الفهرس 
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Abstract
Staphylococci are of the most important pathogens causing nosocomial and community-associated infections worldwide. Lately, emerging resistance in this organism, particularly MRSA to many routinely used antibiotics has led to treatment failure of severe life threatening infections caused by Staphylococci.
Development of drug resistance in Staphylococci has guided the use of older compounds like macrolide, lincosamide and streptogramin B (MLSB) family of antibiotics as therapeutic alternatives.
Macrolides, Lincosamides and Streptogramin B (MLSB) are clinically useful antibiotics They have a narrow spectrum of activity that includes gram-positive cocci (in particular, staphylococci, streptococci, and enterococci). This group of antibiotics are chemically distinct but have a similar mode of action which all bind to 23S rRNA in 50S ribosomal subunits, and subsequently inhibiting the protein synthesis.
The extensive and inappropriate use of these antibiotics has caused increased acquisition of cross-resistance to MLSB antibiotics among Staphylococci.
As a result of this increase in antimicrobial resistance, attention has been directed toward the development of newer antimicrobial agents, including quinupristin-dalfopristin(Streptogramins) for management of these infections.
Resistance to quinupristin-dalfopristin among staphylococcal isolates has been reported in French hospitals where the natural mixture has been used orally and topically for many years. And it requires resistance to the streptogramin A component, in addition to the streptogramin B component as well.
In staphylococci, the genes responsible for resistance to streptogramin B antibiotics include, vgb in addition to ermgene encoding resistant to MLSB agent and msr geneencoding resistance to 14- and the 15- membered macrolides and type B streptogramins only (MSB phenotype).
The genes responsible for resistance to streptogramin A antibiotics include vgaA and vgaB which are most likely involved in active transport of streptogramin A. AndvatA, vatB, and vatC, which encode acetyltransferases that inactivate the antibiotic.
Expression of MLSB resistance can be constitutive or inducible. Constitutive and inducible strains of MLSB-positive S. aureus are susceptible in vitro to quinupristin-dalfopristin.
Inducible resistance is not detectable by routine antimicrobial susceptibility tests. The risk for therapeutic failure is increased as cMLSB may rise from iMLSB during the course of clindamycin therapy in patients with severe Staphylococci infections.Therefore TheCLSI has recommended the erythromycin–clindamycin disc approximation test (D- zone) to detect inducible clindamycin resistance.
The present study was a trial to detect phenotypic and molecular characterization of resistance to Streptogramins in S.aureus isolated from different types of clinical samples.
Over a period of 3 months, Sixty-three S.aureus isolates were identified in Microbiology Department in the Medical Research Institute hospital, Alexandria University from inpatients and outpatients. The isolates were isolated from different types of clinical samples. Fifty S.aureus isolates were selected blindly to be included in the present study.
All S. aureus isolates were subjected to the following:
1- Phenotypic identification based on colony morphology,microscopic examinationand standard biochemical test.
2- All isolates were tested for their antibiotic susceptibility to different classes of antibiotics (11 antibiotics) by disk diffusion method on Muller-Hinton agar based on CLSI guidelines.
3- Molecular Detection of ermA, ermB, ermC, msrA, vatA, vatB, vatC, vgaA and vgbA genes in S. aureus isolates by PCR.
The results of the present study revealed:
1- The 50 S. aureus isolates included in this study were collected from different types of clinical samples [pus (70%), sputum (6%), blood (4%), nasal swab (10%), central venous catheter (C.V.C) (2%), pleural (2%), ascitic (4%) and synovial fluid (2%)].
2- The most active agents by susceptibility were consistently linezolid (100%)followed by Quinupristin/dalfopristin (88%) then Teicoplanin (70%). On the other hand, the isolates exhibited high level of resistance to Cefoxitin (96%), followed by Ceftriaxone (66%) Amoxaicillin/Clavulanic acid (66%) and moderate resistance to macrolides (44%), levofloxacin (40%), Amikacin (30%), clindamycin (28%), and Imipenem (26%).
