-Besides their role as commensals on the skin and mucosal surfaces, staphylococci may be involved in a wide variety of diseases in animals. Staphylococcal infections in animals are mainly treated with antimicrobial agents and as a consequence, staphylococci from animal sources have developed and/or acquired resistance to the respective antimicrobial agents. Resistance statistics obtained from national monitoring programmes on staphylococci from cattle and pigs, but also from surveillance studies on staphylococci involved in diseases in dogs are reported and reviewed with regard to their comparability. This review mainly focusses on the genetic basis of antimicrobial resistance in staphylococci of animal origin. Particular attention is paid to resistance to those antimicrobial agents which are most frequently used in veterinary medicine, but also to antimicrobial agents, such as chloramphenicol and mupirocin, which are used in specific cases for the control of staphylococcal infections in pets and companion animals. In addition, plasmids and transposons associated with the respective resistance properties and their ways of spreading between members of the same or different staphylococcal species, but also between staphylococci and other gram-positive bacteria, are described.antimicrobial resistance / Staphylococcus / animal origin / epidemiology / horizontal gene transfer Vet. Res. 32 (2001) 341

The occurrence of mutations in the genes coding for gyrase (gyrA and gyrB) and topoisomerase IV (parE and parC) of Salmonella typhimurium experimental mutants selected in vitro and in vivo and of 138 nalidixic acid-resistant Salmonella field isolates was investigated. The sequencing of the quinolone resistance-determining region of these genes in highly fluoroquinolone-resistant mutants (MICs of 4 to 16 g/ml) revealed the presence of gyrA mutations at codons corresponding to Gly-81 or Ser-83, some of which were associated with a mutation at Asp-87. No mutations were found in the gyrB, parC, and parE genes. An assay combining allele-specific PCR and restriction fragment length polymorphism was developed to rapidly screen mutations at codons 81, 83, and 87 of gyrA. The MICs of ciprofloxacin for the field isolates reached only 2 g/ml, versus 16 g/ml for some in vitro-selected mutants. The field isolates, like the mutants selected in vivo, had only a single gyrA mutation at codon 83 or 87. Single gyrA mutations were also found in highly resistant in vitroselected mutants (MIC of ciprofloxacin, 8 g/ml), which indicates that mechanisms other than the unique modification of the intracellular targets could participate in fluoroquinolone resistance in Salmonella spp. A comparison of experimental mutants selected in vitro, field strains, and mutants selected in vivo suggests that highly fluoroquinolone-resistant strains are counterselected in field conditions in the absence of selective pressure.

-Isolates of the genera Pasteurella and Mannheimia cause a wide variety of diseases of great economic importance in poultry, pigs, cattle and rabbits. Antimicrobial agents represent the most powerful tools to control such infections. However, increasing rates of antimicrobial resistance may dramatically reduce the efficacy of the antimicrobial agents used to control Pasteurella and Mannheimia infections. This review presents a short summary of the infections caused by Pasteurella and Mannheimia isolates in food-producing animals and the possibilities of preventing and controlling primary and secondary pasteurellosis. Particular reference is given to antimicrobial chemotherapy and the resistance properties of Pasteurella and Mannheimia isolates. The genetic basis of the most predominant resistance properties such as resistance to β-lactam antibiotics, tetracyclines, aminoglycosides, sulfonamides, and chloramphenicol is discussed. This is depicted with reference to the role of plasmids and transposons in the spread of the resistance genes among Pasteurellaceae and members of other bacterial families and genera.

A florfenicol resistance gene almost identical to floR of Salmonella enterica serovar Typhimurium DT104 was detected on 110-to 125-kb plasmids in Escherichia coli isolates of animal origin. Analysis of the floR gene flanking regions of one of the plasmids showed that they were different from those encountered in S. enterica serovar Typhimurium DT104.

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