Growth, sporulation, and germination of Clostridium perfringens in media of controlled water activity

Risk assessment for Clostridium perfringens in ready-to-eat and partially cooked meat and poultry products

An assessment of the risk of illness associated with Clostridium perfringens in ready-to-eat and partially cooked meat and poultry products was completed to estimate the effect on the annual frequency of illnesses of changing the allowed maximal 1-log growth of C. perfringens during stabilization (cooling after the manufacturing heat step). The exposure assessment modeled stabilization, storage, and consumer preparation such as reheating and hot-holding. The model predicted that assuming a 10- or 100-fold increase from the assumed 1-log (maximal allowable) growth of C. perfringens results in a 1.2- or 1.6-fold increase of C. perfringens-caused illnesses, respectively, at the median of the uncertainty distribution. Improper retail and consumer refrigeration accounted for approximately 90% of the 79,000 C. perfringens illnesses predicted by the model at 1-log growth during stabilization. Improper hot-holding accounted for 8% of predicted illnesses, although model limitations imply that this is an underestimate. Stabilization accounted for less than 1% of illnesses. Efforts to reduce illnesses from C. perfringens in ready-to-eat and partially cooked meat and poultry products should focus on retail and consumer storage and preparation methods.

Combined effects of water activity, solute, and temperature on the growth of Vibrio parahaemolyticus

Vibrio parahaemolyticus was grown at 36 C in tryptic soy broth (pH 7.8) containing added levels of NaCl ranging from 0.5 to 7.9% (wt/wt). The fastest generation time was 16.4 min in tryptic soy broth containing 2.9% NaCl (TSBS) which corresponded to a water activity (a(w)) of 0.992 (+/-0.005). Tryptic soy broth containing lower or higher levels of NaCl resulted in higher or lower a(w), respectively, and slower generation times. Growth was measured turbidimetrically at 36 C in TSBS containing added amounts of NaCl, KCl, glucose, sucrose, glycerol, or propylene glycol. The solutes used to reduce a(w) to comparable levels resulted in extended lag times of varied magnitude, dissimilar growth rates, and different cell numbers. Reduction of a(w) with glycerol was less inhibitory to growth than similar a(w) reductions with NaCl and KCl. Sucrose, glucose, and propylene glycol generally had the greatest effect on extending the lag times of V. parahaemolyticus when the addition of these solutes was made to establish similar a(w) levels lower than 0.992. Minimal a(w) for growth at 15, 21, 29, and 36 +/- 0.2 C for each of four strains of V. parahaemolyticus was tested in TSBS containing added solutes. Reduced a(w) was generally most tolerable at 29 C, whereas higher minimal a(w) for growth was required at 15 C. Solutes added to TSBS to achieve reduction in a(w), minimal a(w) for growth after 20 days, and incubation temperatures were as follows: glycerol, 0.937, 29 C; KCl, 0.945, 29 C; NaCl, 0.948, 29 C; sucrose, 0.957, 29 and 36 C; glucose, 0.983, 21 C; and propylene glycol, 0.986, 29 C. Each of the four strains tested responded similarly to investigative conditions. It appears that minimal a(w) for growth of V. parahaemolyticus depends upon the solute used to control a(w).

Tolerance of bacteria to high concentrations of NaCl and glycerol in the growth medium

When compared at similar levels of water activity, glycerol was more inhibitory than sodium chloride to relatively salt-tolerant bacteria and less inhibitory than salt to salt-sensitive species.

Influence of water activity on the growth of Clostridium perfringens

Each of four strains of Clostridium perfringens was grown in modified fluid thioglycolate medium which was adjusted to yield selected water activity (a(w)) levels. The adjustments to secure the desired a(w) levels were made with NaCl, KCl, or glucose. At each a(w) level, further modification was effected to produce four pH values. Cultures were incubated at either 37 or 46 C. The solute used to achieve the reduced a(w) levels appeared to have a definite effect on the magnitude of growth achieved, the rate of growth, and the limiting a(w) at which growth would occur. Use of glucose as the controlling solute permitted growth at the lowest a(w) level tested, 0.960, and yielded the greatest magnitude of growth as measured by turbidity values, at all of the a(w) levels investigated. Cultures grown in the medium with added KCl generally demonstrated the longest lag times and the least amount of growth. Regardless of specific solute used, as the a(w) level was lowered and the pH value decreased within each a(w) level, the rate and amount of growth were lessened. It appeared, however, that low pH values had less effect on inhibiting growth at low a(w) levels than at higher a(w) levels. Those cultures incubated at 46 C generally exhibited shorter lag periods than those at 37 C, although the maximal growth attained was somewhat less than that achieved at 37 C. The response to all of the investigated conditions was similar for each of the four strains tested.