Quaternary ammonium compounds (QACs) are widely used biocides that possess antimicrobial effect against a broad range of microorganisms. These compounds are used for numerous industrial purposes, water treatment, antifungal treatment in horticulture, as well as in pharmaceutical and everyday consumer products as preserving agents, foam boosters, and detergents. Resistance toward QACs is widespread among a diverse range of microorganisms and is facilitated by several mechanisms such as modifications in the membrane composition, expression of stress response and repair systems, or expression of efflux pump genes. Development of resistance in both pathogenic and nonpathogenic bacteria has been related to application in human medicine and the food industry. QACs in cosmetic products will inevitably come into intimate contact with the skin or mucosal linings in the mouth and thus are likely to add to the selection pressure toward more QAC-resistant microorganisms among the skin or mouth flora. There is increasing evidence of coresistance and cross-resistance between QACs and a range of other clinically important antibiotics and disinfectants. Use of QACs may have driven the fixation and spread of certain resistance cassette collectors (class 1 integrons), currently responsible for a major part of antimicrobial resistance in gram-negative bacteria. More indiscriminate use of QACs such as in cosmetic products may drive the selection of further new genetic elements that will aid in the persistence and spread of antimicrobial resistance and thus in limiting our treatment options for microbial infections.

Summary -The normal bacterial flora contains antibiotic resistance genes to various degrees, even in individuals with no history of exposure to commercially prepared antibiotics. Several factors seem to increase the number of antibiotic-resistant bacteria in feces. One important factor is the exposure of the intestinal flora to antibacterial drugs. Antibiotics used as feed additives seem to play an important role in the development of antibiotic resistance in normal flora bacteria. The use of avoparcin as a feed additive has demonstrated that an antibiotic considered "safe" is responsible for increased levels of antibiotic resistance in the normal flora enterococci of animals fed with avoparcin and possibly in humans consuming products from these animals. However, other factors like stress from temperature, crowding, and management also seem to contribute to the occurrence of antibiotic resistance in normal flora bacteria. The normal flora of animals has been studied with respect to the development of antibiotic resistance over four decades, but there are few studies with the intestinal flora as the main focus. The results of earlier studies are valuable when focused against the recent understanding of mobile genetics responsible for bacterial antibiotic resistance. New studies should be undertaken to assess whether the development of antibiotic resistance in the normal flora is directly linked to the dramatic increase in antibiotic resistance of bacterial pathogens. Bacteria of the normal flora, often disregarded scientifically, should be studied with the intention of using them as active protection against infectious diseases and thereby contributing to the overall reduction of use of antibiotics in both animals and humans.normal bacterial flora / antibiotic resistance / feed additives / genetic exchange / preventive management Résumé -Résistance aux antibiotiques dans la flore normale des animaux. La flore bactérienne normale contient des gènes de résistance aux antibiotiques, à des niveaux divers, même chez des individus n'ayant jamais été exposés à des préparations commerciales d'antibiotiques. Plusieurs facteurs semblent augmenter le nombre de bactéries résistantes aux antibiotiques dans les selles. Un facteur important est l'exposition de la flore intestinale aux antibiotiques. Les antibiotiques utilisés comme additif alimentaire semblent avoir un rôle important dans le développement de la résistance aux antibiotiques dans la flore bactérienne normale. L'utilisation de l'avoparcine comme additif alimentaire a montré qu'un antibiotique considéré sans danger est responsable de l'augmentation des taux de résistance aux antibiotiques chez les entérocoques de la flore normale d'animaux ayant reçu de Vet. Res. 32 (2001) 227-241 227

This study shows the occurrence and dissemination of PMQR genes in Salmonella and E. coli in Europe with a defined quinolone resistance phenotype. We also report the first detection of qnrD in Salmonella collected in Europe.

Background: The project "Antibiotic resistance in bacteria of animal origin -II" (ARBAO-II) was funded by the European Union (FAIR5-QLK2-2002-01146) for the period [2003][2004][2005], with the aim to establish a continuous monitoring of antimicrobial susceptibility among veterinary laboratories in European countries based on validated and harmonised methodologies. Available summary data of the susceptibility testing of the bacterial pathogens from the different laboratories were collected.

This study provides strong, novel evidence that humans may introduce methicillin-resistant Staphylococcus aureus CC398 into closed pig populations; it also demonstrates that stringent control and eradication measures were effective and prevented dissemination from pig farms to the general human population.