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.

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Oxytetracycline (OT) is used extensively in the treatment of bacterial diseases in marine f~s h .The standard dose recommended for treatment of fish is 5 to 10 times higher than doses commonly used in medical practice, indicating that OT is poorly absorbed. This is especially true for fish held in sea water, where the intestinal uptake is substantially reduced as compared to fish in fresh water. OT forms complexes with magnesium and calc~um We have determined the con~plex constants and calculated that when therapeutic concentrations of OT are present in sea water, only about 5 O/O of the OT exists in the free form. Of the bound OT, the 1:l OT-magnesium complex predominates. The complex formation in sea water results in a strong reduction in the antibacterial effect of OT This has been demonstrated in vitro. The poor intestinal uptake and reduced antibacterial effect of complex-bound OT is caused by an alteration of the molecular charge that diminishes its ability to cross lipid-rich biological membranes. A major portion of the OT administered to farmed salmonids inevitably ends up in the environment, especially in the sediments under aquaculture facilities. No mechanism is known for biodegradation of OT and thus it can remain in the sediments long enough to affect the indigenous bacterial flora and induce resistance. For these reasons the use of OT in marine environments should be questioned.

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It is not well understood why Atlantic halibut (Hippoglossus hippoglossus) has longer shelf-life than most other white fish species. Our approach was to examine the microbiological diversity of the spoilage microbiota during modified atmosphere (MA) packaging of farmed Atlantic halibut. Portions were packaged with gas mixtures of CO(2):N(2) and CO(2):O(2) (50%:50%) and with air as a reference. The packages were stored at 4 degrees C and samples were taken 6 times during the 23 days of storage. Analyses with molecular techniques (PCR-DGGE) determined profiles of the bacterial populations in the various samples and sequencing detected the bacterial species present. In addition, samples were analysed for microbial, chemical and sensory parameters. The shelf-life was 10-13 days when stored in air and between 13 and 20 days for MA packages, with oxygen-enriched packages suggested as the better gas mixture, based on microbial growth and sensory scores. From sequence analyses of the bacterial population Photobacterium phosphoreum and Pseudomonas spp. were found to dominate in the halibut. Brochothrix thermosphacta was found in most samples at the end of the storage period. Shewanella putrefaciens was found sporadically and in low concentrations based on microbial methods, but not detected by PCR-DGGE.

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