Methicillin-Resistant Staphylococcus aureus Isolates from Filipino Patients (1999-2003)

Infections with methicillin- or oxacillin-resistant Staphylococcus aureus (historically known as MRSA) is a global problem and local investigators have reported the presence of MRSAs in the Philippines. Staphylococcal strains often synthesize β-lactamase that hydrolyzes the β-lactam ring, thus, causing inactivation of β-lactams. This study correlated the in vitro presence of β-lactamase in S. aureus strains and their resistance to penicillin and oxacillin. From 1999 to 2003, 604 local Staphylococcus isolates were collected from patients in three Manila hospitals and assayed for identification, presence of β- lactamase, oxacillin-resistance, and susceptibility to various antibiotics using the Vitek 60. Three hundred seventy-three (373) S. aureus strains were isolated, and results indicated a significant correlation between the presence of the β-lactamase enzyme and resistance to penicillin (Pearson's correlation coefficient, r=1, p-value<0.001). The average resistance rate to oxacillin was 18.0% from 1999-2003. The 68 MRSA strains showed two types of antibiograms: a) 35.3% with multiple-resistance to all β-lactams, β-lactam/β- lactamase-inhibitor combinations, erythromycin and clindamycin, and could include resistance to tetracycline, aminoglycosides and quinolones; b) 64.7% with resistance to all β-lactams and β-lactam/β- lactamase-inhibitor combinations but not to non-β-lactams. Results also indicated that aside from sensitivity to vancomycin, the local MRSAs are still responsive to all the non-β-lactams tested as shown by the 87-100% susceptibility rate in 2002-2003. (Phil J Microbiol Infect Dis 2004; 33(3):105-110)