Erythromycin resistance features and biofilm formation affected by subinhibitory erythromycin in clinical isolates of Staphylococcus epidermidis.

Background/purpose: Subminimal inhibitory concentration (sub-MIC) of antibiotics can modify the phenotype of biofilm formation in bacteria. However, the relationship between resistance phenotypes, genotypes, and the biofilm formation phenotype in response to sub-MIC antibiotics remains unclear.

Methods: Here, we collected 96 clinical isolates of Staphylococcus epidermidis (S. epidermidis) and investigated the erythromycin (ERY) susceptibility, the biofilm formation in respond to sub-MIC ERY, the presence and transcription expression of erm genes. Serial passage of induction resistance was used against ERY-susceptible isolates and biofilm formation in response to their new sub-MIC ERY was determined.

Results: The incidence of biofilm phenotype modification in ERY-resistant isolates was significantly higher than that of ERY-susceptible isolates [27/85 (31.8%) vs. 0/11 (0%), p = 0.031]. Yet, ERY-susceptible isolates displayed the phenomenon of biofilm phenotype modification (7/11), after induction of resistance to ERY. The ermC gene was absolutely dominant among the three macrolide resistant genes including erm (A, B, C) [6/96 (6.2%), 6/96 (6.2%), and 91/96 (94.8%), respectively]. With statistic stratification analysis, a linear and positive correlation was identified between the two factors in the biofilm-enhanced strains, a linear and negative correlation in biofilm-inhibited strains, and a weakly positive correlation in biofilm-unaffected strains (R(2) = 0.4992, 0.3686, and 0.0512, respectively).

Conclusion: The results suggest that the ERY resistance phenotype and the transcription expression of ermC gene could be considered as important signs to estimate whether the biofilm formation phenotype in S. epidermidis clinical isolates can be easily affected by sub-MIC ERY.