Twenty-five distinct tetracycline-resistant gram-negative bacteria recovered from four Chilean fish farms with no history of recent antibiotic use were examined for the presence of tetracycline resistance (tet) genes. Sixty percent of the isolates carried 1 of the 22 known tet genes examined. The distribution was as follows. The tet(A) gene was found in six isolates. The tet(B) gene was found in two isolates, including the first description in the genus Brevundimonas. Two isolates carried the tet(34) and tet(B) genes, including the first description of the tet(34) gene in Pseudomonas and Serratia and the first description of the tet(B) gene in Pseudomonas. The tet(H) gene was found in two isolates, which includes the first description in the genera Moraxella and Acinetobacter. One isolate carried tet(E), and one isolate carried tet(35), the first description of the gene in the genus Stenotrophomonas. Finally, one isolate carried tet(L), found for the first time in the genus Morganella. By DNA sequence analysis, the two tet(H) genes were indistinguishable from the previously sequenced tet(H) gene from Tn5706 found in Pasteurella multocida. The Acinetobacter radioresistens isolate also harbored the Tn5706-associated 1,063-bp IS element IS1597, while the Moraxella isolate carried a 1,026-bp IS-like element whose 293-amino-acid transposase protein exhibited 69% identity and 84% similarity to the transposase protein of IS1597, suggesting the presence of a novel IS element. The distribution of tet genes from the Chilean freshwater ponds was different than those that have previously been described from other geographical locations, with 40% of the isolates carrying unidentified tetracycline resistance genes.Intensive fish farming is done in Chile, which is the secondlargest producer of farm-raised salmon in the world (25). Oxytetracycline is the most frequently used antimicrobial agent in the Chilean salmon industry, which has resulted in increased tetracycline resistance (Tc r ) in gram-negative bacteria associated with all aspects of fish farming, from the water entering and leaving the ponds to the fish food pellets themselves. Previous studies of Tc r Acinetobacter spp., Aeromonas hydrophila, Edwardsiella tarda, Pasteurella piscicida (which has been reclassified as Photobacterium damselae subsp. piscicida) (7), Vibrio anguillarum, and Vibrio salmonicida from fish farms have been characterized in other geographical areas (2,8,23). A few studies have characterized nonpathogenic bacteria isolated from catfish ponds (5) or from polluted and unpolluted marine sediments (1). In these previous studies, 66 to 94% of the total isolates carried one of five known Tc r genes: tet(A), tet(B), tet(C), tet(D), and tet(E).A recent report describes the isolation of Tc r bacteria from fish farm influents, salmon culture tanks, farm effluents, surface water, salmon, and unmedicated fish food pellets (15). From that study, 25 Tc r isolates were selected for characterization of the tetracycline resistance genes by using oligonucleotide...

