الفهرس 
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Abstract
The emergence of MDR, XDR, and PDR Gram-positive organisms is a serious problem facing humanity as a whole, with the peak of the problem being in developing countries. Linezolid is one of the last resort drugs that should be saved for cases suffering from difficult and resistant Gram-positive bacterial infections. Linezolid is active against most clinically important MDR Gram-positive pathogens, such as MRSA, VRSA, VRE and MDR strains of Pneumococci. Linezolid has excellent pharmacokinetic properties, being available for oral administration with 100% bioavailability and excellent distribution. Moreover, it has a very good safety profile. All these factors make linezolid very attractive for being prescribed by the clinicians, leading to abusive prescribing practices. This led to emergence of linezolid-resistant strains worldwide. Linezolid resistance can occur as a result of either chromosomal mutations of genes coding for 23s rRNA, or the horizontal transmission of resistant determining genes carried on MGEs.The cfr, optrA, and poxtA genes can be transmitted by horizontal transmission causing the spread of linezolid resistance among different bacterial species. Thus, the aim of this study was to determine the antibiotic susceptibility profiles of Gram-positive pathogens causing CA and HA infections in Tanta University Hospitals, investigate the prevalence of linezolid resistance among them, and screen for cfr, optrA, and poxtA among linezolid resistant isolates. This study included 400 patients admitted to or visiting outpatient clinics of Tanta University Hospitals during the period from December 2019 to November 2020. The subjects were divided into 2 main groups: • group 1: 232 cases having HA infections. • group 2: 168 cases having CA infections. The isolates were identified and isolated using the standard laboratory methods. HA infections samples gave rise to 10.25% fungal isolates, 63.96% Gram-negative isolates, and 25.79% Gram-positive isolates. On the other hand, the growth from CA samples was distributed as the following: 0.56% fungal isolates, 58.19% Gram-negative isolates, and 41.24% Gram-positive isolates, 73 CA Gram-positive isolates and 73 HA Gram-positive isolates were detected. The antibiotic susceptibility profiles of Gram-positive isolates were determined by disc diffusion method according to CLSI (2019). Only vancomycin susceptibility of staphylococcal isolates was tested by E-test. The percentages of MDR isolates among the Gram-positive isolates were 65.75% and 72.6% in CA and HA isolates, respectively. MRSA isolates were highly prevalent among the two study groups being as high as 72% and 74.36% of CA and HA S. aureus isolates, respectively. The antibiotics showing lowest resistance rates were linezolid, vancomycin, tigecycline. None of the CA isolates showed resistance to them, while 2.74% of the HA isolates showed resistance to each of these drugs. Vancomycin and tigecycline resistances were observed in enterococcal isolates. On the other hand, linezolid resistance was detected in one CoNS isolate and one S.aureus isolate. The total percentage of linezolid resistance among Gram-positive isolates was 1.37%, being 2.74% among HA Gram-positive isolates. Both linezolid-resistant isolates were positive for the cfr gene and negative for both optrA and poxtA genes.