36o 



NATURE 



[February 7, 190T 



London there was, on an average, just one organism to every 

 38,300,000 dust particles present in the air, and in the air of a 

 room, amongst 184,000,000 dust particles, only one organism 

 could be detected. 



These figures illustrate forcibly the poverty of the air in micro- 

 organisms even when very dusty, and likewise the enormous dilu- 

 tion they undergo in the atmosphere. Their continued existence 

 is rendered difficult through the influence of desiccation and sun- 

 light. Desiccation is one of nature's favourite methods for 

 getting rid of bacteria. Moisture is necessary for their develop- 

 ment and their vital processes, and constitutes about 80 per cent, 

 of their cell-substance. When moisture is withdrawn, most 

 bacterial cells, unless they produce resistant forms of the nature 

 of spores, quickly succumb. The organism of cholera air-dried 

 in a thin film dies in three hours. The organisms of diphtheria, 

 typhoid fever and tuberculosis show more resistance, but die in a 

 few weeks or months. 



Dust containing tubercle bacilli may be carried about by air 

 currents, and the bacilli in this way transferred from an affected 

 to a healthy individual. It may, however, be said that drying 

 attenuates and kills most of these forms of life in a comparatively 

 short time. The spores of certain bacteria may, on the other 

 hand, live for many years in a dried condition, e.g. the spores of 

 anthrax bacilli which are so infective for cattle and also for man 

 (wool sorter's disease). Fortunately, few pathogenic bacteria 

 possess spores, and; therefore, drying by checking and destroying 

 their life is a physical agent that plays an important rdle in the 

 elimination of infectious diseases. This process is aided by the 

 marked bactericidal action of sunlight. Sunlight, which has a 

 remarkable fostering influence on higher plant life, does not 

 exercise the same influence on the bacteria. With few excep- 

 tions we must grow them in the dark in order to obtain 

 successful cultures ; and a sure way of losing our cultures is to 

 leave them exposed to the light of day. Direct sunlight is the 

 most deadly agent, and kills a large number of organisms in the 

 short space of one to two hours ; direct sunlight proves fatal to 

 the typhoid bacillus in half an hour to two hours, to the 

 diphtheria bacillus in half an hour to one hour, and to the 

 tucercle bacillus in a few minutes to several hours. Even 

 anthrax spores are killed by direct light in three and a half hours. 

 Diffuse light is also injurious, though its action is slower. 

 By exposing pigment-producing bacteria to sunlight, colourless 

 varieties can be obtained, and virulent bacteria so weakened that 

 they will no longer produce infection. The germicidal action 

 of the sun's rays is most marked at the blue end of the spec- 

 trum, at the red end there is little or no germicidal action. It 

 is evident that the continuous daily action of the sun along with 

 desiccation are important physical agents in arresting the further 

 development of the disease germs that are expelled from the body. 



It has been shown that sunlight has an important effect in the 

 spontaneous purification of rivers. It is a well-known fact that 

 a river, despite contamination at a given point, may show little 

 or no evidence of this contamination at a point further down in 

 its course. Buchner added to water 100,000 colon bacilli per 

 cubic centimetre, and found that all were dead after one houi's 

 exposure to sunlight. He also found that in a clear lake the 

 bactericidal action of sunlight extended to a depth of about six 

 feet. Sunlight must therefore be taken into account as an 

 agent in the purification of waters, in addition to sedimentation, 

 oxidation and the action of algae. 



Air or the oxygen it contains has important and opposite 

 effects on the life of bacteria. In 1861 Pasteur described an 

 organism in connection with the butyric acid fermentation which 

 would only grow in the absence of free oxygen. And since then 

 a number of bacteria, showing a like property, have been 

 isolated and described. They are termed anaerobic bacteria, as 

 their growth is hindered or stopped in the presence of air. The 

 majority of the bacteria, however, are serobic organisms, inasmuch 

 as their growth is dependent upon a free .supply of oxygen. 

 There is likewise an intermediate group of organisms which 

 show an adaptability to either of these conditions, being able to 

 develop with or without free access to oxygen. Preeminent 

 types of this group are to be met with in the digestive tract of 

 animals, and the majority of disease-producing bacteria belong 

 to this adaptive class. When a pigment-producing organism 

 is grown without free oxygen its pigment production is almost 

 always stopped. For anaerobic forms N and Hg give the best 

 atmosphere for their growth, whilst CO^ is not lavourable and 

 may be positively injurious, as, e.g., in the case of the cholera 

 organism. 



