions from a type capable of producing violent and fatal blood poisoning to a type producing only a simple abscess, or even to a type that is entirely innocuous. It is this factor, doubtless, which in a large measure determines the severity of any epidemic of a bacterial contagious disease.
SUSCEPTIBILITY OF THE INDIVIDUAL
The very great modification of our early views has affected our ideas as to the power which individuals have of resisting the invasion of pathogenic bacteria. It has from the first been understood that some individuals are more susceptible to disease than others, and in attempting to determine the significance of this fact many valuable and interesting discoveries have been made. After the exposure to the disease there follows a period of some length in which there are no discernible effects. This is followed by the onset of the disease and its development to a crisis, and, if this be passed, by a recovery. The general course of a germ disease is divided into three stages: the stage of incubation, the development of the disease, and the recovery. The susceptibility of the body to a disease may be best considered under the three heads of Invasion, Resistance, Recovery.
Means of Invasion.—In order that a germ disease should arise in an individual, it is first necessary that the special bacterium which causes the disease should get into the body. There are several channels through which bacteria can thus find entrance ; these are through the mouth, through the nose, through the skin, and occasionally through excretory ducts. Those which come through the mouth come with the food or drink which we swallow ; those which-enter through the nose must be traced to the air; and those which enter through the skin come in most cases through contact with some infected object, such as direct contact with the body of an infected person or his clothing or some objects he has handled, etc. Occasionally, perhaps, the bacteria may get into the skin from the air, but this is certainly uncommon and confined to a few diseases. There are here two facts of the utmost importance for every one to understand: first, that the chance of disease bacteria being carried to us through the air is very slight and confined to a few diseases, such as smallpox, tuberculosis, scarlet fever; etc., and, secondly, that the un-injured skin and the uninjured mucous membrane also is almost a sure protection against the invasion of the bacteria. If the skin is whole, without bruises or cuts, bacteria can seldom, if ever, find passage through it. These two facts are of the utmost importance; since of all sources of infection we have the least power to guard against infection through the air, and since of all means of entrance we can guard the skin with the greatest difficulty. We can easily render food free from pathogenic bacteria by heating it. The material we drink can similarly be rendered harmless, but we can not by any known means avoid breathing air, nor is there any known method of disinfecting the air, and it is impossible for those who have anything to do with sick persons to avoid entirely having contact either with the patient or with infected clothing or utensils.
From the facts here given it will be seen that the individual's susceptibility to disease produced, by parasitic bacteria will depend upon his habits of cleanliness, his care in handling infectious material, or care in cleansing the hands after such handling, upon his habit of eating food cooked or raw, and upon the condition of his skin and mucous membranes, since any kind of bruises will increase susceptibility. Slight ailments, such as colds, which inflame the mucous membrane, will decrease its resisting power and render the individual more susceptible to the entrance of any pathogenic germs should they happen to be present. Sores in the mouth or decayed teeth may in the same way be prominent factors in the individual's susceptibility. Thus quite a number of purely physical factors may contribute to an individual's susceptibility.
Resisting Power of the Body.—Even after the bacteria get into the body it is by no means certain that they will give rise to disease, for they have now a battle to fight before they can be sure of holding their own. It is now, indeed, that the actual conflict between the powers of the body and these microscopic invaders begins. After they have found entrance into the body the bacteria have arrayed against them strong resisting forces of the human organism, endeavouring to destroy and expel them. Many of them are rapidly killed, and sometimes they are all destroyed without being able to gain a foothold. In such cases, of course, no trouble results. In other cases the body fails to overcome the powers of the invaders and they eventually multiply rapidly. In this struggle the success of the invaders is not necessarily a matter of numbers. They are simply struggling to gain a position in the body, where they can feed and grow. A few individuals may be entirely sufficient to seize such a foothold, and then these by multiplying may soon become indefinitely numerous. To protect itself, therefore, the human body must destroy every individual bacterium, or at least render them all incapable of growth. Their marvellous reproductive powers give the bacteria an advantage in the battle. On the other hand, it takes time even for these rapidly multiplying beings to become sufficiently numerous to do injury. There is thus an interval after their penetration into the body when these invaders are weak in numbers. During this interval the period of incubation—the body may organize a resistance sufficient to expel them.
