The in vitro antimicrobial activities of oxolinic acid, flumequine, sarafloxacin, enrofloxacin, and oxytetracycline against strains of bacteria pathogenic to fish (Aerononas salmonicida subsp. salmonicida, atypical A. salmonicida, Vibrio sabnonicida, Vibrio anguillarum, and Yersinia ruckeri) were determined at two different incubation temperatures, 4 and 15°C, by a drug microdilution method. The main objective of the study was to examine the effect of incubation temperature on the in vitro activities of 4-quinolones and oxytetracycline against these bacteria. When tested against A. salmonicida subsp. salmonicida, all of the quinolones examined had MICs two-to threefold higher at 4°C than at 15°C. Similarly, 1.5-to 2-fold higher MICs were recorded for all of the quinolones except sarafloxacin at 4°C than at 15°C when the drugs were tested against V. salmonicida. In contrast to those of the quinolones, the MICs of oxytetracycline were two-to eightfold lower at 4°C than at 15°C against all of the bacterial species tested. Of the antimicrobial agents tested against the bacterial species included in the study, enrofloxacin was the most active and oxytetracycline was the least active. Sarafloxacin was slightly more active than flumequine and oxolinic acid, especially against oxolinic acid-resistant A. salmonicida subsp. salmonicida strains.In recent years, the 4-quinolone antimicrobial compounds oxolinic acid and flumequine, in addition to oxytetracycline, and sulfadiazine-trimethoprim, have been the most frequently used antimicrobial agents in Norwegian aquaculture (18,23).Substantial development of oxolinic acid and tetracycline resistance in bacteria pathogenic to fish (2, 19, 29, 31) has led to a need for new antimicrobial drugs for control of bacterial fish diseases in aquaculture. Aminopenicillins, several sulfonamide-trimethoprim combinations, chloramphenicol analogs such as thiamphenicol and florfenicol, and potent fluoroquinolones have all been proposed and tested as potential new drug candidates (2,3,12,16,21,24,25,30).Several of the new fluoroquinolones generated during the last decade (32) show increased inhibitory (6, 15) and bactericidal (4) activities in vitro against many bacterial pathogens, compared with the old 4-quinolones, such as nalidixic acid and oxolinic acid. Studies have also revealed that the newer compounds show increased potency (2, 25), as well as more effective bactericidal activity (3, 21), against bacteria pathogenic to fish.Along the Norwegian coastline, there are significant seasonal and geographic seawater temperature variations, from just above zero in the winter to well above 18°C in the summer at the same site. The most common bacterial fish diseases in Norway, furunculosis, vibriosis, cold water vibriosis, bacterial kidney disease, yersiniosis, and infections caused by different strains of atypical Aeromonas salmonicida, may all occur at a wide range of temperatures.The general influence of temperature on the pharmacoki-
Multi-drug-resistant coliform bacteria were isolated from feces of cattle exposed to antimicrobial agents and humans associated with the animals. Isolates from both cattle and humans harbored an R plasmid of 65 kb (pTMS1) that may have been transferred between them due to selective antibiotic pressure in the farm environment.The amount of antimicrobial agents used for therapeutic and nontherapeutic purposes in agriculture far exceeds what is used for humans in many parts of the world (11). Since exposure to antimicrobial agents is the most important factor with regard to development of antimicrobial resistance, animals and animal products could thus be significant sources of resistant bacteria for the human population (1,5,12,17,18). Nonpathogenic, multiple-drug-resistant Escherichia coli in the intestine is probably an important reservoir of resistance genes (3,10,13,15,21), and drug-resistant, intestinal E. coli of animal origin may colonize the human intestine, at least temporarily (14,15,20). However, the ease with which bacteria acquire new resistance genes by self-transmissible and mobilizable plasmids and conjugative transposons may represent a more significant contribution to the increasing incidence of resistant strains (19,23,24).In this study, farm inhabitants were investigated for the occurrence of multi-drug-resistant intestinal E. coli. On the farm studied here, various antimicrobials had been used extensively, primarily to treat recurrent Staphylococcus aureus mastitis in dairy cattle (L. Sølverød, personal communication). One family lived on the farm, and one veterinarian had served the animals. During the spring of 1996, fecal swabs (Culturette; Becton Dickinson Europe, Meyland, France) were collected from 13 cattle, three family members, and the local veterinarian. One year later, sampling of the family members and the veterinarian was repeated, and fecal swabs from four other veterinarians operating sporadically in the area were also included. For a primary screening of the total aerobic fecal flora, each swab was plated onto blood agar (blood agar base [Difco Laboratories, Detroit, Mich.] containing 5% citrated bovine blood) and Mueller-Hinton agar (Difco) with Neo-Sensitabs (Rosco Diagnostica, Taastrup, Denmark) containing 33 g of ampicillin (AMP), 80 g of tetracycline (TET), 100 g of streptomycin (STR), 5.2 g of trimethoprim (TMP), 240 g of sulfonamides (SUL), 60 g of chloramphenicol, and 10 g of enrofloxacin. Agar plates were incubated at 37°C for 24 h. A total of six cattle and five human samples exhibited multipledrug-resistant patterns (Amp r Tet r Str r Tmp r Sul r ). From these plates, three colonies were picked from the zones close to the Neo-Sensitabs containing AMP, TET, and SUL, respectively. Colonies were subcultivated on blood agar and BTB-lactose agar plates to assure pure cultures and retested for susceptibility to the above-mentioned antimicrobials. MICs of AMP, TET, STR, TMP, and SUL were determined (16). A total of 39 of 90 lactose-fermenting (coliform) bovine and human iso...
