DISCUSSION: MICROORGANISMS ASSOCIATED WITH MALIGNANCY

It is now apparent that there exists a rather unusual bacterium, or group of similar, but not necessarily identical, bacteria, closely associated with malignant disease in man and other animals. In the case of human disease, there is obviously room for argument about the exact role of the microbe, but there seems little doubt in the n e ~ t ~ ~ J * that the bacterium is capable of producing a tumor under properly controlled conditions. There is significant evidence of a connection between infection and tumor incidence in the mouse,8*9 and Alexander-Jackson2,S has produced evidence to suggest that the so-called virus of the Rous sarcoma is a bacterial L-form, and closely related to her own human cancer isolates. Evidence is beginning to be collected about the pathogenic constitution of these bacteria,17 and their presence in leukaemic blood cells has been directly dernon~trated.2~ It is therefore quite apparent that this organism, or group of organisms, can no longer be dismissed out-of-hand as a casual, secondary invader of tumors. Bacteria are decidedly associated with tumors of animals, and almost certainly of plants also, in a most significant manner, and no automatic claim that human beings are different can be logically sustained: the role of bacteria in producing tumors in human beings is unproved because we cannot use human beings as experimental material. Bacteria of this type are not by any means conlined to malignant disease, but occur commonly, perhaps universally, in the blood and tissues of man and at least some other My own observation is that the difference between the condition in diseased and healthy subjects is quantitative, rather than q u a l i t a t i ~ e ; ~ ~ in cancer, and probably also in such autoimmune conditions as arthritis,l,zO these bacteria are more numerous, and they appear more frequently in the L-form or mycoplasma phase in healthy subjects, where they are commonly disregarded, since they are slow to grow and difficult to isolate. Moreover, despite the recent attention they have received, the exact identity of these bacteria is still problematical, and this uncertainty has contributed greatly to the cautious attitude taken by so many bacteriologists. It could easily be suggested that they are merely secondary invaders, a result of the disease process, rather than the cause, and because they are by no means confined to cancer subjects but are very widespread in man and animals, it could also be argued that they have no relationship at all to the disease; but if they have the potential to cause tumors, their wide distribution is not an argument against a causative role, since this is precisely the manner in which many oncogenic viruses behave." The virus is widely distributed in the animal population, but the tumors occur only in relatively few individuals, and the disease is the final result of an interplay of several factors, both constitutional and environmental, of which the presence of the virus is only one; for example, genetic susceptibility, immunological or hormonal disturbance, chemical carcinogens and irradiation. One of these infective agents of animal tumors, the Rous sarcoma virus, that falls into this category because it is widely distributed but causes tumors in a minority of infected fowls and is encouraged to do so by