Long term monitoring of susceptibility of Streptococcus suis isolates to amoxicillin from clinical cases on Czech swine farms

S. suis is recognised as major swine pathogen associated with the intensification of the swine industry worldwide. Until now limited number of effective vaccines are available. Therefore antimicrobial treatment is considered as one of the most effective approach for the control of streptococcal meningitis and polyarthritis. Beta-lactam antibiotics and especially amoxicillin is considered as drug of first choice based on several criteria such as the PK/PD parameters and susceptibility (MIC) patterns. Aim of this study was long term amoxicillin susceptibility monitoring in clinical isolates of S. suis from Czech farms

50 clinical isolates of S. suis serotype 1 and 2 were chosen in to the study. Isolate was represented one herd per year from the widest random coverage in the Czech Republic in the period 2005-2012. Isolates were obtained from clinical cases of meningitis and or arthritis) in suckling piglets and growers. Identification of S. suis isolates was performed based on biochemical tests (API20Strep, BioMerieux, France) and confirmed by MALDI TOF MS (Bruker). Serotypes 1 and 2, as well as genes responsible for virulence were identified by specific primers using PCR. The MICs were determined using the agar macro-dilution method on Mueller-Hinton agar (Oxoid, UK), with the addition of 5 % sheep blood according to CLSI guidelines (2013), The range of amoxicillin concentration was ≤ 0,015 to &- mg/L. Two independent examinations for each strain were made.

Because interpretative criteria for the susceptibility testing of S. suis of porcine origin have not yet been determined, we have used following criteria for streptococci (viridans group) CLSI M31-A3 (CLSI, 2013): category sensitive ≤ 0.12 mg/l, intermediate 0.25 to 2mg/l, resistant ≥ 4,0 mg/l. Using these criteria 5 ( 10%) strains out of 50 were recognised as intermediately sensitive to amoxicillin and no resistant isolate detected. Both parameters MIC 50 = ≤0,015 mg/l and MIC 90 0,125mg/l belongs to sensitive pattern.

Results obtained in this study demonstrate that pathogenic strains of S. suis isolated from pigs with streptococcal meningitis and arthritis in Czech Republic remain highly susceptible to amoxicillin and there is no evidence for resistance during 7 years period. We can conclude that amoxicillin remains as very effective drug for treatment approach against Czech S. suis isolates within in vitro susceptibility testing which is confirmed by clinical outcome of treatment and is in alignment with other similar studies worldwide.

Disclosure of Interest: None Declared.