Abstract:
Streptococcus suis (S. suis) is an important pathogen in the swine farm. Sometime the use of a single antimicrobial drug has been unsuccessful in the treatment. The use of antimicrobial combinations has been interesting for the treatment of S. suis infected pigs. Therefore, the objective of this study was to develop combined antimicrobial injectable suspension for the treatment of S.suis infected pigs. The susceptibility testing of 16 local S.suis strains collected from Thai swine farms against three antimicrobial groups (β-lactams, aminoglycosides and fluoroquinolones) was performed using agar dilution technique. The Minimum Inhibitory Concentrations (MICs) were determined and the antimicrobials were selected for further studies. The synergistic effects of antimicrobial combinations were studied using checkerboard method and Fractional Inhibitory Concentration (FIC) index calculation. The most potent antimicrobial combination was selected and formulated using various oils and dispersing agents to prepare parenteral suspensions. Physical and chemical stabilities and in vitro release of the suspensions were evaluated. The results demonstrated that S.suis isolates were more sensitive to β-lactam (amoxicillin, cefazolin, ceftriaxone and cefotaxime) and fluoroquinolone ( ciprofloxacin and enrofloxacin) groups than aminoglycoside group (neomicin and gentamicin). The synergistic effects of enrofloxacin and amoxicillin provided the highest antimicrobial activities against S.suis among other antimicrobials. Then, enrofloxacin combined with amoxicillin was selected to formulate as antimicrobial suspensions in oil. Amoxicillin trihydrate and enrofloxacin base suspension containing cottonseed oil and tween 20 as a medium and a dispersing agent was good physical appearance, easily resuspended and physically stable. After 3 months at 30±2 °C and 75±5%RH, percent remainings of amoxicillin trihydrate and enrofloxacin base in the suspension were 101.57±0.71 and 98.81±2.22, respectively. The release profile of enrofloxacin base showed sustained release longer than 24 hours while that of amoxicillin trihydrate reached a plateau 10 hours. Therefore, amoxicillin trihydrate and enrofloxacin base in oil is a potential candidate in developing formulation of combined antibacterials for the treatment of S.suis infected pigs.
