1. The following disinfectants were investigated for use in bar-soaps: Hexachlorophene (G11) (2:2′-dihydroxy-3:5:6:3′5′:6:′-hexachlorodiphenylmethane); Actamer (2:2′-thiobis, 4-6-dichlorophenol); DCMX (dichloro-m-xylenol); TMTD (3:4:5-tetramethyl thiuram disulphide); TCC (3:4:4′-trichlorcarbanilide); TBS (3:4′:5-tribromsalicylanilide) and TCS (3:3′:4′:5-tetrachlorsalicylanilide).
2. The concentration required for bacteriostasis of Staph. aureus was 2·5 p.p.m. for DCMX and 0·1 p.p.m. for TCS, the other disinfectants falling between these values. None were effective against E. coli, 12·5–50 p.p.m. being required for bacteriostasis (Tabel 1).
3. In the presence of serum the concentration required for bacteriostasis was increased (Table 4).
4. The soap inactivation coefficient (S.I.C.) is a value derived from zone diffusion tests. Zone diameters of the same disinfectant are compared in soap and soap-free solution so that the S.I.C. measures the effect of soap on disinfectant activity.
5. DCMX is inactivated by soap (S.I.C. = ∞). The activity of G 11 is much reduced by soap (S.I.C. = 200). Actamer and TMTD are active in soap, whereas TCC, TBS and particularly TCS is more active in soap than soap-free solution (S.I.C. = 0·32) (Table 3).
6. Another test based on zone diffusion is described which measures the effect of soap-soluble skin substances on disinfectant activity. Zone diameters of the same disinfectant are compared in fresh soap and used soap solutions.
7. Of the disinfectants tested only TMTD lost some of its activity in the presence of soap-soluble skin substances (Table 5).
8. Conventional bacteriological plate-counting technique was used to enumerate bacteria in wash-water. The bacteria were removed from hands by using a standardized washing procedure with bar-soap but without scrubbing.
9. Bacteria were not removed uniformly when washing was carried out twelve times in succession, significantly more bacteria being removed at the beginning of of the washing procedure.
10. When the same individuals were tested at weekly intervals the rate at which bacteria were removed in successive washes was found to vary significantly.
11. Using a single standardized wash the variation in the count of twenty individuals was followed for a period of 16 months. The count from one individual could vary by a factor of 10.
12. A design for a handwashing test, to take account of these variables is described. The bacterial counts of the wash-water when subjects used control soap for 1 week was compared with counts obtained from subjects using disinfectant soap. Subjects and soaps were randomized according to multiple Latin squares.
13. This handwashing technique was used to test the disinfectants described in Part I of this paper. All these disinfectants were effective but tetrachlorsalycilanilide (TCS) was the best. Soap containing 0·5% TCS caused an 84% reduction (Table 9).
The authors wish to thank Dr H. Wilkinson for the ideas he contributed and the interest he has shown in this work, and the Directors of Unilever Limited for permission to publish this paper.