We screened 615 gram-positive isolates from 150 healthy children for the presence of the erm(A), erm(B), erm(C), erm(F), and mef(A) genes. The mef(A) genes were found in 20 (9%) of the macrolide-resistant isolates, including Enterococcus spp., Staphylococcus spp., and Streptococcus spp. Sixteen of the 19 gram-positive isolates tested carried the other seven open reading frames (ORFs) described in Tn1207.1, a genetic element carrying mef(A) recently described in Streptococcus pneumoniae. The three Staphylococcus spp. did not carry orf1 to orf3. A gram-negative Acinetobacter junii isolate also carried the other seven ORFs described in Tn1207.1. A Staphylococcus aureus isolate, a Streptococcus intermedius isolate, a Streptococcus sp. isolate, and an Enterococcus sp. isolate had their mef(A) genes completely sequenced and showed 100% identity at the DNA and amino acid levels with the mef(A) gene from S. pneumoniae.The normal flora is thought to act as a reservoir for many bacterial antimicrobial resistance genes, including those that confer macrolide resistance (12). In 1999, there were 20 different rRNA methylases described in the literature, which coded for macrolide-lincosamide-streptogramin B resistance, and 24 efflux and inactivating genes, which coded for one or more of the macrolide-lincosamide-streptogramin B complex of antimicrobials (14). However, relatively few of these 44 genes are found in the majority of macrolide-resistant grampositive bacteria (1, 2, 13). Resistance to macrolides in the absence of resistance to lincosamides and streptogramin B has been associated with the presence of the mef(A) gene in Streptococcus pneumoniae (17,18). The mef(A) gene has become more common than erm(B) in macrolide-resistant S. pneumoniae isolates from North America (7,15). We have shown that the mef(A) gene is present in macrolide-resistant oral Streptococcus spp. and Enterococcus spp. isolated in Seattle, Wash., and Micrococcus luteus and Corynebacterium spp. isolated in the United Kingdom (5), as well as in gram-negative Acinetobacter junii and Neisseria gonorrhoeae (6). All of these species have been able to conjugally transfer the mef(A) genes to a variety of recipients. Recently, two genetic elements, Tn1207.1 (16) and mega (3), have been characterized from macrolide-resistant S. pneumoniae. A highly related gene has been sequenced from Streptococcus pyogenes, while related genes have been identified in Lancefield group C and G streptococci from Finland (4).In this study, we examined randomly selected gram-positive isolates collected from healthy Portuguese children for the presence of the common macrolide resistance genes, erm(A), erm(B), erm(C), erm(F), and mef(A). Representative mef(A)genes were sequenced, and the presence of the other seven open reading frames (ORFs) from Tn1207.1 was investigated.(The data in Table 2 were presented in part at the First Annual Symposium on Resistant Gram-Positive Infections in San Antonio, Tex., 3 to 5 Dec. 2000.) MATERIALS AND METHODSBacterial isolates. A total of 615 rand...

Of the 176 randomly selected, commensal, gram-negative bacteria isolated from healthy children with low exposure to antibiotics, 138 (78%) carried one or more of the seven macrolide resistance genes tested in this study. These isolates included 79 (91%) isolates from the oral cavity and 59 (66%) isolates from urine samples. The mef(A) gene, coding for an efflux protein, was found in 73 isolates (41%) and was the most frequently carried gene. The mef(A) gene could be transferred from the donors into a gram-positive E. faecalis recipient and a gram-negative Escherichia coli recipient. The erm(B) gene transferred and was maintained in the E. coli transconjugants but was found in 0 to 100% of the E. faecalis transconjugants tested, while the other five genes could be transferred only into the E. coli recipient. The individual macrolide resistance genes were identified in 3 to 12 new genera. Eight (10%) of the oral isolates and 30 (34%) of the urine isolates for which the MICs were 2 to >500 g of erythromycin per ml did not hybridize with any of the seven genes and may carry novel macrolide resistance genes.The use of macrolide and related antibiotics (ketolides, oxazolidinones, streptogramins, and lincosamide) has increased dramatically over the last 15 years. A number of different mechanisms of macrolide resistance have been reported for gram-negative bacteria. These mechanisms include two esterase genes [ere(A) and ere(B)] found in Escherichia coli and Enterobacter, Klebsiella, Citrobacter, and Proteus species (1) and more recently, in Providencia stuartii, Pseudomonas species, and Vibrio cholerae (5,17,22). These mechanisms also include three phosphorylase genes [mph(A), mph(B), and mph(D)] found in E. coli (14,15) and Pseudomonas (12) and one rRNA methylase gene [erm(B)] previously found in E. coli and Actinobacillus, Klebsiella, Neisseria, and Wolinella species (1, 4, 18-21). The strains described above were principally clinical isolates from hospital settings and/or from patients with clinical disease (17,18,20,21), with members of the family Enterobacteriaceae and Pseudomonas species primarily isolated from France and Japan

The staphylococcal msr(A) gene, coding for a macrolide efflux protein, was identified in three new grampositive genera and one gram-negative genus. These msr(A) genes shared 99 to 100% identity with each other and the staphylococcal gene. This study demonstrates that the msr(A) gene has a wider host range than previously reported.