130 BACTERIA IN AIR 



infection can be caused in the air by dust coming, say, from 

 infected skin or clothes, etc. Fliigge, in dealing with this 

 subject in an experimental inquiry, distinguishes between large 

 particles of dust which require an air current moving at the rate 

 of 1 centimetre per second to keep them suspended, and the finer 

 dust which can be kept in suspension by currents moving at from 

 1 to 4 millimetres per second. In the former case, when once 

 the particles alight they cannot be displaced by currents of air 

 except when these are moving at, at least, 5 metres per second, 

 but the brushing, shaking, or beating of objects may, of course, 

 distribute them. In the case of the finer dust the particles will 

 remain for long suspended, and when they have settled can be 

 more easily displaced, as by the waving of an arm, breathing, 

 etc. With regard to infection by dust, a most important factor, 

 however, is whether or not the infecting agent can preserve its 

 vitality in a dry condition. In the case of a sporing organism 

 such as anthrax, vitality is preserved for long periods of time, 

 and great resistance to drying is also possessed by the tubercle 

 and diphtheria bacilli ; but apart from such cases there is little 

 doubt that infection is usually necessarily associated with the 

 transport of moist particles, and is thus confined to a limited 

 area around a sick person. Among diseases which may 

 occasionally be thus spread cholera and typhoid have been classed. 

 Considerable controversy has arisen with regard to certain out- 

 breaks of the latter disease, which have apparently been spread 

 by dusty winds, although we have the fact that the typhoid 

 bacillus does not survive being dried even for a short time. 

 It appears, however, that in such epidemics the transport of 

 infection by .means of insects carried by the wind has not been 

 entirely excluded. 



As in the cases of the soil and of water, presently to be described, 

 attempts have been made to obtain indirect evidence of the contamination 

 of the air by man. Thus Gordon has shown that certain streptococci 

 are common in the saliva ; these resemble the streptococcus pyogenes, but 

 are relatively non-pathogenic, grow well at 37 C. and under anaerobic 

 conditions, cause clotting and acid-formation in litmus milk at 37, and 

 in neutral-red media have an action resembling that of b. coli. These 

 characters serve, according to Gordon, to differentiate organisms of 

 human origin from ordinary streptococci occurring in the air and which 

 he states grow better at about 22 C., are facultative anaerobes and 

 do not produce the changes in milk and in neutral-red media. Thus 

 the finding of streptococci of the first group in plates exposed to air 

 would indicate that a human source was probable, and, if the observation 

 were made on air from the neighbourhood of a sick person, that risk of 

 the dissemination of disease germs was present. The value of this as a 

 practical method has yet to be determined. 