The alterations in the balance of the normal intestinal bacterial flora of chickens exposed to acidified wood-derived litter were analyzed and compared to those of a control group exposed to nonacidified litter. A total of 1,728 broilers were divided into two groups, with six replicates in each. One group was exposed to dry wood-derived litter, and the other was exposed to dry wood-derived litter sprayed with a mixture of sodium lignosulfonate, formic acid, and propionic acid. At five different times, five chickens from each pen were killed and the intestinal contents from ileum and caeca were collected. The samples were diluted and plated onto selective media to identify coliforms, Lactobacillus spp., Clostridium perfringens, and Enterococcus spp. Covariance analysis of bacterial counts showed significantly lower counts for C. perfringens in the caeca and the ileum and for Enterococcus spp. and Lactobacillus spp. in the ileum in chickens exposed to the acidified litter. Lactobacillus spp. showed significantly higher counts in the caeca in chickens exposed to acidified litter. There was no difference between the two litters with regard to coliforms in the ileum and the caeca or to Enterococcus spp. in the caeca. The study shows that exposing the chickens to acidified litter lowers the intestinal bacterial number, especially in the ileum, without negative consequences for the chicken's health or performance. Of special interest are the lower counts of C. perfringens and Enterococcus spp. that might reduce the risk of developing clinical or subclinical necrotic enteritis and growth depression.Antibiotic growth promoters (AGPs) have been used in animal production worldwide since 1946, when their positive effects were observed for the first time (29). AGPs are antibiotics added to animal feed at subtherapeutic levels to increase growth, improve feed efficiency, and decrease the incidence of diseases (1, 18).The use of antibiotics over time in human and animal medicine and for growth promotion in animals has caused a large pressure on the microfloras, with the consequent appearance of resistance to these antibiotics among pathogenic bacteria (4,35,36,37,46). A lot of attention is being focused upon this problem, which has resulted in the banning and/or regulation of the use of AGPs by a number of countries and an increasing interest in organic farming. Development of alternative products and improved management is therefore necessary to eliminate the use of AGPs while achieving the same productivity.Organic acids (OAs) have increasingly and successfully been supplemented in feed in swine and broiler production. The way of action of OAs seems to be related to a reduction of pH in the upper intestinal tract, interfering with the growth of undesirable bacteria and modifying the intestinal flora (19). OAs also improve the digestibility of proteins and amino acids and the absorption of minerals (30, 31), modulate endocrine and exocrine secretions, and influence the mucosal morphology (33). Whether these effects can be app...
gyrA mutations in quinolone-resistant isolates of Aeromonas salmonicida have been detected by using PCR to amplify the quinolone resistance-determining region of gyrA and subsequent cloning and sequencing of PCR products. Comparison of nucleotide and derived amino acid sequences of PCR products from quinolone-susceptible and -resistant bacteria revealed a serine 83-to-isoleucine substitution in the gyrase A protein of resistant isolates. One of the resistant isolates differed from the other by a two- to fourfold-higher MIC of the fluoroquinolone enrofloxacin and carried an additional alanine 67-to-glycine substitution, which may contribute to the higher level of resistance.
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