Nature Vol. 281 398 2M3 Thymus Fla. 1 Competition and absorption analysis of NCPl 50 and Pl20. Labelled cell extracts of C57BL/6 thymus and 2M3 cells (an AMuL V in vitro derived clonal lymphoid cellline)6 were prepared and analysed as in Fig. 1. A final volume of 1 ml for each cell extract of thymus (10 7 cells) or 2M3 (S x 10! cells) was used for each tube. All tubes contained 100 j.l.g ml- 1 unlabelled Moloney MuLV virion proteins. Lane a, normal mouse serum (5 j.l.1). Lane b, anti-AbT (5 j.l.1). Lanes c, d and e, anti-AbT (5 r-l) plus unlabelled extract of 2 x 10 , 4xl0 6 or 8xl06 A-MuLV transformed non-producer NIH/ 3T3 cells. Lanes f, g, h, antiAbT (5 j.l.1) plus unlabelled extract of 2X106 , 4X10 6 , or 8x106 Moloney sarcoma virus transformed non-producer NIH 3T3 cells respectively. Lane i, 1: 10 diluted anti-AbT (50 j.l.1) controlmock absorbed. Lane i, 1 : 10 antiAbT (50 j.l.1) pre-absorbed with viable A-MuLV transformed nonproducer NIH 3T3 cells (2 x 108 cells per ml diluted serum-3 cycles each 45 min; O°C). Immune precipitates were prepared and analysed as in Fig. 1. abcdefgh results imply that we are studying different proteins remains unclear. One possibility is that Risser et al. were detecting NCP150 but that the synthetic rate (as monitored by immunoprecipitation) is controlled separately from expression at the cell surface (as monitored by cytotoxicity absorption). We thank Drs Robert Weinberg and David Housman for Moloney sarcoma and spleen focus-forming virus infected nonproducer NIH/3T3 cells, respectively. This work was supported by grant VC-4J from the American Cancer Society (to D.B.), grant CA-24220 from the NCI (to N.E.R.) and grant CA-14051 from the NCI (core grant to Dr S. E. Luria). O.N.W. is a Helen Hay Whitney postdoctoral fellow. N .E.R. is a Research Scholar of the American Cancer Society, Massachusetts Division. D.B. is a Research Professor of the American Cancer Society. Received 16 May; accepted 1 Auauatl979. 1. AbellOn, H. T. &: Rabatein, L. S. Cancer R ... 30, 2208-2212 (1970). 2. Scher, C. D. &: Siealer, R. Nature 153, 729-731 (1975). 3. Ro.enbera, N., Baltimore, D. &: Scher, C. D. hoc. nalrl. Acad. Sci. U.S.A. 72,1932-1936 (1975). 4. Roaenbera, N. &: Baltimore, D. !. up. M.d. 143, 1453-1463 (1976). 5. Shield. A. the.i., M.I.T., Cambrldae, Mall. (1979). 6. Wine, O. N., Roaenbera. N., Pukind, M., Shield., A. &: Baltimore, D. Proc. .alrl. Acad. Sci. U.S.A . 75, 2488-2492 (1978). 7. Reynolda. F. H., Sack., T. L., Deobaakar, D. H. &: Stephen.on, J. R. hoc. nalrl. Acad. Sci. U.S.A. 75, 3974-3978 (1978). 8. Reynold., F. H., van de Ven, W. J. M. &: Stephenlon, J. R. J. Virol. 28, 665-670 (1978). 9. Wille, O. N., ROienber" N. &: Baltimore, D. J. Vlrol. (in the prell). 10. Wille, O. N. &: Baltimore, D. J. VITOI. 16,750-761 (1978). 11. CoUett, M. S., Bruaae, J. S. &: BrlklOn, R. L. C.lllS, 1363-1369 (1978). 12. Oppermann, H., LovinlOn, A. D., VarmUl, H. B., Lovintow, L. &: Bilhop, J. M. Proc. nalrl. Acad. Sci. U.S.A. '76, 1804-1808 (1979). 13. Spector, D. H. " al. C.Il13, 371-379 (1978). 14. RI..er, R., Stockert, B. &: Old L. J. Proc. nalrl. Acad. Sci. U.S.A. '75, 3918-3922 (1978). 0028~836/ 79 /400398-02$OI.OO 4 October 1979 j abcdefgh •••• j - P120 Clostridium botulinum can grow and form toxin at pH values lower than 4.6 G. J. M. Raatjes & J. P. P. M. Smelt Unilever Research Vlaardingen. PO Box 114,3130 AC Vlaardingen. The Netherlands It is generally accepted1 that in Clostridium botulinum both growth and toxin formation are completely inhibited at "H values below 4.6. This critical" H value has been confirmed by many investigators U51n! food as substrate2-$ or culture media 3, - . Occasionally9- 1 growth of C. botulinum and toxin formation at pH values lower than 4.6 have been reported. In these cases the authors ascribed the unexpected outgrowth and toxin formation to local pH differences in inhomogeneous media and growth of C. botulinum before pH equiUbration, or to ~he fact that fungi created microenvironment5 within or adjacent to the mycelial mat, where the" H wu higher than 4.6 as was demonstrated by Odiaug and Pftug l l•l l• We