Abstract
Streptococcus suis is a zoonotic pathogen causing disease in both animals and humans and the emergence of increasingly resistant bacteria to antimicrobial agents has become a significant challenge globally. The objective of this study was to investigate the genetic basis for declining susceptibility to penicillin and other ?-lactams among S. suis. Antimicrobial susceptibility testing and penicillin-binding proteins (PBP1a PBP2a PBP2b and PBP2x) sequence analysis were performed on 225 S. suis isolated from diseased pigs. This study found that a growing trend of isolates displayed reduced susceptibility to ?-lactams including penicillin ampicillin amoxicillin/clavulanic acid and cephalosporins. A total of 342 substitutions within the transpeptidase domain of four PBPs were identified of which 18 substitutions were most statistically associated with reduced ?-lactams susceptibility. Almost all the S. suis isolates which exhibited penicillin-non-susceptible phenotype (71.9%) had single nucleotide polymorphisms leading to alterations of PBP1a (P409T) and PBP2a (T584A and H588Y). The isolates may manifest a higher level of penicillin resistance by additional mutation of M341I in the 339STMK active site motif of PBP2x. The ampicillin-non-susceptible isolates shared the mutations in PBP1a (P409T) and PBP2a (T584A and H588Y) with additional alterations of PBP2b (T625R) and PBP2x (T467S). The substitutions including PBP1a (M587S/T) PBP2a (M433T) PBP2b (I428L) and PBP2x (Q405E/K/L) appeared to play significant roles in mediating the reduction in amoxicillin/clavulanic acid susceptibility. Among the cephalosporins specific mutations strongly associated with the decrease in cephalosporins susceptibility were observed for ceftiofur: PBP1a (S477D/G) PBP2a (E549Q and A568S) PBP2b (T625R) and PBP2x (Q453H). It is concluded that there was genetically widespread presence of PBPs substitutions associated with reduced susceptibility to ?-lactam antibiotics. ? 2023 by the authors.