NO. 1632, VOL. 63] 



The physical conditions favouring the presence and multipli- 

 cation of bacteria in water under natural conditions are a low 

 altitude, warmth, abundance of organic matter and a sluggish 

 or stagnant condition of the water. As regards water-borne 

 infectious diseases such as typhoid or cholera, their transmission 

 to man by water may be excluded by simple boiling or by an 

 adequate filtration. The freezing of water, whilst stopping the 

 further multiplication of organisms, may conserve -the life of 

 disease germs by eliminating the destructive action of commoner 

 competitive forms. Thus the typhoid bacillus may remain 

 frozen in ice for some months without injury. Employment of 

 ordinary cold is not, therefore, a protection against dangerous 

 disease germs. 



As regards electricity, there is little or no evidence of its direct 

 action on bacterial life, the effects produced appear to be of an 

 indirect character due to the development of heat or to the 

 products of electrolysis. 



Ozone is a powerful disinfectant, and its introduction into pol- 

 luted water has a most marked purifying effect. The positive 

 effects of the electric current may therefore be traced to the action 

 of the chemical products and of heat. I am not aware that any 

 direct action of the X-rays on bacteria has up to the present been 

 definitely proved. 



Mechanical agitation, if slight, may favour, and if excessive 

 may hinder bacterial development. Violent shaking or concussion 

 may not necessarily prove fatal so long as no mechanical lesion 

 of the bacteria is brought about. If, however, substances likely 

 to produce triturating effects are introduced, a disintegration and 

 death of the cells follows. Thus Rowland, by a very rapid 

 shaking of tubercle bacilli in a steel tube with quartz sand and 

 hard steel balls, produced their complete disintegration in ten 

 minutes. 



Bacteria appear to be very resistant to the action of pressure. 

 At 300-450 atmospheres putrefaction still takes place, and at 

 600 atmospheres the virulence of the anthrax bacillus remained 

 unimpaired. Of the physical agents that affect bacterial life, 

 temperature is the most important. Temperature profoundly 

 influences the activity of bacteria. It may favour or hinder 

 their growth, or it may put an end to their life. If we regard 

 temperature in the first instance as a favouring agent, very strik- 

 ing differences are to be noted. The bacteria show a most 

 remarkable. range of temperature under which their growth is 

 possible, extending from zero to 70° C. If we begin at the 

 liottom of the scale we find organisms in water and in soil that 

 are capable of growth and development at zero. Amongst these 

 are certain species of phosphorescent bacteria which continue to 

 emit light even at this low temperature. At the Jenner Institute 

 we have met with organisms growing and developing at 34-40° F. 

 The vast majority of interest to us find, however, the best 

 conditions for their growth from 15° up to 37° C. Each species 

 has a minimum, an optimum an I a maximum temperature at 

 which it will develop. It is important in studying any given 

 species that the optimum temperature for their development be 

 ascertained, and that this temperature be maintained. In 

 this respect we can distinguish three broad groups. The first 

 group includes those for which the optimum temperature is 

 from 15-20° C. The second group includes the parasitic forms, 

 viz. those which grow in the living body and for which the 

 optimum temperature is at blood heat, viz. 37° C. We have a 

 third group for which the optimum temperature lies as high as 

 50-55° C. On this account this latter group has been termed 

 thermophilic, on account of its growth at such abnormally high 

 temperatures — temperatures which are fatal to other forms of 

 life. They have been the subject of personal investigation 

 in conjunction with Dr. Blaxall. We found that there existed 

 in nature an extensive group of such organisms to which the 

 term thermophilic bacteria was applicable. Their growth and 

 development occurred best at temperatures at which ordinary 

 protoplasm becomes inert or dies. The best growths were always 

 obtained at 55-65° C. Their wide distribution was of a striking 

 nature. They were found by us in river water and mud, in 

 sewage, and also in a sample of sea water. They were present 

 in the digestive tract of man and animals, and in the surface 

 and deep layers of the soil as well as in straw and in all samples 

 of ensilage examined. Their rapid growth at high temperatures 

 was remarkable, the whole surface of the culture medium being 

 frequently overrun in from fifteen to sevenleeen hours. The 

 organisms examined by us (fourteen forms in all) belonged to 

 the group of the Bacilli. Some were motile, some curdled milk, 

 and some liquefied gelatin in virtue of a proteolytic enzyme. 