show here that the general assumption that C. botulinum does not grow below pH 4.6 is Incorrect. We have observed that growth and toxin formation by C. botulinum can take place in homogeneous protein rich substrates (containinl 3% or more soya or mDk protein) at"H values lower than 4.6. © Macmillan Journals Ltd 1979 Nature Vol. 281 399 4 October 1979 Tablel Influence of pH, type of acid, and protein content on the behaviour of C. botulinum type A and B, if inoculated together with Bacillus spores in substrates based on protein enriched soya extracts Protein content Acid HCl HCI HCI HCl Initial pH (%w/w) 4.0 5.5 4.2 4.4 4.4 1 (1) +- (2) (3) (J (1 ) (2) (3 ) 5.5 5.5 3.0 Citric acid Lactic acid Acetic acid 4.4 4.4 4.4 4.4 Incubation (weeks at 30 ·C) 6 4 8 ++ ++ ++ ++ -+ -+ -+ -+ I I ++ ++ ++ ++ ++ ++ (1) -+ -+ -+ -+ II I 5.5 (J I ++ -+ -+ -+ -+ -+ (J I I NT I (J (J -+ ++ ++ ++ (J (J (J (J (J (J (J (J (1) +- +- (2) (3) ++ -+ ++ ++ ++ ++ -+ -+ ++ ++ ++ ++ (J (J I II II I II II -+ -+ (J (J (J (J (J (J I (J -+ -+ (1) (2) (3) 5.5 -+ -+ (2) (3) (1) (2) (3) 5.5 (J 14 12 NT II (1) 0.6 I +- (1) (2) (3) I I II II I -+ -+ (J (J 10 -+ -+ (J (J (2) (3) HC) 2 (J (J (J (J (J (J (J I (1) Initial inoculum 100 bacilli per ml substrate. At least hundred-fold growth of Bacillus in one sample (+) or two samples (++). Less than hundred-fold growth of Bacillus in one sample (-) or two samples (--). (2) Initial inoculum 1,000 C. botulinum per ml substrate. At least 10-fold growth of C. botulinum in one sample (+) or two samples (++). Less than 10-fold growth of C. botulinum in one sample (-) or two samples (--). (3) (J, Two samples tested for toxin, no sample toxic; I, 1/2 samples toxic; II, 2/2 toxic. NT, Not tested. Media in screwcap bottles containing different levels of soya or milk proteins, in which the pH was adjusted with various acids (Table 1), were inoculated with either a mixture of spores of C. botulinum type A strain 62A, Vh and ZK3 and C. botulinum type B strain B6 and 2345, alone or together with a mixture of spores of Bacillus subtilis and B. licheniformis isolated from soya concentrates. Details are given in Table 1. The inoculated media were filled into screwcap bottles which were subsequently heated for 5 min at 100 ·C. After cooling, the headspace in each bottle was filled with liquid paraffin. Duplicate bottles were examined at various intervals during incubation at 30·C for growth, pH value and toxin; the presence of toxin was tested by intraperitoneal injection of mice. The pH value did not increase from the initial values by more than 0.1 pH unit during incubation. Growth and toxin formation by C. botulinum took place at a pH value as low as 4.0 in the presence of the bacilli if the pH was adjusted with hydrochloric acid. Growth of the bacilli removed residual oxygen and lowered the redox potential, thus creating more favourable conditions for the growth of C. botulinum. When we compared the inhibitory action of hydrochloric, citric, lactic and acetic acids at pH 4.4, growth and toxin formation decreased in that order (Table 1). The level of protein in the media also affected the ability to grow at low pH values-at pH 4.4 a minimum protein level around 3% was necessary for growth and toxin formation. In further experiments with 5.5% soya protein J. S. Crowther (personal communication) observed that in anaerobic conditions spores of C. botulinum are able to outgrow and form toxin at pH 4.2 (if hydrochloric acid was used as acidulant) in the absence of other microorganisms. However the results at this pH were not always repeatable. So far we have not found C. botulinum to grow in a wide range of actual foods with pH value lower than 4.6, but the assumption that C. botulinum does not grow below pH 4.6 has been shown to be incorrect, and the understanding of conditions in which growth may occur, requires further study. Received 23 July; accepted 10 August 1979. I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Edward•• C. C. Fedl R egister 38. 2398- 2410 (1973). Inaram. M. & Robinson, R. H . M. h oc. Soc. awl. 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