Bacterial Silver Resistance Following Repeated Serial Passage and Cross Resistance to Antimicrobials and Antibiotics

Background: The rise in multidrug resistant (MDR) infections is a global concern. Whilst there is relatively less evidence of silver resistance compared to antibiotic resistance, silver containing dressings are widely used in wound care so it is important to assess resistance development and potential cross-resistance to antibiotics.

Purpose: The aim of this study was to investigate the ability of several wound pathogens to develop resistance to a widely used silver compound in wound care, silver sulfadiazine (SSD).

Methods: The minimum inhibitory concentration (MIC) of Pseudomonas aeruginosa and Staphylococcus aureus was determined using a broth microdilution method. The frequency of resistance was used to quantify occurrence of spontaneously arising mutants. Strains were repeatedly passaged in sub-inhibitory concentrations of SSD (0.25 MIC) to develop resistance. The MIC of resistant mutants was determined for SSD and several clinically used antibiotics.

Results: The MIC of SSD against P. aeruginosa and S. aureus was 16 and 128 micrograms/mL, respectively. At a concentration of 4x MIC no mutants could be selected following treatment with SSD. Following serial passage of strains with sub-inhibitory concentrations of SSD some Enterobacter cloacae showed increased tolerance. However, no cross resistance was found with SSD resistant mutants and clinically used antibiotics.

Conclusion: Results indicate cross-resistance to antibiotics following the use of SSD is not a concern at present, although to confirm this a higher number of resistant isolates should be screened.