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Challenge tests and their predictive ability
Published in R. M. Baird, S. F. Bloomfield, Microbial quality assurance in cosmetics, toiletries and non-sterile Pharmaceuticals, 2017
R. E. Leak, C. Morriis, R. Leech
Preservative activity can be measured by assessing the initial rate of kill of the test organism. This can be achieved by determining the D value, the time required for 90% reduction in the population (the decimal reduction time). The preserved product is inoculated with a test organism and the number of viable cells determined at intervals over a short period, e.g. 6 h, 24 h or 48 h. From these data, the D value can be calculated by plotting log number of surviving organisms per gram as a function of the time after inoculation into the product (Orth 1980). Using this method, Orth (1980) calculated the D values of three concentrations of a given preservative in a product and determined the concentration of preservative required to give a specific D value. Orth (1981) suggested that D values of 4 h or less for pathogens and 28 h or less for non-pathogenic bacteria, yeasts and moulds (resulting in a destruction of 106 organisms g−1 in 24 h and 7 days, respectively) should give satisfactory preservation in cosmetics intended for use around the eyes and for baby products. D values can be used for evaluating different preservatives using a range of test organisms (Akers et al. 1984). For preservatives which produce a rapid initial reduction in microbial populations followed by a gradual subsequent loss of viability, Akers et al. (1984) suggested a quadratic expression for calculation of the D value which gave a more representative estimation of the destructive action of the preservative system than the linear model of Orth (1980). Orth and Milstein (1989) concluded from their work with D values and linear regression that preservative efficacy testing depended on several factors; reliability, time available, use of statistical controls, the need for rechallenge testing, the organisms used, the product being tested and the acceptance criteria. Testing the efficacy of the actual preservative was merely one part of the process of evaluating the preservative capacity of a product. Subsequently, Orth (1991) showed that products which complied with the standards of the USP and CTFA but which failed to meet acceptance criteria based on D values, occasionally failed to prevent growth of micro-organisms.
Antimicrobial mechanism and the effect of atmospheric pressure N2 plasma jet on the regeneration capacity of Staphylococcus aureus biofilm
Published in Biofouling, 2018
Jiaquan Wang, Zhiyuan Yu, Zimu Xu, Shuheng Hu, Yunxia Li, Xiaojuan Xue, Qiuchen Cai, Xiaoxia Zhou, Jie Shen, Yan Lan, Cheng Cheng
The culturability of S. aureus biofilms after plasma treatment was evaluated by counting colony forming units (CFUs) grown on the TSB agar plates. The decimal reduction time (D-value) was also studied to investigate the inactivation effects. The plasma-treated S. aureus biofilm was placed in 1 ml of deionized H2O and ultrasonically dispersed for 20 min (Matthes et al. 2013, 2014). Afterwards, 100 µl of the diluted solution were spread on TSB agar plates. The inactivation effects of biofilms were then assessed by counting the cultivable bacterial populations after incubation overnight at 37 °C.