February 7, 1901] 



NA TURE 



359 



bacteriologists in the country, Prof. Theobald Smith, and 

 requested him, not only to prepare antitoxin for the citizens of 

 the State, but also to investigate the best methods of its pre- 

 paration and preservation, besides other cognate and novel 

 but pressing problems in the field of pure science. Here also 

 the most thorough-going utilitarianism has proved to be scientific 

 investigation pushed to its utmost limits. . . . 



THE EFFECT OF PHYSICAL AGENTS ON 

 BACTERIAL LIFE} 



'T'HE fact that life did not exist upon the earth at a remote 

 -'- period of time, the possibility of its present existence as 

 well as the prospect of its ultimate extinction, can be traced to 

 the operation of certain physical conditions. These physical 

 conditions upon which the maintenance of life as a whole 

 depends are in their main issues beyond the control of man. 

 We can but study, predict and, it may be, utilise their effects for 

 our benefit. Life in its individual manifestations is, therefore, 

 conditioned by the physical environment in which it is placed. 

 Life rests on a physical basis, and the main springs of its 

 energies are derived from a larger world outside itself. If these 

 conditions, physical or chemical, are favourable, the functions of 

 life proceed ; if unfavourable, they cease— and death ultimately 

 ensues. These factors have been studied and their effects 

 utilised to conserve health or to prevent disease. It is our pur- 

 pose this evening to study some of the purely physical factors, 

 not in their direct bearing on man, but in relation to much lower 

 forms in the scale of life — forms which constitute in number a 

 family far exceeding that of the human species, and of which we 

 may produce at will in a test-tube, within a few hours, a popula- 

 tion equal to that of London. These lowly forms of life — the 

 bacteria — belong to the vegetable kingdom, and each individual 

 is represented by a simple cell. 



These forms of life are ubiquitous in the soil, air and water, 

 and are likewise to be met with in intimate association with 

 plants and animals, whose tissues they may likewise invade with 

 injurious or deadly effects. Their study is commonly termed 

 bacteriology — a term frequently regarded as synonymous with a 

 branch of purely medical investigation. It would be a mistake, 

 however, to suppose that bacteriology is solely [concerned with 

 the study of the germs of disease. The dangerous microbes are 

 in a hopeless minority in comparison with the number of those 

 which are continually performing varied and most useful functions 

 in the economy of nature. Their wide importance is due to the 

 fact that they ensure the resolution and redistribution of dead 

 and effete organic matter which, if allowed to accumulate, would 

 speedily render life impossible on the surface of the earth. If 

 medicine ceased to regard the bacteria, their study would still 

 remain of primary importance in relation to many industrial 

 processes in which they play a vital part. It will be seen, there- 

 fore, that their biology presents many points of interest to 

 scientific workers generally. Their study as factors that 

 ultimately concern us really began with Pasteur's researches 

 upon fermentation. The subject of this evening's discourse, the 

 effect of physical agents on bacterial life, is important not 

 merely as a purely biological question, though this phase is of 

 considerable interest, but also on account of the facts I have 

 already indicated, viz. that micro-organisms fulfil such an 

 important function in the processes of nature, in industrial 

 operations, and in connection with the health of man and 

 animals. It depends largely on the physical conditions to be 

 met with in nature whether the micro-organisms exercise their 

 functions, and likewise whether they die or remain inactive. 

 Further, the conditions favouring one organism may be fatal to 

 another, or an adaptability may be brought about to unusual 

 conditions for their life. To the technologist the effect of 

 physical agents in this respect is of importance, as a knowledge 

 of their mode of action will guide him to the means to be 

 employed for utilising the micro-organisms to the best advantage 

 in processes of fermentation. The subject is of peculiar interest 

 to those who are engaged in combating disease, as a knowledge 

 of the physical agents that favour or retard bacterial life 

 will furnish indications for the preventive measures to be 

 adopted. With a suitable soil and an adequate temperature 

 the propagation of bacteria proceeds with great rapidity. If the 

 primary conditions of soil and an adequate temperature are not 



1 Discourse delivered at the Royal Institution by Dr. Allan Macfadyen, 

 Director ofthe Jenner Institute of Preventive Medicine. 



