The mef gene, originally described for gram-positive organisms and coding for an efflux pump, has been identified in clinical isolates of Acinetobacter junii and Neisseria gonorrhoeae. These strains could transfer the mef gene at frequencies ranging from 10 ؊6 to 10 ؊9 into one or more of the following recipients: gram-negative Moraxella catarrhalis, Neisseria perflava/sicca and Neisseria mucosa and gram-positive Enterococcus faecalis. Three Streptococcus pneumoniae strains could transfer the mef gene into Eikenella corrodens, Haemophilus influenzae, Kingella denitrificans, M. catarrhalis, Neisseria meningitidis, N. perflava/sicca, and N. mucosa at similar frequencies. The mef gene can thus be transferred to and expressed in a variety of gram-negative recipients.The mef gene encodes a membrane-bound efflux protein and confers resistance to macrolides but not to lincosamides or streptogramin B (24, 25). From geographically diverse areas, two gene variants with 90% nucleotide sequence identity, mefA and mefE, have been identified in gram-positive organisms, including Streptococcus pneumoniae, Streptococcus pyogenes, Corynebacterium spp., Enterococcus spp., Micrococcus luteus, and viridans group streptococci (2,5,9,20,(23)(24)(25). Both genes have now been combined as mef(A) (20).Recently, we have shown that both Neisseria gonorrhoeae and oral commensal Neisseria spp. carry known rRNA methylase genes (20). However, we have also identified several of these Neisseria spp. and oral gram-negative isolates that were macrolide resistant yet did not carry Erm determinants (ErmA, -B, -C, or -F) (21). We selected a gram-negative Acinetobacter junii strain and screened two groups of N. gonorrhoeae in order to determine if the mef gene was present. Because we have shown that the mef gene can be transferred by conjugation in various gram-positive genera, we also wanted to determine the mobility of the mef gene in these gram-negative isolates as well as the ability of gram-positive donors to move the mef gene to gram-negative species (9).(This work was presented, in part, at the 39th Interscience Conference on Antimicrobial Agents and Chemotherapy, 23 to 27 September 1999, San Francisco, Calif.) MATERIALS AND METHODSBacteria. Erythromycin-resistant (Ery r ) Acinetobacter junii strain 329 was isolated from the oral gumline of a child in Portugal (Table 1). The organism was identified by D. Stroman (Alcon Laboratories, Inc., Fort Worth, Tex.) by sequencing the 16S rRNA. Sixteen N. gonorrhoeae isolates with variable susceptibilities to erythromycin (MIC ϭ 0.25 to 4 g/ml) were collected from urethral specimens from adults in Seattle, Wash., and identified by the Neisseria Reference Laboratory at the University of Washington (Table 1). Thirteen N. gonorrhoeae isolates carrying the tetM 25.2-MDa plasmid were collected in Seattle and the eastern United States during 1983 through 1986 and have been previously described (7). Of the three Ery r S. pneumoniae strains carrying the mef gene, two (n011 and 970146) were isolated in Washington...

The presence of macrolide-lincosamide-streptogramin B resistance genes erm(B), erm(C) and erm(F), the macrolide resistance mef(A) gene, and the DNA sequence of a 13 bp repeat in the promoter region of the mtrR gene, were determined in 62 Neisseria gonorrhoeae isolates collected between 1992 and 1999 in Seattle, Washington, USA. Eleven isolates with erythromycin and azithromycin MICs of < or =0.06 mg/L, had no acquired genes or deletions in the 13 bp repeat region. Among 44 isolates with erythromycin MICs 1.0-16.0 mg/L, and azithromycin MICs 0.06-4.0 mg/L, 16 carried the 1 bp deletion in the mtrR promoter region alone, nine carried one or more of the four acquired macrolide resistance genes alone, and 14 carried both acquired macrolide resistance genes plus the 1 bp deletion in the mtrR promoter region. Three isolates with erythromycin MICs > or = 8 mg/L, and azithromycin MICs of 4.0 mg/L, carried only erm genes. Five isolates with MICs of 1-2 mg/L did not carry the 1 bp deletion, or any of the acquired resistance genes examined. Our data suggest that the 1 bp deletion in the mtrR promoter region is not found in all erythromycin-resistant (MIC > or = 1.0 mg/L) N. gonorrhoeae.

Seventy-six Neisseria gonorrhoeae isolates, isolated between 1940 and 1987, and seven Neisseria meningitidis isolates, isolated between 1963 and 1987, were screened for the presence of acquired mef(A), erm(B), erm(C), and erm(F) genes by using DNA-DNA hybridization, PCR analysis, and sequencing. The mef(A), erm(B), and erm(F) genes were all identified in a 1955 N. gonorrhoeae isolate, while the erm(C) gene was identified in a 1963 N. gonorrhoeae isolate. Similarly, both the mef(A) and erm(F) genes were identified in a 1963 N. meningitidis isolate. All four acquired genes were found in later isolates of both species. The mef(A) gene from a 1975 N. gonorrhoeae isolate was sequenced and had 100% DNA and amino acid identity with the mef(A) gene from a 1990s Streptococcus pneumoniae isolate. Selected early isolates were able to transfer their acquired genes to an Enterococcus faecalis recipient, suggesting that these genes are associated with conjugative transposons. These isolates are the oldest of any species to carry the mef(A) gene and among the oldest to carry these erm genes.

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