A Study on resistance of non-PBP 2a type to oxacillin of Staphylococcus aureus

Haifeng SHAO; Wei XIE; Weiping WANG; Zhenda LI

ABSTRACT

Objective:

To investigate whether there is another resistance mechanism besides mecA gene in oxacillin-resistant(OR) isolates of Staphylococcus aureus(S.aureus).

Methods:

There were 130 oxacillin-resistant phenotype isolates of S. aureus. Of these isolates 125 were resistant to more than 3 of 5 non-?-lactams (gentamicin, ciprofloxacin, clindamycin, tetracycline, erythromycin) (OR-R) and 5 susceptible to more than 3 of the 5 non-?-lactams (OR-S) and 14 were oxacillin-susceptible (OS) phenotype isolates of S. aureus and resistant to more than 3 of 5 non-?-lactams (OS-R). All the strains were detected by the two disks diffusion tests with oxacillin (OXA) and oxacillin/clavulanic acid (OXA/CA), by the three-dimensional extract tests of penicillin (PEN) and OXA for penicillinase and oxacillinase, and by PCR tests for mecA.

Results:

The 130 OR and 14 OS isolates were all oxacillinase-negative with the two disks diffusion tests, all pencillinase-positive and oxacillinase-negative in the three-dimensional extract tests. The mecA gene was detected in 125 OR-R-type and 2 of the 5 OR-S-type isolates, but was not detected in the other 3 OR-S-type nor in any of the 14 OS-R-type isolates by PCR.

Conclusion:

In a few Staphylococcus aureus strains which are phenotype of oxacillin-resistant and are susceptible to mostly non-?-lactam agents there are both mecA-negative and oxacillinase-negative strains, 2.3% (3/130) of the OXA-resistant strains. The resistance mechanism may be associated with reduced binding capacity of the modified Preexisting PBPs with OXA.