Transfer of plasmid and chromosomal glycopeptide resistance determinants occurs more readily in the digestive tract of mice than in vitro and exconjugants can persist stably in vivo in the absence of glycopeptide selection.

OBJECTIVES AND METHODS: The transferability of vanA and vanB glycopeptide resistance determinants with a defined plasmid (n = 9) or chromosomal (n = 4) location between Enterococcus faecium strains of human and animal origins was compared using filter mating (in vitro) and germ-free mice (in vivo) as experimental models. Moreover, the stability of exconjugants in vivo in the absence of antibiotic selection was examined. RESULTS: Higher transfer rates were observed in vivo for four of six vanA and five of six vanB donor strains. For plasmid-encoded resistance, several log higher transfer frequencies were observed in vivo for some strains. Moreover, the in vivo model supported transfer of plasmid-encoded vanB (1 x 10(-7) exconjugants/donor) when repeated in vitro experiments were negative (estimated < 1 x 10(-9) exconjugants/donor). Readily detectable transfer of plasmid-located vanA and vanB as well as large chromosomal (>200 kb) vanB elements was observed after 24 h. The number of plasmid-mediated vanA exconjugants generally decreased markedly after 3 days. However, exconjugants containing a plasmid harbouring the vanA transposon Tn1546 linked to the post-segregational killing system omega-epsilon-zeta persisted stably in vivo in the absence of glycopeptides for more than 20 days. CONCLUSIONS: The overall results support the notion that the in vitro model underestimates the transfer potential. Rapid transfer of vanA plasmids from poultry- and pig-derived strains to human faecal E. faecium shows that even transiently colonizing strains may provide a significant reservoir for transfer of resistance genes to the permanent commensal flora. Newly acquired resistance genes may be stabilized and persist in new populations in the absence of antibiotic selection.

mef(A), mef(E) and a new mef allele in macrolide-resistant Streptococcus spp. isolates from Norway.

OBJECTIVES: To type mef genes in a nationwide collection of clinical isolates of Streptococcus pneumoniae and Streptococcus pyogenes as well as pharyngeal carrier strains of viridans streptococci in Norway. METHODS: Erythromycin-resistant mef-positive multilocus sequence-typed (MLST) clinical isolates of S. pneumoniae (n = 36) and S. pyogenes (n = 12) from the National Surveillance Program for Antimicrobial Resistance (NORM) as well as viridans streptococci (n = 20) from healthy adults were included. PCR-amplified mef genes were initially discriminated by BamHI digestion. Selected mef genes from representatives of different sequence types (STs) of S. pneumoniae (n = 11) and S. pyogenes (n = 4), and viridans group streptococcal species (n = 8) were typed by sequencing and their strains examined for co-resistances. Hydropathy plots of different mef-encoded proteins were performed. RESULTS: A predominance of mef(A) was detected in S. pneumoniae (23/36) and S. pyogenes (9/12) due to the clonal spread of ST9 and ST39, respectively. mef(E) was the most widely distributed mef determinant occurring in nine different STs of S. pneumoniae and in four different viridans species. A new mef allele was identified in two STs of S. pyogenes. CONCLUSIONS: mef(E) is the most widely distributed mef determinant in Norwegian clinical strains of S. pneumoniae and pharyngeal carrier strains of various viridans streptococci. However, mef(A) is more prevalent in S. pneumoniae and S. pyogenes due to clonal spread. A new mef allele was found in two different STs of S. pyogenes.