The water activity of common liquid bacteriological media

Performance evaluation of a model describing the effects of temperature, water activity, pH and lactic acid concentration on the growth of Escherichia coli

A square root-type model for Escherichia coli growth in response to temperature, water activity, pH and lactic acid was developed by Ross et al. [Int. J. Food Microbiol. (2002).]. Predicted generation times from the model were compared to the literature data using bias and accuracy factors, graphical comparisons and plots of residuals for data obtained from both liquid growth media and foods. The model predicted well for 1025 growth rate estimates reported in the literature after poor quality or unrepresentative data (n=215) was excluded, with a bias factor of 0.92, and an accuracy factor of 1.29. In a detailed comparison to two other predictive modes for E. coli growth, Pathogen Modeling Program (PMP) and Food MicroModel (FMM), the new model generally performed better. The new model consistently gave better predictions than the other models at generation times </=5 h. Inclusion of the lactic acid term in the model is proposed to account for the consistently good performance of the model for comparisons to growth in meat, a parameter that is not explicitly included in the other models considered in the comparisons.

Effect of water activity on production of beta-lactam antibiotics by Streptomyces clavuligerus in submerged culture

The amount of available water in the environment of micro-organisms, defined as water activity (aw), has been shown to affect growth, respiration, enzyme synthesis, sporulation and other physiological functions. The aim of this study was to examine the impact of aw on production/excretion of a secondary metabolite. For this purpose, the production of beta-lactam antibiotics and biomass of Streptomyces clavuligerus was studied in relation to the aw-depressing agents glucose, sorbitol and NaCl. These were chosen because NaCl and sorbitol are often used to depress aw and glucose was not thought to be taken up by S. clavuligerus. The filamentous bacterium S. clavuligerus NRRL 3585 (ATCC 27064) is a prokaryotic producer of penicillin N, cephalosporins including cephamycin C and clavulanic acid. Under water stress conditions, a greater effect upon antibiotic biosynthesis than upon growth was consistently observed. When aw was decreased to below 0.997, antibiotic production began to decrease. For growth, inhibition was much more gradual and did not become intensive until an aw of 0.990 was reached.

Minimal water activity for growth of Listeria monocytogenes as affected by solute and temperature

Two strains of Listeria monocytogenes, Scott A (serotype 4b) and Brie 1 (serotype 1b) were characterized in terms of minimum aw requirements for growth at 4 and 30 degrees C, using NaCl, glycerol and sucrose as test solutes. Both strains grew well at 30 degrees C in glycerol-supplemented tryptic soy broth (TSB), but not in NaCl- and sucrose-supplemented TSB at aw 0.90. Sucrose was more inhibitory than NaCl and glycerol. The Brie 1 strain was less tolerant to sucrose compared to the strain Scott A at both 4 and 30 degrees C. The effects of all three solutes were magnified at 4 degrees C, where tolerance to low aw was markedly less than at 30 degrees C. Results confirm that the type of solute as well as the osmotic conditions created by the solute affects the ability of Listeria monocytogenes to grow at 4 and 30 degrees C.

Effect of water activity and pH on growth and toxin production by Clostridium botulinum type G

The combined effect of water activity (aw) and pH on growth and toxin production by Clostridium botulinum type G strain 89 was investigated. The minimum aw at which growth and toxin formation occurred was 0.965, for media in which the pH was adjusted with either sodium chloride or sucrose. The minimum pH (at the optimum aw) for growth and toxin production of C. botulinum type G was found to be 5.6. Optimum conditions for toxin activation were a trypsin concentration of 0.1%, a pH of the medium of 6.5, and an incubation for 45 min at 37 degrees C. These data did not show evidence of heat-labile spores, since a heat shock of 75 degrees C for 10 min did not significantly decrease the spore count of strain 89G in media at pH 7.0 or 5.6. It was frequently observed that cells grown at reduced aw or pH experienced severe morphological changes.

Some factors affecting the efficiency of settle plates

An evaluation has been made of some of the factors which may affect the efficiency of settle plates. Water loss was found to be linear with time. Although the count was reduced over an 8 h period the reduction was not statistically significant. No difference in total bacterial counts could be detected between four, 1/2 h exposures and one, 2 h exposure. The addition of water and the surface area of the plates had no effect on the total count.