Death, microbial

It is necessary to estabUsh a criterion for microbial death when considering a sterilization process. With respect to the individual cell, the irreversible cessation of all vital functions such as growth, reproduction, and in the case of vimses, inabiUty to attach and infect, is a most suitable criterion. On a practical level, it is necessary to estabUsh test criteria that permit a conclusion without having to observe individual microbial cells. The failure to reproduce in a suitable medium after incubation at optimum conditions for some acceptable time period is traditionally accepted as satisfactory proof of microbial death and, consequentiy, stetihty. The appHcation of such a testing method is, for practical purposes, however, not considered possible. The cultured article caimot be retrieved for subsequent use and the size of many items totally precludes practical culturing techniques. In order to design acceptable test procedures, the kinetics and thermodynamics of the sterilization process must be understood. 

Nonoxidizing Antimicrobials. Nonoxidizing antimicrobials usually control growths by one of two mechanisms. In one, microbes are inhibited or killed as a result of damage to the ceU membrane. In the other, microbial death results from damage to the biochemical machinery involved in energy production or energy utilization. 

As predicted by the Arrhenius equation (Sec. 4), a plot of microbial death rate versus the reciprocal or the temperature is usually linear with a slope that is a measure of the susceptibility of microorganisms to heat. Correlations other than the Arrhenius equation are used, particularly in the food processing industry. A common temperature relationship of the thermal resistance is decimal reduction time (DRT), defined as the time required to reduce the microbial population by one-tenth. Over short temperature internals (e.g., 5.5°C) DRT is useful, but extrapolation over a wide temperature internal gives serious errors. 

The activation energy (E) associated with microbial death is larger than the thermal inactivation of chemical compounds in fermentation broths (see Table 24-4). Thus by sterthzing at high temperatures for short times (HTST), overcooking of nutrients is minimized. 

These have been ascribed to seasonal variability in organic inputs through rhizodeposition, microbial death and residue inputs and/or seasonal variability in soil microbial activity and thus metabolism of the... 

A novel bioassay for nystatin based on the use of a microbial sensor was recently reported. Nystatin is believed to bind to the steron present in the membranes of sensitive cells, leading to the formation of pores. The subsequent death of the microorganism is preceded by leakage of cellular materials. Microbial death can be detected by means of an oxygen electrode. 

Ultra-high-temperature treatment (UHT) is now the most widely exploited method in the food industry to stabilize microbiologically any foodstuff. It consists of heating at an ultra high-temperature for a short period of time for example, a treatment at 145°C for 2 seconds is sufficient to assure a total microbial- and spore inactivation. The microbial death is principally due to irreversible cell damage (e.g., of proteins, DNA, RNA, vitamins) enzymes are inactivated by heat which modifies their active sites. 

From the data reported above, it is clear that the semi-continuous system is much more efficient than the batch one. If the main cause of microbial death is the CO2 concentration in the liquid phase, in the flux process high gas concentrations are reached faster, as the bubbling enhances and favours the mass transport of CO2 from the gas- to the liquid phase. In the semi-continuous process, a total sterilization is obtained with an operating pressure of 74 bar and an exposure time of 10 minutes, while in a batch process, 60 minutes are needed with an operating pressure of 200 bars. Tables 9.10-4 and 9.10-5 give results for other experiments run with the semi-continuous system. 

The D value is a single quantitative expression of the rate of killing of microorganisms. The D term refers to the decimal point in which microbial death rates become positive time values by determining the time required to reduce the microbial population by one decimal point. This is also the time required for a 90% reduction in the microbial population. Hence, the time or dose it takes to reduce 1000 microbial cells to 100 cells is the D value. The D value is important in the validation of sterilization processes for several reasons. 

The following equation typically describes the order of microbial death ... 

Fig. 1 Microbial death rate curves that illustrate concept of decimal reduction (D values) and probability of survivors. (From Ref. l)...

Fig. 6 Microbial death rate (D determination using direct enumeration of survivors) for B. Subtilis var. niger on paper (D = 4310 min), aluminum (D = 6143 min), and plastic (D = 12,300 min) carrier.

There are times when a researcher wants to predict a specific x value from a y value as well as generate confidence intervals for that estimated x value. For example, in microbial death kinetic studies (D values), a researcher often wants to know how much exposure time (x) is required to reduce a microbial population, say, three logs from the baseline value. Alternatively, a researcher may want to know how long an exposure time (x) is required for an antimicrobial sterilant to reduce the population to zero. In these situations, the researcher will predict x from y. Many microbial death kinetic studies, including those using dry heat, steam, ethylene oxide, and gamma radiation. 

The emeraldine salt forms of copolymers appear to be more antimicrobially effective than the emeraldine base forms of the same copolymer. It seems that the presence of an acidic functional group (-COOH) in the polymer chain improves the antibacterial efficacy of the copolymer. This could, in theory, be due to the acidic dopants on the molecular chains of the copolymers reacting with the bacteria (or other relevant microbial organism) which result in microbial death. Alternatively, it could be due to the electrostatic adherence between copolymer molecules and the bacteria, which carry charges of different polarity, the bacteria walls may break and the bacterial contents become exposed or leak out, which cause the bacteria to die [17]. 

At low values of T, the second term is negligible, and the rate will formally follow the simple Arrhenius law with E. Over a certain narrow range of T, the two terms will be of the same order of magnitude. After that the negative second term far outweighs the first the rate will fall rapidly to zero with a fornial activation energy of microbial death E 2-... 

Negative Biokinetic Rates—The Case of Microbial Death and Endogenous Metabolism... 

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