Sterilization treatment resistance

The micro-organisms must have high resistance to the sterilization treatment which they are being used to validate. This does not mean that they must be the most resistant micro-organism known to man. B. stearothermophilus has D i-values of l min according to conditions of culture and the substrate upon which they are mounted. This is higher than most spores of Bacillus spp., which tend to have Di2i-values below 0.5 min. 

The micro-organisms must have stable resistances to the sterilization treatment which they are being used to validate. There are data from commercial suppliers of Bis to show that spores of B. stearothermophilus survive and retain stable resistances over long periods of crudely controlled storage. The micro-organisms must be easily culturable and preferably be easily identifiable in culture. Very few micro-organisms share with B. stearothermophilus the ability to grow in simple culture media at 55-60°C. 

In the first instance, the intensity or durtUion of the sterilization treatment may be very high, possibly loo high to be tolerated by some products. This begs the practical question of why such an intense process must be used. It might be argued that the actual contaminants on the product are far less resistant to the sterilization treatment than the reference microorganisms. 

The impact of the proposal may reopen a debate on the way in which SALs appear to be applied to aseptic processing. The origins of the SAL concept lie in terminal sterilization and rest heavily on the extrapolated effects of uniform sterilization treatments to populations of contaminants defined in terms of resistance and of numbers of contaminants (biobuiden). The process of aseptic manufacture is a process of contamination control the frequency of occurrence of a contaminated item within a population of aseptically filled items is a measure of bioburden, not a measure of the SAL. The SAL is the probability of those contaminants surviving, and this is a function of the types of contaminants and the formulation of the product. Formulations can be made to be antimicrobial. In principle this is no different from chemical sterilization. 

Physical and thermal properties PP is thermoplastic. It has low density and low strength but has poor impact resistance. Reinforcing with glass, chalk or talc increases flexibility, impact resistance and strength. PP has a higher melting temperature than PE so can be used to contain hot materials and to withstand steam sterilizing treatments. ... 

Sterilization and radiation resistance 16 Growing use of POs in medical applications, radiation-intensive sterilization treatments Better antioxidants and stabilization against radiation... 

The estabhshment of safe thermal processes for preserving food in hermetically sealed containers depends on the slowest heating volume of the container. Heat-treated foods are called commercially sterile. Small numbers of viable, very heat-resistant thermophylic spores may be present even after heat treatment. Thermophylic spores do not germinate at normal storage temperatures. 

Irradiation. Although no irradiation systems for pasteurization have been approved by the U.S. Food and Dmg Administration, milk can be pasteurized or sterilized by P tays produced by an electron accelerator or y-rays produced by cobalt-60. Bacteria and enzymes in milk are more resistant to irradiation than higher life forms. For pasteurization, 5000—7500 Gy (500,000—750,000 tad) are requited, and for inactivating enzymes at least 20,000 Gy (2,000,000 rad). Much lower radiation, about 70 Gy (7000 tad), causes an off-flavor. A combination of heat treatment and irradiation may prove to be the most acceptable approach. 

Bacterial spores are the most resistant of all microbial forms to chemical treatment. The majority of antimicrobial agents have no useful sporicidal action, with the exception of the aldehydes, halogens and peroxygen compounds. Such chemicals are sometimes used as an alternative to physical methods for sterilization ofheat sensitive equipment. In these circumstances, correct usage of the agent is of paramount importance since safety margins are lower in comparison with physical methods of sterilization (Chapter 20). 

Ideally, when considering the level of treatment necessary to achieve sterility a knowledge of the type and total number of miciooiganisms present in a product, together with their likely response to the proposed treatment, is necessary. Without this information, however, it is usually assumed that organisms within the load are no more resistant than the reference spores or than specific resistant product isolates. In the latter case, it must be remembered that resistance may be altered or lost entirely by laboratory subculture and the resistance characteristics of the maintained strain must be regularly checked. 

The resistance of an organism to a sterilizing agent can be described by means of the D-value. For heat and radiation treatments, respectively, this is defined as the time taken at a fixed temperature or the radiation dose required to achieve a 90% reduction in viable cells (i.e. a 1 log cycle reduction in survivors Fig. 20.2k). The calculation of the D-value assumes a linear type A survivor curve (Fig. 20.1), and must be corrected to allow for any deviation from linearity with type B or C curves. Some typical D-values for resistant bacterial spores are given in Table 23.2 (Chapter 23). 

Organisms are more resistant to ethylene oxide treatment in a dried state, as are those protected from the gas by inclusion in crystalline or dried organic deposits. Thus, a further condition to be satisfied in ethylene oxide sterilization is attainment of a minimum level of moisture in the immediate product environment. This requires a sterilizer humidity of 30-70% and frequently a preconditioning of the load at relative humidities of greater than 50%. 

There is concern regarding administration of dexamethasone to patients with pneumococcal meningitis caused by penicillin- or cephalosporin-resistant strains, for which vancomycin would be required. Animal models indicate that concurrent steroid use reduces vancomycin penetration into the CSF by 42% to 77% and delays CSF sterilization due to reduction in the inflammatory response.23 Treatment failures have been reported in adults with resistant pneumococcal meningitis who were treated with dexamethasone, but the risk-benefit of using dexamethasone in these patients cannot be defined at this time. Animal models indicate a benefit of adding rifampin in patients with resistant pneumococcal meningitis whenever dexamethasone is used.21,23... 

Testing of decay resistance can be performed in a laboratory environment or in outdoor field trials, and there are many standards defined for these tests. The first objective of a laboratory-based test is to provide a methodology for the rapid screening of a candidate wood preservative, treatment or modification in order to assess which ones exhibit decay resistance. Broadly speaking, laboratory-based tests can be divided into sterile (pure culture) tests and unsterile tests (such as fungal cellar, soil burial etc.). 

Later, Timson and Short started systematic experiments to test the resistance of bacterial spores, and tried to inactivate them completely to obtain a total sterilization. These authors studied the behaviour of spores under a high-pressure treatment with a long residence time at constant pressure in a range of temperature between - 25°C and 95°C. They noted the high insensitivity to pressure of the spores compared to vegetative forms [8]. 

Once a new case of resistance has been confirmed, detection will then most likely rely on the reduction of egg counts after treatment (faecal egg count reduction test (FECRT)). Allowance must be made for temporary sterilization of the worms, or in the case of tapeworms, destrobilation. Interpretation of results may be further complicated by development of immature forms in cases where drugs only kill adult worms. No such tests have been adequately investigated for detecting resistance in cestodes or trematodes. 

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