This study was concerned with the green synthesis of gold nanoflowers (AuNFs) using the bioactive constituents of Rosmarinus officinalis (rosemary) and Helichrysum italicum (immortelle) extracts, as reducer and stabilizer agents along with the determination of their antibacterial and antibiofilm activity against E. coli, S. aureus, and S. epidermidis. The AuNFs were characterized using STEM, UV–Vis, DLS, ZETA, FESEM-EDX, and FTIR techniques. The antibacterial and antibiofilm activity of the AuNFs were evaluated by microdilution broth and microtiter plate (MTP) tests, respectively. STEM and DLS analysis confirmed the flower-like morphology of gold nanoparticle clusters of R. officinalis-AuNFs (R-AuNFs) and H. italicum-AuNFs (H-AuNFs) with a size of 20–130 nm and 15–90 nm, respectively. The MICs of R-AuNFs were found to be 40 µg/mL for E. coli and S. epidermidis and 160 µg/mL for S. aureus. The MICs of H-AuNFs against all bacterial strains were 20 µg/mL. All tested AuNFs exhibited a strong dose-dependent antibiofilm activity against the test strains, and H-AuNFs was more effective than R-AuNFs. The green synthesis of AuNFs from the rosemary and immortelle extracts can be applied as a potential agent to overcome the growth of biofilm-producing microorganisms in food industries.

1. The Shiga toxin-producing Escherichia coli (STEC) is a hazardous zoonotic agent for chicken meat consumers. This study determined the serogroups and evaluated the virulence genes, antibiotic resistance, biofilm-forming profiles and genetic relationships of STEC isolates in chicken meat. 2. A total of 100 samples belonging to dressed-whole chicken and different parts of the chicken (wing, breast, thigh, drumstick) were collected between September and November 2019 from different retail markets in Kayseri, Türkiye. 3. Phenotypic (identification, disc diffusion test, Congo red agar and microtitre plate tests) and molecular tests (identification, serogrouping, virulence factors, biofilm, antibiotic susceptibility, 16S rRNA sequencing and enterobacterial repetitive intergenic consensus-PCR for typing of the isolates) were carried out. 4. E. coli was isolated from 35% of the samples and 35% of the samples harboured at least one STEC. Among 35 STEC isolates, 3 (8.5%), 6 (17.1%), 2 (5.7%) and 3 (8.5%) were found to be positive for fliCH2, fliCH8, fliCH11, fliCH19 genes, respectively. Out of 35 STEC positive isolates, 4 (11.4%) were identified as E. coli O157, from which 2 (5.7%) were E. coli O157:H7. E. coli O157 was detected in two (10%), one (5%), one (5%) of the thigh, drumstick and whole chicken samples, respectively. 5. Biofilm-forming ability was reported in 33 (94.2%) of 35 E. coli isolates, whilst the biofilm-associated genes detected among 35 STEC isolates included csgA (88.5%), fimH (88.5%), bcsA (85.7%), agn43 (14.2%) and papC (8.5%). The STEC strains showed resistance against ampicillin (88.5%) and erythromycin (88.5%), followed by tetracycline (74.2%) and gentamicin (25.7%). However, the distribution of isolates harbouring bla_CMY, ere(A), tet(A) and aac(3)-IV antibiotic resistance genes was found to be 17.1%, 11.4%, 85.7% and 5.7%, respectively. 6. ERIC-PCR showed that E. coli strains obtained from different parts and whole of chicken samples had genetic diversities. ERIC-PCR patterns grouped strains of 35 STEC into eight clusters designated A-H, with 73% similarity. Proper hygiene measures and staff training are essential for public health during poultry processing and in retail stores to control STEC.