Journal of Food and Nutrition Research Vol. 47, 2008, No. 1, pp. 18-22 18 © 2008 VÚP Food Research Institute, Bratislava Staphylococcus aureus is a Gram-positive coc- cus, which forms grape-like clusters. This bac- terium is facultatively anaerobic, mesophilic, catalase-positive and oxidase-negative. It grows within a temperature range from 7 °C to 48 °C and in a pH range from 4.2 to 9.3. Its distinctive fea- ture is halotolerance equivalent to minimal water activity (a w ) values ranging from 0.83 to 0.85 [1]. Strains of these bacteria are characterized by pro- duction of heat-stable enterotoxins, which may be produced at a w values as low as 0.86 under aerobic conditions. S. aureus is generally associated with skin and mucous membranes of warm-blooded animals, in- cluding humans. BAIRD-PARKER [2] considered this organism to be transient and not usually becoming a part of the resident flora of the skin. Irrespective of this fact, growth of S. aureus is connected with artisanal bryndza cheese produced from raw ewes’ milk in Slovakia. The level of milk contamination by these pathogenic bacteria may be increased when mastitis is present in the herd. During milking and primary treatment of milk on farms, S. aureus may colonize the parts of technological equipment that are difficult to clean [3]. For exam- ple, typical counts of S. aureus in properly drawn ewes’ milk are between 100 and 200 CFU.ml -1 [4], and S. aureus is considered to be one of the ubiquitous contaminants. The presence of toxin- producing strains in ewes’ lump cheese or bryndza cheese produced from raw milk is dependent on various factors that include animal health status and environmental, hygienic and technological conditions during production. On the other hand, fermentation and ripening of cheese reduce the probability of enterotoxin production. According to [5], increase in the bacterial population during initial 24 h of cheese production in normal con- ditions is about 1.5 to 3 log CFU.ml -1 . The maxi- mum contents of S. aureus of m = 10 4 CFU.g -1 and M = 10 5 CFU.g -1 , (n = 5, c = 2; where n is number of units comprising the sample, c – number of sample units giving values greater than m or be- tween m and M) is a criterion of process hygiene for cheeses produced from raw milk according to Regulation (EC) No. 2073/2005 [6]. When analysing a bacterial growth curve, lag time and the exponential growth phase are the most important for growth assessment. At present, the Baranyi model and Gompertz function are the most widely used primary models for the descrip- tion of microbial growth [7, 8]. The growth of a culture of a particular micro- bial strain in foods is determined by its intrinsic Variability of growth parameters of Staphylococcus aureus in milk ĽUBOMÍR VALÍK – ALŽBETA MEDVEĎOVÁ – BARBORA BAJÚSOVÁ – DENISA LIPTÁKOVÁ Summary Growth of 15 strains of Staphylococcus aureus in milk at 15 °C was studied in 87 experiments, in order to determine the reproducibility of the growth parameters measured. The average growth rate (Gr) at 15 °C was 0.067 log CFU.h -1 , ranging from 0.54 to 0.77 log CFU.h -1 , which was equivalent to a doubling time ranging from 12.8 h to 9.0 h. Using a standard inoculation of an 18 h culture, the initial counts in milk of 3.68 log CFU.ml -1 and the duration of the lag phase of 14.4 h were determined with the standard deviations of 0.2 log CFU.ml -1 and 2.4 h, respectively. Based on ANOVA tests, the growth rates of individual strains did not produce different variances at the statistically significant 5% level. The results show that despite the errors connected with cultivation methods of determination microbial counts, the coefficient of variation (CV) of the growth rates for all 15 strains was as low as 7.1%. When 6 growth curves for a single strain were studied in two independent experiments, a CV of 1.2% for the growth rate was determined. Keywords growth parameters; variability; Staphylococcus aureus; predictive microbiology Ľubomír Valík, Alžbeta Medveďová, Barbora Bajúsová, Denisa Liptáková, Institute of Biochemistry, Nutrition and Health Protection, Department of Nutrition and Food Safety, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinského 9, SK – 812 37 Bratislava, Slovakia. Correspondence author: Ľubomír Valík, e-mail: lubomir.valik@stuba.sk