Evaluation of a new 3-h hybridization method for detecting the mecA gene in Staphylococcus aureus and comparison with existing genotypic and phenotypic susceptibility testing methods
Rl. Skov et al., Evaluation of a new 3-h hybridization method for detecting the mecA gene in Staphylococcus aureus and comparison with existing genotypic and phenotypic susceptibility testing methods, J ANTIMICRO, 43(4), 1999, pp. 467-475
A new 3-h hybridization assay for detection of the staphylococcal mecA gene
and the Staphylococcus aureus nuclease gene was evaluated by comparing the
assay with existing genotypic and phenotypic methods. A total of 275 S. au
reus strains were tested, including 257 epidemiologically unrelated strains
(135 mecA-positive and 122 mecA-negative; collection I), and 18 strains wi
th known borderline resistance to methicillin (collection II). Complete agr
eement was obtained for both collections when comparing the new assay with
genotypic methods. We further evaluated a range of phenotypic susceptibilit
y methods recommended in Europe and/or USA using the presence of the mecA g
ene as the defining standard. For collection I a high degree of agreement w
as found for both Etests (256 strains) and the oxacillin screen plate test
(255 strains); the degree of agreement was lower for agar dilution methicil
lin (250 strains) and oxacillin 1 mu g discs (239 strains). For the borderl
ine strains a high degree of agreement was only obtained by the oxacillin s
creen plate test (17 of 18 strains). The other tests were less accurate, in
the following order: agar dilution methicillin, Etest methicillin, Etest o
xacillin and oxacillin discs with disagreement for four, five, nine and 13
strains, respectively. In conclusion, the new hybridization assay is a rapi
d and exact method for detecting the mecA gene and the S. aureus nuclease g
ene. This study confirms that phenotypic tests for methicillin resistance i
n S. aureus strains creates both false-susceptible and false-resistant resu
lts, especially for borderline resistant strains.