Immediately after their introduction in the beginning of the fourties of the previous century, the agents used to combat infectious diseases caused by bacteria were regarded with suspicion, but not long thereafter antibiotics had the status of miracle drugs. For decades mankind has lived under the impression that infectious diseases were no longer a threat to human health. This optimism was so high at a certain moment that antibiotics were also used against viral infections, whereas viruses are not even sensitive to antibiotics. This wrong use, or if one likes, misuse of antibiotics took also place in animal husbandry, where many tons of antibiotics were added to the feed of healthy animals, just because they grew so nicely from these additives. However, also in the use of antibiotics an ancient law in physics, “action equals reaction” turned out to be applicable. Bacteria reacted to the fact that they were attacked by changing their hereditary properties (through mutation) or by taking up parts of the hereditary properties of organisms (bacteria and fungi) able to produce certain antibiotics themselves. As a result of this reaction, already a short while after the introduction of antibiotics, the first bacteria could be isolated that had become insensitive (immune) for particular antibiotics. The bacteria in fact, had even more surprises in store. They turned out to be fanatic collectors of the pieces of hereditary properties that made them immune for antibiotics and like a stamp collector puts his stamps in an album, they also put their collection in an album (an integron). In this way, the best collectors have now become insensitive to more than ten different types of antibiotics. At the moment there are even bacteria that are not sensitive anymore to whatever type of antibiotic and for these bacteria treatment with antimicrobial agents is no longer available. Where, “work together, live together” is the current motto of the Dutch government, “work together to survive together” might be the motto of bacteria. They put this into practice by passing on their album with its integron collection from one bacterial species to the other. In this way a bacterium that used to be sensitive and could very well be treated with antibiotics can in one stroke become resistant to sometimes thirteen different antimicrobial agents, resulting in the fact that an infection with such a bacterium becomes untreatable. In this thesis research with respect to the sensitivity of the bacterium Salmonella, which can cause intestinal infections in human and animals, for antimicrobial agents is described. Since the (wrong) use of antibiotics can influence the development of resistance to antibiotics, in these studies a comparison has been made between Salmonella bacteria isolated from human, pigs, cattle and poultry in Vietnam and The Netherlands. Whereas in The Netherlands antibiotics are only available on prescription by a physician or veterinarian, antibiotics can be purchased over the counter in Vietnam. This leads to a significantly different attitude in both countries with respect to handling antibiotics. Examples are i.e. not taking a course of antibiotics of the correct dose, not taking a course of antibiotics of sufficient duration, not only taking a course of antibiotics in the case of bacterial infections and the continuing use of antibiotics as growth promoters in Vietnam. Resistance to antibiotics in Salmonella bacteria isolated in Vietnam turned out to occur frequently. In the Netherlands where the development of resistance has been monitored and registered for years the problem was hardly less. In Salmonella isolates from some animal species even resistance to antibiotics for which the use of that antibiotic is not allowed in that animal, was observed. In the current studies Salmonella bacteria have been isolated, both in Vietnam and in the Netherlands that have a collection in their integron album which is unique and has not been described before. At the end of the thesis the measures that could be taken to counteract the development of antibiotic resistance are discussed. The necessity of continuously making an inventory of the situation at local, regional, national and global level is accentuated, as is the shared responsibility that the government and civilians have with respect to the improper use of antibiotics
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