TITLE
Mannitol Salt Agar-Cefoxitin
Combination as a screening
Medium for MethicillinResistant Staphylocuccus
aureus.
INTRODUCTION
 In
disk diffusion tests, cefoxitin is now
considered a better indicator than
oxacillin for the presence of the mecA
gene in staphylococcus aureus.
 A logical
extension of this work is the
incorporation of cefoxitin in to media
selective for MRSA.

Culture media selective for MRSA have
traditionally been based on blood agar ,
mannitol salt agar or Bird-Parker agar
containing oxacillino or methicillin or
combination with other antibiotics.
These screening media for MRSA have
never achieved the acceptable level of
sensitivity and specificity.

The paper describes a selective media
based on mannitol salt agar containing
4mg/l cefoxitin support the growth of
unique collection of mecA positive S
aureus and inhibit the growth of mecAnegative strain
Materials and Methods
The following groups of strain were
tested in this study:
1.
100 strain collection of 75 MRSA
strain and 25 MSSA strain .
2. 64 S aureus strain that tested negative
for the mecA gene ( 14 were
classified as BORSA strain on
account of their elevated MICs for
oxacillin)
3. 8 mecA positive strains which have
proved difficult or impossible to
classify as such by the use of cefoxitin
disks.
4. An additional four MRSA strains
that were initially classify as BORSA
strains but that were later found to be
positive for the mceA gene.
 The
presence or absence of mecA gene
and nuc gene were determined for all
strain by PCR.
 The
MICs for the 100 strain collection
were determined by using Etest on Isosensitest agar.
(oxoid,Basingstoke,United Kingdom).
 MSA plate
were prepared with cefioxitin
at concentration of 2,3,or 4mg/l.the test
strains were applied at concentration of
102 and 105 viable organisms and
incubation was carried out in air
aerobically at 35C to 37C and the result
was read at 18 and 48 h. The growth of
any visible colonies after incubation was
recorded as a positive result.
RESULT
The MIC result for the 100 collection were as
follows:
 All 25 MSSA MIC of cefoxitin-  2mg/l.
 All
68 MRSA MIC of cefoxitin- 4mg/l.
 Seven
mecA positive strains MIC of
cefoxitin – > 4mg/l.
 MSA plate
with 2mg/l cefoxitin allowed
the growth of all MSSA and MRSA strains
after 48h incubation
 MSA plate
with 3mg/l cefoxitin allowed
the growth of four MSSA strains and all
MRSA strains.
 MSA plate
with 4mg/l cefoxitin allowed
growth of none of the 25 MSSA and all 75
MRSA strains.
DISCUSSION
Methicillin or oxacillin has been the agent of
choice in selective media for mceA-positive for s
aureus over 20 years.
 The results of this study demonstrated that
cefoxitin is far superior to oxacillin when it is
used in this particular mannitol salt media .
 By the use of a 48h incubation and a 10 2–
CFU/ml inoculam, the sensitivity and specificity
of cefoxitin MAS plate method were
100%(compared to 90.7% with oxacillin) and
100% ( compared to 96.0% for oxacillin)
respectively.


How ever more frequently patient specimens
contain methicillin resistant CoNS .This are
capable to grown MAS plate and produce pink
colonies which are easily distinguishable from
the yellow colonies of mannitol positive S
aureus.

In highly prevalence area of MRSA cefoixtin
plate can easily detected MRSA and methicilin
resistant CoNS will not be perceived as a
problem.