الفهرس 
IntroductionOnly 14 pages are availabe for public view
Abstract
Staphylococci are opportunistic pathogens when they gain
entry to the host tissue through trauma of the skin barriers,
inoculation by needles or direct implantation of medical device.
The most frequently isolated staphylococci are S. aureus
which is coagulase positive and S. epidermidis which is coagulase
negative.
Staphylococci have been implicated in a wide range of
infections making them a major pathogen of hospital- and
community-acquired infections as a result of having a variety of
virulence factors causing toxin production, toxin invasion of
distant organs and biofilm formation that protect the bacterial
cells from antibiotics and immunity.
Beta-lactam antibiotics are used for treatment of
staphylococcal infections. They act by inhibiting the synthesis of
the peptidoglycan layer of the bacterial cell wall. They are
divided into 2 groups: B-lactam antibiotics which include 4
subgroups: penicillins, cephalosporins, monobactams and
carbapenems while the other group is B-lactamase inhibitors
including: sulbactam, clavulanate and tazobactam.
Beta-lactamase inhibitors act on a group of enzymes called
Beta-lactamases that cause destruction of the β-lactam ring the β-lactam antibiotic inactive. In order to understand
the relationship of these enzymes to one another, two
classification systems have been developed; the Bush (functional)
classification and the Ambler (molecular) classification. Bush
classified them into four groups according to substrate and
inhibitor profiles, while Ambler classified them into four classes
(A-D) based on the nucleotide and amino acid sequences. Classes
A, C, and D act by a serine based mechanism, whereas class B or
metallo- β-lactamases need zinc for their action.
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, after penicillin resistance had developed, and because
resistance to this antibiotic implies resistance to all ß-lactam
antibiotics.
Staphylococci resist B-lactam antibiotics through 2
mechanisms either acquisition or hyperexpression of Blactamases
causing degradation of the antibiotic, or acquisition of
foreign Penicillin Binding Protein (PBP2a) with low affinity to
methicillin. MecA gene is the gene encodes a modified PBP2
protein (PBP2a) with low affinity for methicillin and all ß-lactam
antibiotics except the newest 5th generation cephalosporin. This gene is located in a mobile gene cassette (SCCmec) which is
composed of two components mec complex and ccr complex.
Eight types have been discovered (SCCmec typeI-VIII). Most of
the hospital-acquired methicillin resistant S. aureus (HA-MRSA)
harbor SCCmec type (I-II-III) whereas community-acquired
MRSA (CA-MRSA) harbor SCCmec type (IV).
It is important to control the spread of MRSA. Phenotypic
typing methods are good for initial screening and for
identification of known epidemic strains while molecular methods
are capable of differentiating epidemic strains from endemic
strains.
The control of MRSA is a multipronged approach, combining
prevention, decolonization, vaccination and treatment efforts.
Currently, there is no US FDA-approved vaccine to prevent or
treat S. aureus diseases.
There are several methods for detecting methicillin resistance,
including classical methods (disk diffusion, E-test, or broth
dilution), screening techniques with solid culture medium
containing oxacillin e.g ORSAB (containing 2mg/L oxacillin),
selective chromogenic agar which has generated improvements
over routine culture media and methods that detect the mecA gene
e.g PCR or its protein product (PBP2a protein). 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 screening methods
are incorporated into routine clinical practice.
The main objective of this study was to evaluate cefoxitin disk
diffusion test (FOX.DD test), Oxacillin resistant screen agar base
(ORSAB) and oxacillin E- test MIC methods in relation to realtime
PCR as a gold standard method to evaluate the most accurate
method among them for routine diagnosis of methicillin resistant
staphylococci in clinical laboratories.
In our study, 50 mecA positive staphylococci were confirmed
by sybergreen real-time PCR which were used as the gold
standered method. Cefoxitin disk diffusion test and oxacillin Etest
showed sensitivities of 97%, 80.6% for detection of MRSA
and 100%, 100% for detection of MRCoNS; respectively.
According to Oxoid manual, ORSAB test was done only for S.
aureus and showed a sensitivity of 94% in detecting methicllin
resistance.
We concluded that FOX DD is an overall accurate method for
detection of methicillin resistance which is better than the Oxacillin E-test in detecting the heterogenous strains and the
ORSAB is better in detection of MRSA from S aureus isolates
than from clinical samples (that needs further identification of S.
aureus strains from CoNS strains) as CoNS give blue colonies
like that of MRSA on ORSAB media
We recommended that PCR is the best rapid method for
sceening of methicillin resistance in staphylococci to control
spread of infection. In case of unavailability of PCR technique,
FOX DD can be alternative for PCR as Cefoxitin is a better
inducer of the expression of the mecA gene than oxacillin. The
FOX DD is easy to perform, do not require special technique,
media preparation and more cost effective in comparison to other
methods )PCR, E-test).
Also, we recommend combining another test with Oxacillin Etest
for detection of methicillin resistant staphylococci as it is
alone not an accurate method for detection of heterogenous
strains strains. ORSAB can be a good test for screening of MRSA
if used on clinical isolates but for screening from clinical samples,
MRSA identification should be done to exclude CoNS and
increasing the test sensitivity.