Bactericidal activity of orally available agents against methicillin-resistant Staphylococcus aureus

The recent proliferation of community-acquired (CA) methicillin-resistant Staphylococcus aureus (MRSA) has led to a marked increase in the need for outpatient treatment of MRSA infections. Many oral agents active against MRSA have been available for years, and a paucity of literature compares them, leaving physicians with little guidance for choosing among them. The purpose of the present study was to compare the bactericidal effects of orally available antibiotics against MRSA and to determine whether there were differences in antimicrobial killing activity against CA-MRSA and hospital-acquired (HA) MRSA isolates.

A total of 12 unique patient MRSA isolates were studied. Six strains were CA, carrying the staphylococcal chromosomal cassette (SCCmec) type IVa, while six were HA and carried SCCmec type II. Time-kill methods were used to study the bactericidal activity of the orally available antimicrobials linezolid, rifampicin, trimethoprim/sulfamethoxazole, clindamycin, minocycline, and moxifloxacin alone and in combination in vitro.

Trimethoprim/sulfamethoxazole was rapidly bactericidal resulting in >2 log(10) cfu/mL decrease at 8 h and >3 log(10) cfu/mL decrease at 24 h in vitro. No antibiotic combination exhibited better killing than trimethoprim/sulfamethoxazole alone. Adding rifampicin to trimethoprim/sulfamethoxazole showed a trend towards antagonism in vitro. There were no differences in the bactericidal activity of any antimicrobial or antimicrobial combination against MRSA isolates carrying SCCmec type IVa versus those carrying SCCmec type II.

Trimethoprim/sulfamethoxazole is rapidly bactericidal against MRSA in vitro when compared with most other orally available antimicrobials. No differences in bactericidal activity were detected when activities against CA-MRSA and HA-MRSA were compared.