Gilda de Cespédès

Nucleotide sequence of the chloramphenicol resistance determinant of the streptococcal plasmid pIP501.

We have sequenced the chloramphenicol resistance determinant (cat) of plasmid pIP501 from Streptococcus agalactiae to investigate its relationship with other cognate cat determinants. Sequence analysis revealed that it exhibits a high degree of similarity with the cat genes of plasmids pC221 and pUB112 from Staphylococcus aureus and pSCS1 from Staphylococcus intermedius. These genes, however, display several differences in their regulatory and coding regions. These results demonstrate that the cat determinant of plasmid pIP501 belongs to the pC221 subgroup of CAT variants.

Dispersal of a plasmid-borne chloramphenicol resistance gene in streptococcal and enterococcal plasmids

Plasmids coding for chloramphenicol resistance, five isolated from streptococci of groups A, B, and G, ten from enterococci (Enterococcus faecalis, Enterococcus faecium), and two from staphylococci, were tested for sequence homology with the chloramphenicol resistance gene of pIP501, a 30-kb plasmid originally isolated from a group B Streptococcus. The 6.3-kb HindIII fragment of pIP501, known to carry the chloramphenicol resistance gene, was cloned into pBR322. A 1.6-kb portion of the cloned fragment, which included most of the chloramphenicol resistance gene, was used as probe in DNA-DNA hybridization experiments. Sequence homology was detected between the probe and four of the streptococcal, seven of the enterococcal, and one of the staphylococcal plasmids. The absence of hybridization between this probe and one plasmid isolated from a group B Streptococcus, as well as three isolated from E. faecalis, indicated that there are at least two different plasmid-borne chloramphenicol resistance determinants in the streptococci and in the enterococci.

Tn3702, a conjugative transposon in Enterococcus faecalis

Enterococcus faecalis strain D434 was found to carry on its chromosome a determinant encoding tetracycline-minocycline resistance (Tcr-Mnr) and to harbor both an R plasmid and a cryptic conjugative plasmid, pIP1141. The determinant coding for Tcr-Mnr was located on a conjugative transposon, designated Tn3702. The transposition of Tn3702 on to both pIP1141 and the hemolysin plasmid pIP964 yielded different derivatives each of which contained an 18.5-kilobase insert. The structure of Tn3702 is similar to that of the conjugative transposon Tn916.

Heterogeneity of chromosomal genes encoding chloramphenicol resistance in Streptococci

The DNA of 21 chloramphenicol-resistant plasmid-free streptococci was tested for sequence homology with the genes encoding chloramphenicol acetyltransferase (cat) of the staphylococcal plasmids pC194 and pC221. Homology to the cat gene of pC194 was detected in 11 strains, including the 8 strains of Streptococcus pneumoniae examined, and homology to cat of pC221 was found in 3 strains. The DNA of 7 strains did not detectably hybridize with either probe.

Genetic Analysis of Enterococcus faecalis Chromosome Carrying Mobile Elements

Chromosome-borne antibiotic resistance, first described by us in 198111 in group A, B, and G streptococci, appears to be a common trait shared by different species of antibiotic- resistant streptococcal strains. A number of our previous investigations have demonstrated that in 90% of streptococci of group A, B, C, and G8 and 100% of Streptococcus bovis 7, Streptococcus pneumoniae 7, Streptococcus anginosus 3, Streptococcus mitis 1, and Streptococcus oralis 1 strains studied, antibiotic resistance determinants (chloramphenicol, macrolides and related drugs, tetracycline-minocycline, and high levels of aminoglycosides including streptomycin, kanamycin-neomycin, and gentamicin-kanamycin) are located on the chromosome. In most cases, chromosomal antibiotic resistance is mediated by mobile genetic elements10 such as Tn916-like structures6 or Tn3701-like composite structures12. More recently, we have identified two other categories of chromosomal elements, designated Tn916-modified1 which have sequences homologous to the conjugative transposon Tn916 6 but HincII stuctures different from that of Tn916: (i) in 14 (9.3%) of 151 streptococcal strains studied, the HincII profile varies from a strain to another but, apparently, these elements may transpose as Tn916 since they share DNA-DNA homology with the integrase gene and the two extremities of Tn916 5; (ii) four (7.0%) of 58 enterococcal strains studied possess similar if not identical HincII structures among themselves. The enterococcal Tn916-modified elements do not display DNA-DNA homology with the integrase gene or the extremities of Tn916. We suggest that the enterococcal Tn916-modified structures, unlike those detected in streptococci, belong to a new family of mobile genetic elements; the prototype of these elements is Tn3708 5.

Genetic and Molecular Characterization of High-Level Gentamicin Resistance in Enterococcus faecalis Strains Isolated in Romania

Antibiotic resistance in enterococci is becoming a major clinical problem. Enterococcus faecalis has emerged as a pathogen associated with severe nosocomial infections. The treatment of choice for such infections is usually the synergistic combination of a penicillin or a glycopeptide with an aminoglycoside, most commonly gentamicin or streptomycin. However, the efficacy of such combinations has been compromised by the emergence of enterococcal strains displaying multiple antibiotic resistance, including resistance to penicillins, to high levels of aminoglycosides, and to glycopeptides.