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Natural antimicrobials are an alternative to the use of chemically synthesized preservatives and other technological treatments. They have the advantage of not being rejected by consumers because of their natural origin. However, prior to their usage at a food factory scale, their precise effects on microorganisms have to be known. Nisin, a bacteriocin, and carvacrol and thymol, phenolic compounds present in the essential oil fraction of Oreganum and Thymus plants, have been shown to inhibit growth of different bacteria. This research was conducted to show the effect of a thermal treatment, applied previously, on the growth of B. megaterium cells in a culture medium with and without nisin, carvacrol and thymol. It has been shown that a thermal treatment able to kill 90 % of the spore population of B. megaterium had little effect on the growth of the survivors in nutrient broth at 30 °C. Nisin, in a concentration of 0.05 mM, did not affect greatly the growth of unheated B. megaterium. When the same concentration of nisin was applied to the survivors of the heat treatment, lag phase was further increased and growth rate decreased. Thymol, applied at a concentration of 0.6 mM, increased the lag phase duration almost three times, although it did not change the growth rate. Although carvacrol, at the same concentration (0.6 mM), did reduce the growth rate to half its original value, it did not increase the lag phase duration significantly (only twice), resulting in a quite similar growth curve. The combination of thymol and carvacrol at these high doses (0.6 mM) resulted in a further increase of the lag phase and significant decrease of the growth rate. When carvacrol and/or thymol were combined with a previous thermal treatment (able to kill 90 % of the population), the growth of the survivors was inhibited for at least seven days. Therefore a combination of moderate conditions could be effective to control spoilage by sporeforming bacteria., Prirodna antimikrobna sredstva su alternativa kemijski sintetiziranim konzervansima i drugim tehnološkim postupcima. Njihova je prednost u tome što su zbog svoga prirodnog podrijetla prihvatljivi potrošačima. Prije njihove primjene u prehrambenoj industriji, potrebno je znati kako djeluju na mikroorganizme. Nisin, bakteriocin, te karvakrol i timol, fenolni spojevi prisutni u frakcijama eteričnog ulja biljaka Oreganum i Thymus, inhibiraju rast raznih bakterija. Opisano je istraživanje provedeno da bi se utvrdio učinak prethodno provedene toplinske obrade na rast stanica B. megaterium u podlozi s nisinom, karvakrolom i timolom te bez njih. Pokazalo se da toplinska obrada može usmrtiti 90 % populacije spora B. megaterium, a gotovo nikako ne utječe na rast preživjelih u hranjivoj podlozi pri 30 °C. 22 P.M. PERIAGO et al.: B. megaterium Spore Germination and Growth Inhibition, Food Technol. Biotechnol. 44 (1) 17–23 (2006) Nisin koncentracije od 0,05 mM nije bitno utjecao na rast toplinski neobra|enog B. megaterium. Kada se ista koncentracija nisina primijeni na preživjele spore, produlji se lag faza, a smanji brzina rasta. Timol koncentracije od 0,6 mM produžio je trajanje lag faze skoro 3 puta, a nije promijenio brzinu rasta. Iako je karvakrol iste koncentracije (0,6 mM) smanjio brzinu rasta na polovinu originalne vrijednosti, dužinu lag faze produžio je samo za 2 puta, što je rezultiralo skoro jednakom (0,6 mM) krivuljom rasta. Kombinacijom tih velikih koncentracija timola i karvakrola znatno je produljena lag faza i smanjena brzina rasta. Kada se karvakrol i/ili timol kombiniraju s prethodnom toplinskom obradom (pri čemu je uništeno 90 % populacije), rast preživjelih bio je inhibiran barem 7 dana. Stoga kombinacija jednostavnih uvjeta može biti djelotvorna u kontroli onečišćenja sa sporulirajućim bakterijama

Minimal processing implementation greatly depends on a detailed knowledge of the effects of preservation factors and their combinations on the spoilage and foodborne pathogenic microorganisms. The effectiveness of mild preservation conditions will become increasingly dependent on a more stochastic approach linking microbial physiological factors with product preservation factors. In this study, the validity of frequency distributions to efficiently describe the inactivation and growth of Bacillus cereus in the presence of natural antimicrobials (essential oils) has been studied. For this purpose, vegetative cells were exposed to 0.6 mM of thymol or cymene, obtaining survival curves that were best described by the distribution of Weibull, since a tailing effect was observed. B. cereus was also exposed in a growth medium to a low concentration (0.1 mM) of both antimicrobials, separately or combined, and the lag times obtained were fitted to a normal distribution, which allowed a description of dispersion of the start of growth. This allowed a more efficient evaluation of the experimental data to establish safe processing conditions according to accurate parameters and their implementation in risk assessment.