الفهرس 
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Abstract
Mycoplasma is a significant pathogen in veterinary medicine, particularly in the poultry sector. Mycoplasma synoviae (MS) and Mycoplasma gallisepticum (MG) are most common pathogenic strains that can infect poultry causing significant financial losses all over the world. To lessen the severity of the disease, positive breeder flocks should be eliminated, medicated, and immunized. The current study aims to bacteriological, molecular detection of MG and MS isolated from suspected broiler and layer chicken cases representing five Egyptian governorates (AL-Sharkia, Giza, AL-Qaliobia, AL-fayoum, & AL-Dakahlia) from March 2020 to January 2022. Two hundred and fifty samples (125 for MG & 125 for MS) representing 25 flocks/governorate including 15 broiler chicken farms and 10-layer farms. Some of the recovered strained were subjected for in vitro and in vivo evaluation the effectiveness of most common antibiotics used against mycoplasma infection including tilmicosin, tylosin, erythromycin, spiramycin, lincomycin, and doxycycline also, guava (Psidium guajava) leaves extract was evaluated as a natural antibiotic alternative against MG. For in vivo study, 510-day-old chicks were randomly divided into 17 groups then at 3 days old, the birds were experimentally infected, and at 18 days old after the appearance of clinical signs, they were given various treatments in drinking water for 5 days in consecutive days. Mortalities, clinical signs, and postmortem lesion scoring were daily recorded from either freshly dead or slaughtered birds also, tissue samples were collected for re-isolation to detect the shedding of MG and MS. The isolates were characterized using conventional bacteriological techniques and molecularly identified using 16S rRNA, mgc2 specific primers for MG and 16S rRNA, vlha specific primers for MS. Only 101/125 (80.8%) samples presented as fried egg colonies on PPLO agar for MG and 67/125 (53.6%) for MS. Only 88 out of 101 (87.1%) were positive by conventional PCR for MG and 55/67 (82%) were recognised as MS. For in vitro antibiotic sensitivity examination, the MIC revealed that tilmicosin was the most effective against MG isolates with MIC values ranging from 0.0078 to 0.0156 g/ml, and spiramycin was the most effective against MS isolates with MIC values ranging from 0.015625 to 16 g/ml. The MIC value of GLE was 0.25 g/ml, which is moderately sensitive to MG infection. For in vivo results, tilmicosin treated birds showed significant sensitivity against MG at p-value ≤ 0.05 also, spiramycin treated birds demonstrated significant sensitivity against MS at p-value ≤ 0.05 in comparison with control positive groups. There were no significant differences in the cumulative mortality rate between treated birds at p-value = 0.73 for MG groups and at p-value = 0.77 for MS groups in comparison with control positive groups. Tilmicosin and spiramycin were significantly reduced the clinical signs score, air-sac lesion score, and re-isolation rate of MG and MS, respectively. Additionally, Tilmicosin treated birds for MG infection and spiramycin treated birds for MS infection revealed significant increase in the mean body weights at p-value ≤ 0.05 in contrast with control positive groups. Our results showed that MG and MS are still circulating in the field representing serious hazards, and the most widely used medications such as erythromycin, tylosin have no longer worked. The guava leaves extract showed a moderate antibacterial activity against MG isolates. Therefore, molecular monitoring of circulating MG and MS field isolated with MIC testing is recommended to overcome mycoplasmal multidrug resistance also, further studies are recommended to evaluate the efficacy of GLE nano-preparation of against MG.