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Susceptibility to antimicrobial agents of Streptococcus suis capsular type 2 strains isolated from pigs
1Department of Microbiology and Infectious Diseases, Veterinary Faculty, University of Zagreb, Zagreb, Croatia.
Abstract:
The minimal inhibitory concentrations (MICs) for thirty-three epidemiologicaly unrelated clinical isolates of Streptococcus suis capsular type 2 were determined in relation to ampicillin, ampicillin-sulbactam, amoxicillin, clavulanate-amoxicillin, penicillin G, cephalexin, gentamicin, streptomycin, erythromycin, tylosin and doxycycline, using the microtitre broth dilution procedure described by the U.S. National Committee for Clinical Laboratory Standards (NCCLS). Gentamicin was the most active compound tested, with an MIC for 90% of the strains tested (MIC(90)) of 0.4 mg/L. Overall, 70% of strains were resistant to doxycycline (MIC(90) > or = 100.0 mg/L), followed by penicillin G (51% of strains) (MIC(90) + or = 100.0 mg/L). Resistance to amoxicillin and ampicillin was 36.4% (MIC(90) 12.5 mg/L) and 33.3% (MIC(90) 50.0 mg/L), respectively. 15.2% of S. suis strains were resistant to streptomycin, tylosin and cephalexin with MIC90 values of 25.0 mg/L, 12.5 mg/L and 25.0 mg/L, respectively. A combination of ampicillin and sulbactam (MIC(90) 6.3 mg/L) and a combination of amoxicillin and clavulanate (MIC(90) 3.1 mg/L) as well as erythromycin (1.6 mg/L) were of the same efficacy, with a total of 9.1% resistant S. suis strains. This high percentage of resistance to doxycycline and penicillin G precludes the use of these antibiotics as empiric therapy of swine diseases.
Insights
This study evaluated antibiotic resistance in Streptococcus suis type 2. Gentamicin showed high efficacy, while doxycycline and penicillin G resistance limits their use in swine.
Area of Science:
- Veterinary Microbiology
- Antimicrobial Resistance
- Infectious Diseases
Background:
- Streptococcus suis, particularly capsular type 2, is a significant pathogen in swine.
- Understanding antimicrobial susceptibility is crucial for effective treatment and disease control.
- Previous data on resistance patterns in clinical isolates of S. suis type 2 is limited.
Purpose of the Study:
- To determine the minimal inhibitory concentrations (MICs) of various antibiotics against clinical isolates of Streptococcus suis capsular type 2.
- To assess the prevalence of antibiotic resistance in these isolates.
- To evaluate the suitability of different antibiotics for empirical therapy in swine diseases caused by S. suis.
Main Methods:
- Minimal inhibitory concentrations (MICs) were determined for 33 unrelated clinical isolates of S. suis capsular type 2.
- Antibiotics tested included ampicillin, amoxicillin, penicillin G, cephalexin, gentamicin, streptomycin, erythromycin, tylosin, and doxycycline, along with their combinations.
- The microtitre broth dilution method, following U.S. National Committee for Clinical Laboratory Standards (NCCLS) guidelines, was employed.
Main Results:
- Gentamicin demonstrated the highest activity (MIC90 = 0.4 mg/L).
- High resistance rates were observed for doxycycline (70%) and penicillin G (51%).
- Resistance to amoxicillin (36.4%) and ampicillin (33.3%) was also significant, while ampicillin-sulbactam, amoxicillin-clavulanate, and erythromycin showed lower resistance (9.1%).
Conclusions:
- High resistance to doxycycline and penicillin G renders them unsuitable for empirical therapy of S. suis type 2 infections in swine.
- Gentamicin and the beta-lactam/beta-lactamase inhibitor combinations (ampicillin-sulbactam, amoxicillin-clavulanate) appear to be more effective therapeutic options.
- Continuous monitoring of antimicrobial resistance patterns in S. suis is essential for guiding treatment strategies.