A microbial inhibition assay in microplates using Bacillus licheniformis for detection of enrofloxacin and sulfamethazine in chicken spiked kidney, liver and muscle tissues

BACKGROUND

The presence of antimicrobial drugs residues in animal products at levels higher than the maximum residue level (MRL) may have adverse effects on consumer health such as allergic reactions and resistance development. Therefore, it is necessary to monitor animal products for the presence of antimicrobial residues.

OBJECTIVES

The aim of this study was to evaluate the detection limit of microbial inhibition assay (MIA) in microplate by using of Bacillus licheniformis as indicator microorganism for two antibiotics, enrofloxacin (ENR) and sulfamethazine (SMT), in broiler chicken's kidney, liver and muscle tissue samples.

METHODS

Spiked tissues samples for the two antibiotics were analysed separately by this method. The results of the assay were evaluated by the determination of the absorbance after mean 3.47 h of incubation at 45°C.

RESULTS

Results showed that the detection limits of MIA for ENR and SMT in kidney (124.03 and 23.21 μg/kg, respectively) and liver (90.02 and 62.03 μg/kg, respectively) as well as SMT in muscle (46.95 μg/kg) were lower than EU (European Union) - MRL, whereas the detection limit for ENR in muscle was slightly higher than MRL (136.3 μg/kg compared to 100 μg/kg MRL). Furthermore, the MIA in the current study was found to be more sensitive to SMT than ENR (92% and 88% sensitivity rate, respectively). No false-positive was observed in the assay.

CONCLUSIONS

Based on the results, the MIA investigated in this study had the potential to detect ENR and SMT residues in broiler chicken kidney, liver and muscle tissues at levels below or close to EU - MRL but offered lower capability for the detection of ENR compared with SMT in kidney and muscle tissue samples.

Site-specific gene knock-in and bacterial phytase gene expression in Chlamydomonas reinhardtii via Cas9 RNP-mediated HDR

In the present study, we applied the HDR (homology-directed DNA repair) CRISPR-Cas9-mediated knock-in system to accurately insert an optimized foreign bacterial phytase gene at a specific site of the nitrate reductase (NR) gene (exon 2) to achieve homologous recombination with the stability of the transgene and reduce insertion site effects or gene silencing. To this end, we successfully knocked-in the targeted NR gene of Chlamydomonas reinhardtii using the bacterial phytase gene cassette through direct delivery of the CRISPR/Cas9 system as the ribonucleoprotein (RNP) complex consisting of Cas9 protein and the specific single guide RNAs (sgRNAs). The NR insertion site editing was confirmed by PCR and sequencing of the transgene positive clones. Moreover, 24 clones with correct editing were obtained, where the phytase gene cassette was located in exon 2 of the NR gene, and the editing efficiency was determined to be 14.81%. Additionally, site-specific gene expression was analyzed and confirmed using RT-qPCR. Cultivation of the positive knocked-in colonies on the selective media during 10 generations indicated the stability of the correct editing without gene silencing or negative insertion site effects. Our results demonstrated that CRISPR-Cas9-mediated knock-in could be applied for nuclear expression of the heterologous gene of interest, and also confirmed its efficacy as an effective tool for site-specific gene knock-in, avoiding nuclear positional effects and gene silencing in C. reinhardtii. These findings could also provide a new perspective on the advantageous application of RNP-CRISPR/Cas9 gene-editing to accelerate the commercial production of complex recombinant proteins in the food-grade organism "C. reinhardtii".