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DISINFECTION.

DISINFECTION.

THE LANCET.

LONDON: SATURDAY, JULY 5, 1930.

THE first volume of the Medical ResearchCouncil’s " System of Bacteriology

" is of more

general interest than the four highly technicalinstalments which have actually been publishedbefore it. Prof. W. BuLLOCH deals with the historyof bacteriology with his usual accomplishedmastery of the literature. Prof. F. W. ANDREWES,discusses the classification of bacteria and thecriteria which give valid distinctions between

genera and species, and Dr. J. A. ARKWRIGHTwrites an extraordinarily interesting account ofbacterial variation. Both these articles have wide

biological implications, and we hope they will beread by many scientific workers who have noimmediate connexion with microbiology. Thesame is true of the article by Miss HARRIETTECHICK, D.Sc., on the theory of disinfection, whichnot only indicates how practical people may usedisinfectants to the best advantage, but opens upthe great problem of how animals are killed byharmful conditions of the environment, in otherwords, the principles of mass destruction throughwhich natural selection operates. We wonderwhether these products of the bacteriologicallaboratory are as familiar to general biologists asthey ought to be. Miss CHICK points out that,when a disinfectant is applied to a large populationof bacteria, the microbes are not all killed at once.The killing process takes time, even if that timeis very short, and if the disinfectant is strong enoughto kill the great majority in a minute or two itwill hardly succeed in killing all of them in half anhour. If the killing time is reduced by increasingthe strength of the disinfectant, or by raising thetemperature at which it acts, the same relationholds ; some microbes still survive much longerthan the majority. Incidentally, we suspect thatthe influence of high temperatures in increasingthe efficacy of disinfectants is hardly sufficientlyrealised except perhaps by those who insist onhaving hot water to wash their hands with ; thekilling effect of carbolic acid is enhanced aboutten times by a rise of 10° C., and soap, at thehottest temperature which the average hand canbear (a little below 50° C.), is probably three orfour times as fatal to micro-organisms as it is atbody temperature. The killing being in all casesa time process, it cannot be as correct as it isconvenient to talk of this or that organism havinga certain " thermal death point " ; all that is

really ascertained is that at a given temperature,&c., a certain proportion--e.g., 90 per cent.-of theindividuals are dead at the end of a certain time.

So much for the immediate practical applications.The more detailed analysis of this time process i

takes us into the fundamentals of life. It isfound (and Miss CHICK herself has been mainlyresponsible for the discovery) that the individualsin a uniform population of bacteria die logarith-mically, which is to say that if 50 per cent. die inthe first minute of disinfection, 25 per cent. willdie in the second minute, 12t per cent. in the thirdminute, and so on ; the number which die in anyinterval of time is always a constant proportion ofthose alive at the beginning of the interval. The

extraordinary thing is that this is just the way inwhich molecules of cane sugar break up when theyare boiled with hydrochloric acid, with the

implication that the whole of the microbe is

analogous to a molecule. Prof. A. E. BOYCOTTcommented some time ago on the significance ofthis analogy in our conception of the units of whichthe world is made. It may be-the suggestionwas made too recently to be incorporated in thearticle-that the death of the organism is deter.mined by the destruction of some one particularmolecule. This would, perhaps, make the analogyeasier to accept, but it would leave the explanationof the facts where it is, since physical chemistscan " explain " the behaviour of the cane sugarmolecules only by assuming that they are subjectto rhythmical changes in susceptibility to hydro-chloric acid. This logarithmic process of death isalso flatly discordant with the general rule that allmeasurable qualities of live organisms are

distributed on something like the normal curve ofvariation. If resistance to disinfectants varieson the same plan as human stature or the diametersof blood corpuscles, there should be a few verysusceptible organisms and a few exceptionallyresistant, with the majority in intermediate

positions. Killing off a population like this doesnot give a logarithmic curve, but one in which therate of disinfection increases as the process proceeds.And it is necessary to note that though the

logarithmic result has been obtained in a majorityof the most careful observations, a result compatiblewith the normal curve of resistance has beenreached several times. The logarithmic death-rateis, therefore, not an invariable rule, and Dr.HENDERSON SMITH has published figures whichindicate that the mode may vary with the strengthof the disinfectant.The matter is puzzling enough, and Prof.

C. EIJKMAN made it more curious when hediscovered that bacterial spores germinate in thesame logarithmic fashion. But there is no doubtabout the fundamental importance of the problem,and the more people puzzle about it the better.They cannot do better than start with MissCHICK’S summary account.

THE I.M.S. AND THE SIMON REPORT.IT is chiefly through the medium of the Indian

Medical Service, which still contains a large Britishelement, that this country has come into touchwith medical affairs in India, and the future ofthat service is a matter of concern not only tothose who are serving and those who contemplate