Salmonella Typhimurium is an important pathogen affecting both animal production and public health, with pigs representing a major reservoir. The increasing prevalence of antimicrobial resistance highlights the need for alternative or complementary therapeutic strategies. Carvacrol, a phenolic constituent of Origanum vulgare essential oil, has recognized antimicrobial activity and may contribute to the modulation of antimicrobial resistance mechanisms. The present study investigated the antimicrobial activity of carvacrol and tetracycline, individually and in combination, against swine-derived S. Typhimurium strains. Minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) were determined by broth microdilution. Interactions between both compounds were assessed by checkerboard assays using the fractional inhibitory concentration index (FICI). Relative expression of tetracycline resistance genes was analysed by quantitative real-time PCR in selected isolates exposed to tetracycline alone or combined with carvacrol. Carvacrol MIC values ranged from 0.5 to 16 mg/mL, with MIC₅₀ and MIC₉₀ values of 2 and 4 mg/mL, respectively. MBC and MIC values were identical in 84.9% of the isolates, indicating a strong bactericidal effect. Tetracycline MIC₅₀ and MIC₉₀ values were 256 and 512 µg/mL, respectively. Checkerboard assays showed synergistic interactions in 14.3% of strains, additive effects in 50.0%, indifferent responses in 28.6%, and antagonism in 1%. Combined exposure reduced tetA and tetB expression in most tested isolates compared with tetracycline alone, although responses were strain dependent. These findings demonstrate that carvacrol exhibits antimicrobial activity against swine-derived multidrug-resistant S. Typhimurium and may enhance tetracycline activity while modulating resistance-associated gene expression. Carvacrol could represent a promising natural adjuvant for antimicrobial control strategies in swine production.

Stray cats act as reservoirs of pathogens and resistance genes, representing a risk to public health, especially in urban and peri-urban areas due to their mobility and, in some cases, the lack of sanitary control. One of the most relevant zoonotic agents is Escherichia coli, a pathogen of the gastrointestinal tract of humans and animals that causes infection/disease and can be transmitted from one species to another. We investigated a total of 169 rectal swabs from feline colonies in the municipality of Benalmadena (province of Málaga, south Spain). The microbiological identification process included bacterial isolation on selective media (XLD), standard biochemical tests and the MALDI-TOF identification system. A total of 68 E. coli strains were thus confirmed (40.23%). Subsequently, an antimicrobial resistance analysis was carried out using automatized methods (Sensititre); each isolate was evaluated by MIC against 15 antimicrobial agents. E. coli strains showed a high percentage of sensitivity. The most relevant finding was the resistance of the strains to azithromycin (100%), as well as a moderate resistance to sulfonamides (25%) and ampicillin (20.6%). The percentage of multidrug resistance (MDR) was found in 13.23% of the strains studied. In relation to the rest of the antimicrobials evaluated, the strains presented high percentages of sensitivity. It can be concluded that our research represents the first confirmation of commensal Escherichia coli in stray cats in Spain indicating a moderate-high prevalence as well as a low MDR levels. The control of the health status of these animals under a One Health approach is necessary due to their direct impact on public health.

Stray cats may act as reservoirs of antimicrobial-resistant and potentially pathogenic bacteria, representing a potential public health concern within a One Health framework. This study investigated the occurrence of antimicrobial resistance (AMR), multidrug resistance (MDR), and virulence-associated traits in commensal Escherichia coli isolated from free-living cat colonies in southern Spain. A total of 169 rectal swabs were collected from cats belonging to feline colonies and shelters in Benalmádena (Málaga, Spain). Bacterial isolation and identification were performed using selective culture media, conventional biochemical tests, and MALDI-TOF mass spectrometry. Antimicrobial susceptibility was determined by minimum inhibitory concentration (MIC) testing against 15 antimicrobial agents, and isolates were screened for selected virulence-associated genes associated with major diarrheagenic E. coli pathotypes. A total of 68 E. coli isolates (40.2%) were recovered. The highest resistance frequencies were observed for sulfamethoxazole (25.0%) and ampicillin (20.6%), whereas all isolates showed high susceptibility rates to most of the antimicrobials tested, including azithromycin (100%), as well as gentamicin, amikacin, cefotaxime, ceftazidime, meropenem, colistin, chloramphenicol and tigecycline (98.5%). Six isolates (8.8%) were classified as multidrug-resistant. In addition, six isolates (8.8%) were identified as atypical enteropathogenic E. coli (aEPEC), although none exhibited a multidrug resistance phenotype. These findings demonstrate the presence of both antimicrobial resistance and virulence-associated traits among commensal E. coli circulating in free-living cats. The inclusion of feline colonies or shelters in antimicrobial resistance surveillance programmes may contribute valuable information for integrated One Health monitoring strategies.