Increased uptake and improved intracellular survival of a teicoplanin-resistant mutant of methicillin-resistant Staphylococcus aureus in non-professional phagocytes.

Background/aims: Endogenous development of glycopeptide-intermediate resistance is linked to multiple genetic and phenotypic changes in clinical and laboratory isolates of Staphylococcus aureus. This study evaluated endocytic uptake and intracellular survival of a teicoplanin-resistant derivative of S. aureus in a human epithelial cell line, and compared these to the isogenic teicoplanin-susceptible parent or a spontaneously derived, susceptible revertant.

Methods: Endocytic uptake of teicoplanin-resistant and teicoplanin-susceptible strains by human embryonic kidney 293 cells was estimated by a lysostaphin protection assay. Differential intracellular survival of all S. aureus strains from 2 to 24 h was evaluated by colony-forming unit counts of Triton X-100-lysed 293 cells, following lysostaphin inactivation.

Results: Endocytic uptake of the teicoplanin-resistant strain increased by approximately 4-fold over its teicoplanin-susceptible counterparts. Furthermore, the teicoplanin-resistant strain showed an 11-fold increase in intracellular colony-forming unit counts from 2 to 24 h, compared to its teicoplanin-susceptible counterparts that showed marginal (<2-fold) changes during the same time period. Infected host cells showed no significant viability loss at 24 h, as assessed by Trypan blue dye exclusion.

Conclusions: Intracellular location might confer a significant fitness benefit to glycopeptide-intermediate isolates of methicillin-resistant S. aureus and further protect them from cell wall-active antibiotics whose intracellular activity is limited.