The effect of ultrasonic treatment on the heat resistance of Bacillus spores differs widely both with the species and the strain tested. Ultrasonic waves do.
APPLIED AND ENVIRONMENTAL MICROBIOLOGY, July 1976, Copyright ©D 1976 American Society for Microbiology
p. 183-184
Vol. 32, No. 1 Printed in U.S.A.
Effect of Ultrasonic Waves on the Heat Resistance ofBacillus Spores J. A. ORDONEZ AND J. BURGOS*
Cdtedra de Bioquimica y Tecnologia de los Alimentos, Facultad de Veterinaria, Le6n, Spain Received for publication 1 March 1976
The effect of ultrasonic treatment on the heat resistance of Bacillus spores differs widely both with the species and the strain tested. Ultrasonic waves do not affect the heat resistance of some strains, whereas they greatly reduce that of other strains of the same species. The heat-sensitizing effect of ultrasonic waves is more pronounced when short heating periods are used. Ultrasonic treatment does not seem to affect the "Z value."
Ultrasonic treatment has been used in the past to eliminate vegetative cells in spore suspensions (2, 3) and has been recommended for breaking aggregates of Clostridium spores prior to heat resistance determinations (2). However, Burgos et al. (1) have reported that it seems unadvisable to use this kind of treatment with Bacillus spore suspensions since it markedly decreased the heat resistance of B. cereus and B. licheniformis strains tested in their work. The purpose of the present study was to investigate whether this heat-sensitizing effect of ultrasonic waves is a general characteristic of Bacillus or is restricted to some strains, and to determine the possible effect of ultrasonic treatment on the "Z value" (number of degrees change in temperature necessary to achieve a 10-fold change in the decimal reduction time, "D value"). Four strains of B. cereus isolated by G. Sudrez (Barcelona University) from sterilized milk and four more (strain of B. subtilis, 189 supplied by Institute de Bacteriologie D'Hygiene et Virologie, Faculte de M6decine, Lausanne, B. subtilis ATCC 5633, B. subtilis Shenley, and the strain provided by Difco Laboratories) were tested. Spore production, harvesting, ultrasonic and heat treatment, and survivor counting were performed as described previously (1). Even after 30 min of ultrasonic treatment (0°C; 20 kc; 1.2 A; ultrasonic disintegrator MSE, 60 W), the heat resistance at 100°C of one of the B. cereus strains isolated (X-89-F) did not change; those of the other three were reduced to various extents, T-27-S being affected the most. In all of the affected B. cereus strains, the heatsensitization caused by ultrasonic treatment effects a very large reduction in the number of survivors during the first 5 min of heating and a moderate decrease in the D value afterwards. 183
The responses of the four strains of B. subtilis tested varied widely. A linear relationship between the log survivors and time was demonstrated with B. subtilis 189 at three tempera8
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232 236 224 228 Temperatufe (F) FIG. 1. (A) Heat destruction of B. subtilis 189. Symbols: A, without ultrasonic treatment; *, after application of ultrasonic waves (20 kc, 1.2 A, 10 min, volume 5 ml). Each point is the average of three determinations. Heat treatment at 105°C (221°F), 1100C (230°F), ----; and 1120C (233.60F), (B) Z values of B. subtilis 189 without ultrasonic treatment (-) and after application of ultrasonic waves (20 kc, 1.2 A, 10 min, volume 5 ml) (A).
184
NOTES
tures. At each temperature, the ultrasonically treated spores were more susceptible to heat than the untreated spores (Fig. 1A). B. subtilis ATCC 5663 gave heat resistance curves indicating that ultrasonic treatment greatly increases killing during the first 5 min of heating but does not reduce the D value of the survivors. The response of the other two strains was intermediate between that of ATCC 5663 and strain 189. The effect of ultrasonic treatment on the Z value was studied with B. subtilis 189, a strain chosen because of the straight-line plot of survivors against heating time in control and ultrasonically treated spore suspensions. Figure 1
APPL. ENVIRON. MICROBIOL.
demonstrates that 10 min of ultrasonic treatment, although it reduces D values at all tested temperatures, does not affect the Z values esti-
mated at 10°F (5.55°C). LITERATURE CITED 1. Burgos, J., J. A. Ordonez, and F. Sala. 1972. Effect of ultrasonic waves on the heat resistance of Bacillus
and Bacillus licheniformis spores. Appl. Microbiol. 24:497-498. 2. Goodnedough, E. R., and M. Solberg. 1972. A technique for producing large yields of vegetative cellfree refractile Clostridium perfringens spores of unaltered heat resistance. Appl. Microbiol. 23:429-430. 3. Heiligman, F., N. W. Desrosier, and H. Broumand. 1956. Spore germination. I. Activators. Food Res. 21:63-69. cereus
