Streptococcus suis is a porcine and zoonotic pathogen that causes severe systemic infection in humans and pigs. The treatment of S. suis infection relies on antibiotics; however, antimicrobial resistance (AMR) is an urgent global problem, pushing research attention on the surveillance of antibiotic-resistant S. suis to the fore. This study investigated the antimicrobial susceptibility of 246 S. suis strains isolated from diseased pigs in Thailand from 2018–2020. The major sources of S. suis strains were lung and brain tissues. PCR-based serotyping demonstrated that the most abundant serotype was serotype 2 or ½, followed by serotypes 29, 8, 9, and 21. To the best of our knowledge, this is the first report describing the distribution of AMR S. suis serotype 29 in diseased pigs. The antimicrobial susceptibility test was performed to determine the minimum inhibitory concentrations of 35 antimicrobial agents. The results showed that important antimicrobial agents for human use, amoxicillin/clavulanic acid, daptomycin, ertapenem, meropenem, and vancomycin, were the most effective drugs. However, a slight decrease in the number of S. suis strains susceptible to amoxicillin/clavulanic acid and vancomycin raised awareness of the AMR problem in the future. The data indicated a tendency of reduced efficacy of available veterinary medicines, including ampicillin, cefepime, cefotaxime, ceftiofur, ceftriaxone, chloramphenicol, florfenicol, gentamicin, penicillin, and tiamulin, for the treatment of S. suis infection, thus emphasizing the importance of the prudent use of antibiotics. The widespread of multidrug-resistant S. suis strains was identified in all serotypes and from different time periods and different regions of the country, confirming the emergence of the AMR problem in the diseased pig-isolated S. suis population.

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 β-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 β-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 β-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 β-lactam antibiotics.

Feline urinary tract infections (UTIs) present a common challenge in veterinary practice, underscoring the importance of understanding local bacterial pathogens and antimicrobial resistance (AMR). This study determined bacterial prevalence and antimicrobial susceptibility in cats at Kasetsart University’s Veterinary Teaching Hospital in Bangkok, Thailand. Of the 543 cystocentesis urine samples collected from 428 cats, 115 (21.2%) tested positive for bacterial cultures, leading to a diagnosis of UTIs in 95 cats (22.2%). The most prevalent isolates included Escherichia coli (24.8%), Staphylococcus species (19.2%), Proteus mirabilis (13.6%), Pseudomonas aeruginosa (12.0%), and Enterococcus species (12.0%). Staphylococcus felis (8.8%) and Staphylococcus pseudintermedius (5.6%) were the predominant Staphylococcus species. Rare pathogens such as Corynebacterium urealyticum and Lactococcus garvieae were also identified. Antimicrobial testing revealed alarming resistance, with 69.2% of isolates exhibiting multidrug resistance (MDR). Escherichia coli and Proteus mirabilis showed high resistance to amoxicillin/clavulanic acid (AMC) (45.2–70.6%) and sulfamethoxazole/trimethoprim (SXT) (51.6–52.9%). Enterococcus faecium exhibited 85.7% resistance to AMC. Methicillin resistance was identified in 41.7% of Staphylococcus isolates, particularly high in Staphylococcus epidermidis (75.0%) and Staphylococcus pseudintermedius (71.4%). High fluoroquinolone resistance among MDR isolates further exacerbates AMR concerns. These results indicate that MDR Gram-negative, Staphylococcus, and Enterococcus species complicate the empirical treatment of feline UTIs, highlighting significant implications for AMR in veterinary practice.

Background/Objectives: Multidrug-resistant (MDR) strains of Streptococcus suis are increasingly prevalent and present significant challenges in clinical management. Given that the development of new antibiotics is a resource-intensive process and time-consuming, there is an urgent need for alternative therapeutic strategies to address resistance in the short term. One promising approach is the use of combination therapy, which involves pairing potent antibiotics with agents that may be less effective on their own, to enhance therapeutic efficacy and potentially overcome resistance mechanisms. This study aimed to investigate the in vitro antibacterial activity of combining two classes of antibiotics with distinct mechanisms of action—cell wall synthesis inhibitors and protein synthesis inhibitors—against MDR S. suis strains isolated from diseased pigs. Methods: A total of 36 MDR S. suis strains were tested using a microbroth dilution checkerboard assay to determine the minimum inhibitory concentration (MIC) of four cell wall synthesis inhibitors —amoxicillin/clavulanic acid (AMC), ampicillin (AMP), penicillin G (PEN), and vancomycin (VAN)— in combination with four protein synthesis inhibitors —gentamicin (GEN), neomycin (NEO), tilmicosin (TMS), and tylosin (TYL). Time–kill curve assays were conducted to evaluate the in vitro bactericidal activity of synergistic antibiotic combinations (PEN–GEN and AMP–NEO) against Beta-lactam-resistant and Beta-lactam-susceptible MDR S. suis strains. Results: Checkerboard analysis revealed that penicillin-gentamicin combination exhibited the most effective synergistic activity against the MDR S. suis strains (10/19, 52.6%), with ∑FIC values of 0.25–1.06 and MIC reductions from resistant to susceptible levels. Time-kill assays further confirmed the synergistic bactericidal effect of the combination, demonstrating complete bacterial clearance within 6–9 h, markedly rapid bacterial killing compared to monotherapy. Conclusions: This study demonstrates that antibiotic combinations, particularly Beta-lactams combined with aminoglycosides, show synergistic activity against pig-isolated S. suis MDR strains. The PEN-GEN combination exhibited strong synergistic and bactericidal effects, supporting combination therapy as a potential strategy to address antimicrobial resistance. Further evaluation in diverse strain backgrounds and prudent antibiotic use are essential to confirm efficacy and limit the emergence of antibiotic resistance.

Background/Objectives: The emergence of antimicrobial-resistant (AMR) pathogens in aquaculture ecosystems poses a significant risk to both food security and human health. Shewanella species are recognized as significant AMR reservoirs, yet their prevalence and resistance mechanisms within a shrimp-related ecosystem remain poorly characterized. This study aimed to perform a genotypic and phenotypic characterization of S. algae VK101, isolated from wild-caught black tiger shrimp (Penaeus monodon) broodstock. Methods: A complete 5.21 Mb genome was generated using a hybrid Illumina and Oxford Nanopore sequencing approach. Antimicrobial susceptibility was evaluated for 21 antibiotics via Minimum Inhibitory Concentration (MIC) testing. Comparative pangenomics and genome-wide association studies (GWAS) across 125 S. algae genomes were conducted to identify novel resistance determinants. Results: MIC analysis revealed that VK101 was resistant to ampicillin (>16 µg/mL) and colistin (8 µg/mL), while showing intermediate susceptibility to imipenem and ciprofloxacin. In silico analysis identified 205 antimicrobial resistance genes (ARGs), including a perfect hit for the fluoroquinolone resistance gene qnrA3. Notably, no mcr genes were detected. Although VK101 exhibited moderate resistance (8 µg/mL), GWAS across the broader S. algae population linked a specific lptA mutation (K140N) to high-level resistance (64 µg/mL). Other GWAS-identified genes (e.g., czcA, ampC, and oprM) likely represent indirect associations driven by genetic linkage or clade-specific markers rather than direct causal factors. Conclusions: These findings highlighted the presence of multidrug-resistant S. algae in wild-caught P. monodon broodstock, reflecting the occurrence of antimicrobial resistance in aquatic environments. Colistin resistance in these isolates was primarily mediated by chromosomal variants rather than mobile mcr elements, indicating the need for integrated genomic surveillance within the aquaculture value chain.