We do not as yet thoroughly understand the forces which the human organism is able to array against these invading foes. Some of its methods of defence are, however, already intelligible to us, and we know enough, at all events, to give us an idea of the intensity of the conflict that is going on, and of the vigorous and powerful forces which the human organism is able to bring against its invading enemies.
In the first place, we notice that a majority of bacteria are utterly unable to grow in the human body even if they do find entrance. There are known to bacteriologists to-day many hundreds, even thousands of species, but the vast majority of these find in the human tissues conditions so hostile to their life that they are utterly unable to grow therein. Human flesh or human blood will furnish excellent food for them if the individual be dead, but living human flesh and blood in some way exerts a repressing influence upon them which is fatal to the growth of a vast majority of species. Some few species, however, are not thus destroyed by the hostile agencies of the tissues of the animal, but are capable of growing and multiplying in the living body. These alone are what constitute the pathogenic bacteria, since, of course, these are the only bacteria which can pro-duce disease by growing in the tissues of an animal. The fact that the vast majority of bacteria can not grow in the living organism shows clearly enough that there are some conditions existing in the living tissue hostile to bacterial life. There can be little doubt, moreover, that it is these same hostile conditions, which enable the body to resist the attack of the pathogenic species in cases where resistance is successfully made.
What are the forces arrayed against these invaders ? The essential nature of the battle appears to be a production of poisons and counter poisons. It appears to be an undoubted fact that the first step in repelling these bacteria is to flood them with certain poisons which check their growth. In the blood and lymph of man and other animals there are present certain products which have a direct deleterious influence upon the growth of micro-organisms. The existence of these poisons is undoubted, many an experiment having directly attested to their presence in the blood of animals. Of their nature we know very little, but of their repressing influence upon bacterial growth we are sure. They have been named alexines, and they are produced in the living tissue, although as to the method of their production we are in ignorance. By the aid of these poisons the body is able to prevent the growth of the vast majority of bacteria which get into its tissues. Ordinary micro-organisms are killed at once, for these alexines act as antiseptics, and common bacteria can no more grow in the living body than they could in a solution containing other poisons. Thus the body has a perfect protection against the majority of bacteria. The great host of species which are found in water; milk, air, in our mouths or clinging to our skin, and which are almost omnipresent in Nature, are capable of growing well enough in ordinary lifeless organic foods ; but just as soon as they succeed in finding entrance into living human tissue their growth is checked at once by these antiseptic agents which are poured upon them. Such bacteria are therefore not pathogenic germs, and not sources of trouble to human health.
There are, on the other hand, a few species of bacteria which may be able to retain their lodgment in the body in spite of this attempt of the individual to get rid of them. These, of course, constitute the pathogenic species, or so-called disease germs." Only such species as can overcome this first resistance can be disease germs, for they alone can retain their foothold in the body.
But how do these species overcome the poi-sons which kill the other harmless bacteria ? They, as well as the harmless forms, find these alexines injurious to their growth, but in some way they are able to counteract the poisons. In this general discussion of poisons we are dealing with a subject which is somewhat obscure, but apparently the pathogenic bacteria are able to overcome the alexines of the body by producing in their turn certain other products which neutralize the alexines, thus annulling their action. These pathogenic bacteria, when they get into the body, give rise at once to a group of bodies which have been named lysines. These lysines are as mysterious to us as the alexines, but they neutralize the effect of the alexines and thus overcome the resistance the body offers to bacterial growth. 'The invaders can now multiply rapidly enough to get a lasting foothold in the body and then soon produce the abnormal symptoms which we call disease. Pathogenic bacteria thus differ from the non-pathogenic bacteria primarily in this power of secreting products which can neutralize the ordinary effects of the alexines, and so overcome the body's normal resistance to their parasitic life.
