In vitro activity of tigecycline against tetracycline-resistant Gram-positive cocci isolated from patients with bacteraemia

Abstract number: p1321

Forsthuber  S., Lagler  H., Stich  K., Reichmann  S., Georgopoulos  A., Graninger  W., Presterl  E.

Objectives: 

The aim of this study was (i) to evaluate the in vitro activity of tigecycline against tetracycline-resistant Streptococcus pneumoniae (SPN), Staphylococcus aureus (SA) and viridans group streptococci (VGS) isolated from patients with bloodstream infections in Austria and (ii) to detect the genetic mechanism of tetracycline resistance.

Methods: 

A total of 153 tetracycline-resistant (TetR) gram-positive cocci (51 SPN, 54 SA, 48 VGS) were isolated from patients with bacteraemia. Isolates were collected between 2001 and 2004. MICs (minimal inhibitory concentrations) of tetracycline (TET), penicillin (PEN), erythromycin (ERY), clarithromycin (CLR), azithromycin (AZM), clindamycin (CLI), telithromycin (TEL), ciprofloxacin (CIP), levofloxacin (LVX), moxifloxacin (MXF), linezolid (LZD), gentamicin (GEN), fusidic acid (FUS) and vancomycin (VAN) were determined by the microdilution method according to the Clinical and Laboratory Standards Institute (CLSI). MIC of tigecycline (TIG) was tested by the Etest method (AB Biodisk). Tigecycline susceptible breakpoint was defined as <=2. The presence of tetracycline resistance genes (tetM, tetO, tetK) was determined by PCR.

Results: 

The MICs of tigecycline ranged from 0.125 to 0.25 mcg/ml (MIC50 = 0.25, MIC90 = 0.5) for all isolates of TetR S. pneumoniae, including 34 tet(M) positive strains. Similar findings were observed for tetracycline-resistant VGS. The range of MICs was 0.125–1 mcg/ml, including 28 tet(M) and 1 tet(O) positive VGS. 30 S. aureus isolates contained tet(K) and two tet(M). The tigecycline MICs for tet(M/O) positive strains were higher (range, 0.25–1 mcg/ml) than for isolates which lacked tet determinants (range, 0.25–0.5 mcg/ml). All tested pathogens were sensitive to vancomycin and linezolid.

Conclusions: 

(i) The present study suggest that tigecycline may be an effective alternative to commonly prescribed antimicrobials in the treatment of gram-positive cocci. (ii) There was no relation between the presence or absence of tetracycline resistance determinants and the MICs of tigecycline in TetR VGS and SPN, whereas tet (K/M) positive S. aureus isolates showed higher MICs.