The selection of a primary model to describe microbial growth in predictive food microbiology often appears to be subjective. The objective of this research was to check the performance of different mathematical models in predicting growth parameters, both by absorbance and plate count methods. For this purpose, growth curves of three different microorganisms (Bacillus cereus, Listeria monocytogenes, and Escherichia coli) grown under the same conditions, but with different initial concentrations each, were analysed. When measuring the microbial growth of each microorganism by optical density, almost all models provided quite high goodness of fit (r 2 > 0.93) for all growth curves. The growth rate remained approximately constant for all growth curves of each microorganism, when considering one growth model, but differences were found among models. Three-phase linear model provided the lowest variation for growth rate values for all three microorganisms. Baranyi model gave a variation marginally higher, despite a much better overall fitting. When measuring the microbial growth by plate count, similar results were obtained. These results provide insight into predictive microbiology and will help food microbiologists and researchers to choose the proper primary growth predictive model.

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The stability of ceftriaxone in undiluted human serum and in human serum diluted 1/20 in 0.1 M phosphate buffer (pH 6.0) was measured at -70, -40, -20, 4, and 37°C. Ceftriaxone in diluted human serum contained at least 90% of the initial activity after 3 months of storage at -20, -40, and -70°C; unbuffered human serum contained 82. 41, 84.92, and 88.96% of the initial activity, respectively. Ceftriaxone in unbuffered human serum and in diluted human serum showed 80.33 and 86.25% of the initial activity, respectively, after 55 days at 4°C. After 120 h at 37°C, this antibiotic in unbuffered human serum contained 33.08% of the initial activity, whereas samples of diluted human serum contained more than 60%. Consequently, the stability of antibiotic in human serum diluted 1/20 in 0.1 M phosphate buffer (pH 6.0) is increased.Ceftriaxone (Ro 13-9904), a 2-aminothiazoyl-methoxyimino semisynthetic cephalosporin, has been shown to have good in vitro and in vivo activities against many genera of bacteria (7). What distinguishes ceftriaxone from the other newer cephalosporins is its unusually long plasma half-life, which is 4 to 10 times longer than those of other cephalosporins (9). A simple method for buffering human serum at about pH 6.0 was developed by using 0.1 M phosphate buffer. This dilution method is useful when the samples, from clinical studies, cannot be shipped to the analyst after their collection or cannot be evaluated immediately. This study describes the stability characteristics of ceftriaxone sodium in undiluted human serum and in human serum diluted 1/20 in 0.1 M phosphate buffer (pH 6.0).Ceftriaxone standard powder was kindly provided by Roche Products, S.A. We used batch 012037 with a potency of 820 ,ug as free acid per mg. Serum from healthy human subjects not receiving antibiotics was pooled. The final pH was 7.4. The fluid was tested before use to ensure that it was devoid of antimicrobial activity. The pooled human serum was diluted 1/20 in 0.1 M phosphate buffer (pH 6.0; 1.69 g of Na2HPO4 per liter, 11.69 g of KH2PO4 per liter). This dilution buffers the serum pH at about 5.9 to 6.1. Samples were prepared with 50 ,ug of ceftriaxone sodium per ml in both pooled human serum and diluted human serum. Of each sample, 0.5 ml was withdrawn and placed in a sterile glass vial (2 ml). All solutions were prepared in a laminar-flow hood by using aseptic techniques and then were incubated at 37 and 4°C for 120 h and 55 days, respectively, and at -20, -40, and -70°C for 3 months. Samples were removed at appropriate intervals and assayed after being thawed for 2 h at room temperature when they were conserved at freezing temperatures or after several minutes when they were stored at 37 and 4°C. After sampling, the specimens were discarded. The concentration of the antibiotic was determined, in triplicate, by a disk plate assay procedure using Escherichia coli SQ 12155 as the assay microorganism when we evaluated human serum samples and Bacillus subtilis ATCC 6633 spores to determine the stability of antibiotic ...