NO. 1632, VOL. 63] 



present, the organisms will not multiply ; they remain quiescent 

 or they die. The surface layers of the soil harbour the vast 

 majority of the bacteria, and constitute the great storehouse in 

 nature for these forms of life. They lessen in number in the 

 deeper layers of the soil, and few or none are to be met with at 

 a depth of 8-10 feet. As a matter of fact, the soil is a most 

 efl[icient bacterial filter, and the majority of the bacteria are 

 retained in its surface layers and are to be met with there. In 

 the surface soil, most bacteria find the necessary physical condi- 

 tions for their growth, and may be said to exist there under 

 natural conditions. It is in the surface soil that their main 

 scavenging functions are performed. In the deeper layers, the 

 absence of air and the temperature conditions prove inimical to 

 most forms. 



Amongst pathogenic bacteria the organisms of lockjaw and of 

 malignant oedema appear to be eminently inhabitants of the 

 soil. As an indication of the richness of the surface soil in 

 bacteria I may mention that i gramme of surface soil may 

 contain from several hundred thousand to as many as several 

 millions of bacteria. The air is poorest in bacteria. The 

 favouring physical conditions to be met with in the soil are not 

 present in the air. Though bacteria are to be met with in the 

 air they are not multiplying forms, as is the case in the soil. 

 The majority to be met with in air are derived from the soil. 

 Their number lessens when the surface soil is moist, and it in- 

 creases as the surface soil dries. In a dry season the number of 

 air organisms will tend to increase. 



Town air contains more bacteria than country air, whilst they 

 become few and tend to disappear at high levels and on the sea. 

 A shower of rain purifies the air greatly of bacteria. The 

 organisms being,as I stated, mainly derived from the surface of the 

 ground, their number mainly depends on the physical condition 

 of the soil, and this depends on the weather. Bacteria cannot 

 pass independently to the air, they are forcibly transferred to 

 it with dust from various surfaces. The relative bacterial purity 

 ofthe atmosphere is mainly, therefore, a question of dust. Even 

 when found floating about in the air the bacteria are to be met 

 with in much greater number in the dust that settles on exposed 

 surfaces, eg. floors, carpets, clothes and furniture. Through a 

 process of sedimentation the lower layers of the air becotne 

 richer in dust and bacteria, and any disturbance of dust will 

 increase the number of bacteria in the air. 



The simple act of breathing does not disseminate disease germs 

 from a patient ; it requires an act of coughing to carry them into 

 the air with minute particles of moisture. From the earliest 

 times great weight has been laid upon the danger of infection 

 through air-borne contagia, and with the introduction of 

 antiseptic surgery the endeavour was made to lessen this danger 

 as much as possible by means of the carbolic spray, &c. In the 

 same connection numerous bacteriological examinations of air 

 have been made with the view of arriving at results of hygienic 

 value. The average number of micro-organisms present in the 

 air is 5CX)-iooo per 1000 litres ; of this number only 100-200 

 are bacteria, and they are almost entirely harmless forms. The 

 organisms of suppuration have been detected in the air, and the 

 tubercle bacillus in the dust adhering to the walls of rooms. 

 Investigation has not, however, proved air to be one of the 

 important channels of infection. The bactericidal action of 

 sunlight, desiccation, and the diluting action of the atmosphere 

 on noxious substances, will always greatly lessen the risk of 

 direct aerial infection. 



The physical agents that promote the passage of bacteria into 

 the air are inimical to their vitality. Thus, the majority pass 

 into the air, not from moist, but from dry surfaces, and the 

 preliminary drying is injurious to a large number of bacteria. It 

 follows that if the air is rendered dust-free, it is practically 

 deprived of all the organisms it may contain. As regards 

 enclosed spaces, the stilling of dust and more especially the 

 disinfection of surfaces liable to breed dust or to harbour 

 bacteria are more important points than air disinfection, and 

 this fact has been recognised in modern surgery. In an investi- 

 gation, in conjunction with Mr. Lunt, an estimation was arrived 

 at of the ratio existing between the number of dust particles and 

 bacteria in the air. We used Dr. Aitken's Dust-counter, which 

 not only renders the dust particles visible, but gives a means of 

 counting them in a sample of air. In an open suburb of London 

 we found 20,000 dust particles in I cubic centimetre of air ; in 

 a yard in the centre of London about 500,000. The dust con- 

 lamination we found to be about 900 per cent, greater in the 

 centre of London than in a quiet suburb. In the open air of 



