In Vitro Efficacy of Silver Carbene Complexes, SCC1 and SCC22, Against Some Enteric Animal Pathogens

Abstract

Silver carbene complexes (SCCs), a group of novel silver-based compounds capable of gradually ‎releasing ‎silver ions, have shown significant antimicrobial activity against a wide range of ‎bacterial pathogens mainly ‎isolated from human cases. The antimicrobial activity against ‎animal isolated pathogens has yet been done. ‎The in vitro efficacy of two SCCs with different ‎carrier molecules (SCC1 with a methylated caffeine backbone ‎and SCC22 with a ‎dichloroimidazolium backbone) was investigated against three important animal and ‎human ‎pathogen species. SCC1 and SCC22 exhibited bacteriostatic and bactericidal effects against ‎multidrug ‎resistant Salmonella Typhimurium (poultry isolate), E. coli 843 and E. coli 1568 ‎‎(swine isolates), and the ‎poultry field isolates Salmonella Heidelberg, Salmonella Enteritidis, and ‎Salmonella Montevideo with MICs ‎and MBCs ranged from 16-21 µM (6-8 µg/mL) and 16-32 µM ‎‎(6-12 µg/mL), respectively. Clostridium ‎perfringens type A was sensitive to both SCC1 and ‎SCC22 with the MICs being 11 (4 µg/mL) and 21 µM ‎‎(8 µg/mL), respectively. These values were ‎comparable to the MICs and MBCs for silver acetate. The MBCs ‎against C. ‎perfringens was >85 µM for ‎SCCs and >192 µM for silver acetate (>32 µg/mL for all compounds). Ten hours incubation ‎of C. ‎perfringens with ‎‎40 µg/mL of all three products showed down regulation of virulence genes plc and netB, ‎‎suggesting viable cells and silver can modulate the virulence. Treating the C. ‎perfringens with higher ‎concentration (100 ‎‎µg/mL) of each SCC for 10 hours inhibited more bacteria compared to the ‎untreated bacterial cells, however, no ‎differences in the ultrastructure of lysed bacteria were seen ‎and this concentration might not induce viable ‎but non-culturable (VBNC) state as suggested by ‎transmission electron microscopy findings. SCCs showed a ‎broad antimicrobial activity against ‎all bacterial species tested including multidrug resistant pathogens. Both ‎SCCs demonstrated ‎inhibitory effect against the Gram-positive anaerobic C. perfringens type A ‎which ‎could have a high accumulation capacity for silver ion. These data suggest that SCCs may ‎represent a ‎novel class of broad-spectrum antimicrobial agents, which may be used to reduce the ‎burden of pathogenic ‎bacteria in the gastrointestinal tract of poultry‎‎‎.