الفهرس 
StaphylococciOnly 14 pages are availabe for public view
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Abstract
Staphylococci is the most common causes of nosocomial or community-based infections, leading to serious illnesses with high rates of morbidity and mortality. In recent years, the increase in the number of bacterial strains that show resistance to methicillin (MRSA) has become a serious clinical and epidemiological problem because this antibiotic is considered as the first option in the treatment of staphylococci infections, and because resistance to this antibiotic implies resistance to all ß-lactam antibiotics. For these reasons, accuracy and promptness in the detection of methicillin resistance is of key importance to ensure correct antibiotic treatment in infected patients as well as control of MRSA isolates in hospital environments, to avoid their spreading.
Methicillin Resistant Staphylococcus aureus (MRSA) strains harbour the mecA gene, which encodes a modified PBP2 protein (PBP2a) with low affinity for methicillin and all ß-lactam antibiotics. Phenotypic expression of methicillin resistance may alter depending on the growth conditions for staph. aureus, such as temperature or osmolarity of the medium, and this may affect the accuracy of the methods used to detect methicillin resistance. Heteroresistant bacterial strains may evolve into fully resistant strains and therefore be selected in those patients receiving ß-lactam antibiotics, thus causing therapeutic failure. from a clinical point of view, they should, therefore, be considered fully resistant.
There are several methods for detecting methicillin resistance, including classical methods for determining MICs (disk diffusion, E-test, or broth dilution), screening techniques with solid culture medium containing oxacillin e.g ORSAB (containing 2mg/L oxacillin), and methods that detect the mecA gene e.g PCR or its protein product (PBP2a protein) e.g MRSA screen test.
Detection of the mecA gene is considered as the reference method for determining resistance to methicillin. However, many laboratories throughout the world do not have the capacity or the experienced staff required to develop molecular techniques for detecting MRSA isolates and it is therefore essential that other, more useful, screening methods are incorporated into routine clinical practice. The main objective of this study was to evaluate Oxacillin disk diffusion test (OX.DD test), Oxacillin resistant screen agar base (ORSAB) and anti PBP2a latex agglutination test (MRSA Screen Test) in relation to Cefoxitin disk diffusion test (Fox.DD test) to study the value of slide latex agglutination test(MRSA screen test) as a rapid method for detection of methicillin resistance in staphylococcal isolates in routine work and its suitability as routine methods for detecting MRSA isolates in clinical microbiology laboratories.
In our study Oxacillin disk diffusion test, Oxacillin Resistant Screen Agar Base(ORSAB ) and anti PBP2a latex agglutination tsest (MRSA screen test) showed sensitivities of 87.5% , 87.5%, 96% for Staph aureus and 100%, 75%, 87.5 % for CoNS respectively and specificities of 61.54%, 61.5 % ,92.3% for Staph aureus and 62.5%, 80.0%, 80% for CoNS respectively.
The low specificity of OX DD test and ORSAB medium prevent the use of each alone to predict methicillin resistance in staphylococci.
The MRSA-Screen latex agglutination test is a rapid, simple, and accurate method to determine the presence of oxacillin resistance mediated by the mecA gene in Staph.aureus. The test has the major advantage over the other phenotypic methods of not being influenced by the various levels of expression of the resistance, a parameter in which highly heterogeneously resistant isolates tends to render classical and automated methods less accurate.
So we recommend its use in routine work and in infection control program for rapid detection of MRSA as it shorten the delay for their detection to 1 day, versus 2 or 3 days for conventional methods.
On the other hand its use in detection of MRCoNS needs an additional enhancement of PBP2a expression which requires the use of oxacillin disk during over night subcultivation and this could delay results by 24 hours which is considered the principle difficulty of MRSA-Screen latex agglutination test.
Nevertheless, we recommend its use also in detection of MRCoNS as there is no alternative technique with shorter time could be used for routine work.