EFFECT OF ANTISEPTICS AND DISINFECTANTS ON BACTERIA. 33 



the nutrient medium, or when a silk thread, which had been dipped 

 in a solution, was directly transferred to the new soil, enough of the 

 supposed disinfectant might be mechanically carried over to retard 

 the development of the bacteria, though it was ineffectual in 

 destroying them. From a growth not appearing, it was concluded 

 that the spores or the bacteria had been affected, and so a 

 mistake occurred. To avoid this, Koch made a point of transfer- 

 ring a minimum of the disinfected growth to as large a cultivation 

 area as possible, so that any chemical substance mechanically 

 carried over would be so diluted as to be inert. For the same 

 reason, threads, after withdrawal from the disinfecting solution, 

 were rinsed in sterilised water, or weak alcohol, and then trans- 

 planted ; or, instead of judging from the development on nutrient 

 gelatine, the effect of inoculation in a healthy animal was made 

 the test. 



A few examples may be quoted in illustration. Silk threads, 

 impregnated with anthrax spores, were placed in bottles containing 

 carbolic acid of various strengths. A thread was removed from each 

 on successive days, and transferred to nutrient gelatine, and the 

 result noted. It was found that immersion of the thread in a 5 per 

 cent, solution of carbolic acid was sufficient in two days to effect 

 complete sterilisation, and seven days in a 3 per cent, solution was 

 equally efficacious. Since for practical purposes a strength should 

 be selected which would be effectual in twenty-four hours, Koch 

 recommended that for general use, allowing for deterioration by 

 keeping, a solution containing not less than 5 per cent, should be 

 employed, and for complex fluids probably a still higher percentage 

 would be necessary. In the case of sporeless bacilli the results were 

 very different. Blood containing the bacilli, from an animal just 

 killed, was dried on threads, and after exposure for two minutes to 

 a 1 per cent, solution, was completely sterilised; and fresh blood 

 mixed with a 1 per cent, carbolic solution produced no effect when 

 inoculated. On the other hand, when the blood was mixed with a 

 5 per cent, solution, the virulence was not destroyed. The facility 

 with which the bacilli are destroyed, compared with their spores, 

 illustrates how easily errors may occur, when mere arrest of growth 

 or loss of motility is regarde_d as a sign of the efficacy of disinfection. 



To test vapours, Koch exposed anthrax spores or the spores 

 which occur in garden earth by suspending them over solutions, 

 such as bromine or chlorine, in a closed vessel. After a time they 

 were transferred to a nutrient medium to test their vitality. To 

 test the power of sulphurous acid gas, the spores were spread about 



3 