3- Out of 50 S. aureus isolates 2 isolates were Methicillin Sensitive S. aureus (MSSA) (4%) and they were MSB phenotype and 48 were Methicillin Resistant S. aureus (MRSA) (96%). and their MLSB resistance phenotypes were as follows, 9(18.75%) cMLSB, 5(10.5%) iMLSB,6(12.5%) MSB phenotype,25(52%) isolates were (S/S phenotype) and 3(6.25%) (LSA phenotype).
4- The results of antibiotic sensitivity revealed that out of the 50 S. aureus isolates, 44(88%) were sensitive to quinupristin-dalfopristin (6 cMLSB, 5 iMLSB, 8 MSB, 3 LSA and 22 S/S) and 6(12%) were with intermediate resistance to quinupristin-dalfopristin (3 cMLSB and 3 S/S) were found.
5- Out of 50 S. aureus isolates, ,22(44%) isolates were expected to be streptogramin B resistant of them; 3(13.6%) isolates had intermediate resistance to Quinupristin-Dalfopristin by disc diffusion method and the other 19(86.4%) isolates were sensitive to Quinupristin–Dalfopristin. and there were 3(6%) isolates (LSA phenotype) expected to be streptogramin A resistant, but they were sensitive to Quinupristin–Dalfopristin.
6- Among the 50 S. aureus isolates, the ermC gene was the predominant gene, it was detected in (17/50) (34%). Out of them, 8(47.2%) had cMLSB resistance phenotype.The association between ermC gene and cMLSB phenotype of S. aureus
was statistically significant (p≤ .010). ermA was detected in (2/50) (4%)and they had iMLSB, while ermB gene wasn’t detected in any of the tested S. aureus isolates.
7- msrA gene was found in (6/50) (12%) isolates, 5(83.4%) of them were MSB phenotype and 1 (16.6%) was iMLSB resistance phenotype. The association between msrA gene and MSB phenotype of S. aureus. was statistically significant. (p≤ .010)
8- The PCR results revealed no streptogramin A or B resistance genes (vatA, vatB, vatC, vgaA and vgbA) were detected among the 50 S. aureus isolates included in this study.
9- None of the 25 S.aureus isolates expected to be resistant to streptogramin A (3 isolates) or B (22 isolates) showed resistance to quinupristin-dalfopristin by disc diffusion method (only 3 with intermediate resistance) in addition none of them carried the resistance genes (vatA, vatB, vatC, vgaA and vgbA).
10- The discrepancies between the genotypic and the phenotypic pattern were found in 6(12%) S.aureus isolates when comparing the results of antibiotic susceptibility by disk diffusion method with gene analysis results; 3(6%) S. aureus erythromycin sensitive ermC gene positive isolates and 3 (6%) erythromycin resistant isolates (1 cMLSB and 2 MSB) MLS resistance genes negative.
11- No discrepancy was found between phenotypic and genotypic results regarding susceptibility of the S.aureus isolates to streptogramin combination. The isolates with positive phenotypic pattern (P+) have intermediate resistance and was not fully resistant to Q-D, on the other hand, the isolates with positive genotypic pattern (G+) the amplified genes wereerm and/ ormsr genes not vatA, vatB, vatC, vgaA and vgbA.
In Conclusion:
1. High resistance to antibiotics commonly used to treat S.aureus infections.
2. Inducible resistance to clindamycin was 5(10%)isolates.
3. The ermC gene was the predominant gene and was significantly associated withcMLSB phenotype. (p≤.010).
4. The msrA gene was significantly associated with MSB phenotype of S. aureus. (p≤.010).
5. No resistance to quinupristin-dalfopristin was detected in any of the tested S. aureus isolates, this result was confirmed by absence of genes related to quinupristin-dalfopristin (VatA, VatB, VatC, VgaA and vgbA genes)
6. There were discrepancies between the genotypic and the phenotypic MLSBpattern when comparing the results of antibiotic susceptibility by disk diffusion method with gene analysis results in S. aureus isolates.
7. No discrepancy was found between phenotypic and genotypic results regarding susceptibility of the S.aureus isolates to streptogramin combination.