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Comparative in vitro activity of 16 antimicrobial agents against Actinobacillus pleuropneumoniae
H Yoshimura1, M Takagi, M Ishimura
1National Veterinary Assay Laboratory, Ministry of Agriculture, Forestry and Fisheries, Tokyo, Japan.
Abstract:
Sixteen antimicrobial agents were tested for their activity against 68 isolates of Actinobacillus pleuropneumoniae by determining the minimum inhibitory concentrations (MICs). Ceftiofur and the fluoroquinolones danofloxacin and enrofloxacin were the most active compounds, with a MIC for 90% of the isolates (MIC90) of (0.05 microg/ml. The MIC90 values of benzylpenicillin, amoxicillin and aspoxicillin were 0.78 units/ml, 0.39 microg/ml and < or = 0.05 microg/ml, respectively. Three isolates (4.4%) were resistant to penicillins, but aspoxicillin was as active as ceftiofur against the susceptible isolates, with MICs of < or = 0.05 microg/ml for all isolates. Resistance to oxytetracycline, chloramphenicol and thiamphenicol occurred in 22 (32.4%), 14 (20.6%) and 15 (22.1%) of the isolates, respectively. Doxycycline was more active than oxytetracycline, with a MIC90 of 1.56 microg/ml as against 25 microg/ml. Florfenicol was not only as active as thiamphenicol, with a MIC for 50% of the isolates (MIC50) of 0.39 microg/ml, but also active against thiamphenicol-resistant isolates. All the isolates were susceptible to florfenicol. All the isolates were also susceptible to gentamicin, spectinomycin, tilmicosin, colistin and tiamulin. Of these, spectinomycin was the least active, with a MIC50 of 25 microg/ml, followed by tiamulin, with a MIC50 of 6.25 microg/ml. Of the 68 isolates tested, 49 (72.0%) were of serotype 2; 14 (20.5%) were of serotype 1; 2 each (3.0%) were of serotypes 5 and 6; and one was of serotype 7. Of the isolates, 23 (33.8%) were resistant to one or more of the major antibiotics. Antibiotic resistance was found only infrequently among serotype 2, with 5 (10.2%) of 49 isolates being resistant to chloramphenicol and/or oxytetracycline, while it occurred in 18 (94.7%) of the 19 isolates of other serotypes.
Insights
Ceftiofur and fluoroquinolones show high activity against Actinobacillus pleuropneumoniae. Florfenicol is effective against all isolates, including resistant strains, while resistance to other antibiotics varies by serotype.
Area of Science:
- Veterinary Microbiology
- Antimicrobial Resistance
- Animal Health
Background:
- Actinobacillus pleuropneumoniae is a significant pathogen in swine respiratory disease.
- Antimicrobial susceptibility testing is crucial for guiding treatment decisions.
- Understanding resistance patterns is vital for effective disease management.
Purpose of the Study:
- To evaluate the in vitro activity of sixteen antimicrobial agents against Actinobacillus pleuropneumoniae isolates.
- To determine the minimum inhibitory concentrations (MICs) for various antibiotics.
- To assess antibiotic resistance prevalence and its correlation with serotypes.
Main Methods:
- Sixty-eight isolates of Actinobacillus pleuropneumoniae were collected.
- Minimum Inhibitory Concentrations (MICs) were determined for 16 antimicrobial agents.
- Isolates were serotyped to investigate potential links between serotype and resistance.
Main Results:
- Ceftiofur, danofloxacin, and enrofloxacin demonstrated the highest activity (MIC90 ≤ 0.05 μg/ml).
- Florfenicol was effective against all isolates, including thiamphenicol-resistant strains.
- Antibiotic resistance was more prevalent in non-serotype 2 isolates (94.7%) compared to serotype 2 (10.2%).
Conclusions:
- Ceftiofur, fluoroquinolones, and florfenicol are promising therapeutic options for Actinobacillus pleuropneumoniae infections.
- Significant variation in antibiotic resistance exists among different serotypes.
- Targeted antibiotic selection based on susceptibility data and serotype is recommended.