<i>Enterococcus faecium</i> growth model

Enterococci are bacteria commonly found in humans. However, these bacteria can cause severe infections in susceptible individuals. The strains of <i>Enterococcus faecium</i> have demonstrated an increasing resistance to antibiotics, which is considered an important virulence factor. The contribution of <i>E. faecium</i> to the infection-related illnesses has recently increased, which involves most of the isolated Vancomycin-Resistant Enterococcus (VRE) strains. Enterococci are common contaminants of human milk processed in milk banks, and the consumption of contaminated milk can cause severe infection-related complications if the control mechanisms fail to detect the contamination. Extensive data are available on the growth curves of <i>E. faecium</i> in broth at pH values between 6 and 7, at temperatures of 5°C to 20°C, and for water activity values <i>a</i>_w of 0.97 to 0.997. These growth curves were replaced with non-linear Gompertz curves for microorganism growth, the parameters of which were correlated with the temperature and pH values. A mathematical relationship to water activity could not be established since only two water activity levels have been experimentally tested and the resulting model would be highly inaccurate. The issue of water activity was resolved by the development of two separate models, one for each of the water activity values. The models correspond very well with the experimental growth curve data from which they were developed. The model for the water activity level of 0.997 was used to predict the growth of <i>E. faecium</i> in cow and human milks (these two fluids have practically identical water activity), and the prediction was compared to the experimental data. A good agreement between the predicted and experimental data was achieved for cow milk. With human milk, the model usually predicted a more rapid growth rate than that seen experimentally. The model was thus on the conservative side in all cases. The inhibitory agents naturally present in human milk might be responsible for the slower growth rates.