Even if the bacteria do thus overcome the alexines the battle is not yet over, for the individual has another method of defence which is now brought into activity to check the growth of the invading organisms. This second method of resistance is by means of a series of active cells found in the blood, known as white blood-corpuscles (Fig. 33 a, b). They are minute bits of protoplasm present in the blood and lymph in large quantities. They are active cells, capable of locomotion and able to crawl out of the blood-vessels. Not infrequently they are found to take into their bodies small objects with which they come in contact. One of their duties is thus to en-gulf minute irritating bodies which may be in the tissues, and to carry them away for excretion. They thus act as scavengers. These corpuscles certainly have some agency in warding off the at-tacks of pathogenic bacteria. Very commonly they collect in great numbers in the region of the body where invading bacteria are found. Such invading bacteria exert upon them a strong attraction, and the corpuscles leave the blood-vessels and sometimes form a solid phalanx completely surrounding the invading germs. Their collection at these points may make itself seen externally by the phenomenon we call inflammation.
There is no question that the corpuscles en-gage in conflict with the bacteria when they thus surround them. There has been not a little dispute, however, as to the method by which they carry on the conflict. It has been held by some that the corpuscles actually take the bacteria into their bodies, swallow them, as it were, and subsequently digest them (Fig. 33 C, d, e). This idea gave rise to the theory of phagocytosis, and the corpuscles were consequently named phagocytes. The study of several years has, however, made it probable that this is not the ordinary method by which the corpuscles destroy the bacteria. Ac-cording to our present knowledge the method is a chemical one. These cells, when they thus collect in quantities around the invaders, appear to secrete from their own bodies certain injurious products which act upon the bacteria much as do the alexines already mentioned. These new bodies have a decidedly injurious effect upon the multiplying bacteria ; they rapidly check their growth, and, acting in union with the alexines, may perhaps entirely destroy them.
After the bacteria are thus killed, the white blood-corpuscles may load themselves with their dead bodies and carry them away (Fig. 33 d, e). Sometimes they pass back into the blood stream and carry the bacteria to various parts of the body for elimination. Not infrequently the white corpuscles die in the contest, and then may accumulate in the form of pus and make their way through the skin to be discharged directly. The battle between these phagocytes and the bacteria goes on vigorously. If in the end the phagocytes prove too strong for the invaders, the bacteria are gradually all destroyed, and the attack is repelled. Under these circumstances the individual commonly knows nothing of the matter. This conflict has taken place entirely without any consciousness on his part, and he may not even know that he has been exposed to the attack of the bacteria. In other cases the bacteria prove too strong for the phagocytes. They multiply too rapidly, and sometimes they produce secretions which actually drive the phagocytes away. Commonly, as already noticed, the corpuscles are attracted to the point of invasion, but in some cases, when a particularly deadly and vigorous species of bacteria invades the body, the secretions produced by them are so powerful as actually to drive the corpuscles away. Under these circumstances the invading hosts have a chance to multiply unimpeded, to distribute themselves over the body, and the disease rapidly follows as the result of their poisoning action on the body tissues.
It is plain, then, that the human body is not helpless in the presence of the bacteria of disease, but that it is supplied with powerful resistant forces. It must not be supposed, however, that the outline of the action of these forces just given is anything like a complete account of the matter; nor must it be inferred that the resistance is in all respects exactly as outlined. The subject has only recently been an object of investigation, and we are as yet in the dark in regard to many of the facts. The future may require us to modify to some ex-tent even the brief outline which has been given.
But while we recognise this uncertainty in the details, we may be assured of the general facts. The living body has some very efficacious resistant forces which prevent most bacteria from growing within its tissues, and which in large measure may be relied upon to drive out the true pathogenic bacteria. These resistant forces are in part associated with the productions of body poisons, and are in part associated with the active powers of special cells which have been called phagocytes. The origin of the poisons and the exact method of action of the phagocytes we may well leave to the future to explain.
These resisting powers of the body will vary with conditions. It is evident that they are natural powers, and they will doubtless vary with the general condition of vigour of the individual. Robust health, a body whose powers are strong, well nourished, and vigorous, will plainly furnish the conditions for the greatest resistance to bacterial diseases. One whose bodily activities are weakened by poor nutrition can offer less resistance. The question whether one shall suffer from a germ disease is not simply the question whether he shall be exposed, or even the question whether the bacteria shall find entrance into his body. It is equally dependent upon whether he has the bodily vigour to produce alexines in proper quantity, or to summon the phagocytes in sufficient abundance and vigour to ward off the attack. We may do much to prevent disease by sanitation, which aids in protecting the individual from at-tack ; but we must not forget that the other half of the battle is of equal importance, and hence we must do all we can to strengthen the resisting forces of the organism.