Microorganisms inducible resistance

Antibiotics such as penicillin, streptomycin, and amphotericin B can also be added to the culture medium to prevent microbial growth. However, their use should be kept to a minimum level, because some antibiotics can have cytotoxic effect, even when high serum concentrations are used. Besides, the continuous use of antibiotics can result in the selection of resistant microorganisms, inducing a chronic contamination of the culture (Freshney, 1992). 

In human medicine, selection pressure is at its most intense in hospitals, where antibiotics are extensively used. The major cause of problems of antimicrobial resistance in humans arises from overuse of antimicrobials at therapeutic levels in humans. It is generally accepted that drug resistance that develops in a bacterium as a result of mutation is only of importance within the individual host and a single bacterial strain. Because the determinant is chromosomal, the resistance cannot be transferred between different bacterial species and genera. In addition, the mutationally resistant microorganism is not usually as viable as the wild ones hence once the selective antibiotic is removed from the environment, the proportions of the mutant decrease. If exposure to the antibiotic continues, however, the mutants can become life-threatening to the patient. It should be understood that the antibiotic does not induce the mutation. The mutant simply takes advantage of its fortuitous spontaneous appearance to flourish in the presence of a selected antibiotic. 

Phenanthroline, on the other hand, stimulates the activity of some enzymes, perhaps by removing a metal that is inhibitory to the enzyme.523 524 It can induce porphyrin synthesis525 and improve the rate of ascorbate oxidation.526 It induces lysis of sensitized sheep erythrocytes.527 It is reported to reverse the resistance of microorganisms to penicillins.528 It binds to an immunoglobulin.529 It also protects rat liver from injury induced by dimethylnitrosamine530 and ethionine.531... 

More recently it has become apparent that plants respond dynamically at a metabolic level to insect feeding, and that these induced responses can be of sufficient diversity and intensity to confer resistance (6-11). Many of these responses in the plant s chemical composition are identical to those induced as a result of attack by microorganisms (9.12-14). For example, insect feeding... 

Phytohormones and phytotoxins are two major classes of compounds produced and secreted by microorganisms that act on plants. These will be discussed in another chapter (4.10). Recendy, production of 2,3-butanediol (186) and acetoin (187) by two strains of plant growth-promoting rhizobacteria (PGPR) was reported. These compounds promote growth of A. thaliana and trigger induced systemic resistance (ISR) in the plant.108 109... 

Reduction of microorganisms in milk before cheese production must be achieved in such a way that the functionality of the milk proteins is not affected. Heat-resistant spores, such as Clostridium tyrobutyricum and C. sporogenes can cause severe spoilage of the cheese by late fermentation that can result in the production of H2 and CO2 gases, and unpleasant smelling fermentation products [102]. Although heat sterilization reduces these spores, because of the heat-induced complexation between 3-Lg and K-casein, UHT milk normally does not form a rennet gel and consequently could not be used efficiently for cheesemaking [87]. 

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