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Antimicrobial Agents and Chemotherapy, November 2002, p. 3690-3691, Vol. 46, No. 11
0066-4804/02/$04.00+0 DOI: 10.1128/AAC.46.11.3690-3691.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
The Macrolide Efflux Genetic Assembly of Streptococcus pneumoniae Is Present in Erythromycin-Resistant Streptococcus salivarius

LETTER
Macrolide resistance in pneumococci and oral streptococci is
a common phenomenon (
8). The macrolide resistance determinants
erm(B) and
mef(A) are widespread (
4,
7,
9). We characterized
the erythromycin resistance determinant of a
Streptococcus salivarius strain isolated from a healthy person with no exposure to antibiotics
6 months prior to isolation. It was identified as
mef(A) conferring
an erythromycin MIC of 6 µg/ml (
6).
mef(A) was part of
a macrolide efflux genetic assembly (mega) recently identified
in
Streptococcus pneumoniae (GenBank accession no.
AF274302 [
3]) in the United States. The nucleotide sequence of the mega
from
S. salivarius Sp6 (GenBank accession no.
AJ318993) was
99.8% identical to that of the pneumococcal element (Fig.
1A).
Deletions resulted in a 5,511-bp element in
S. salivarius compared
to 5,532 bp in
S. pneumoniae. orf1 is identical to
mef(A) from
S. pneumoniae (U83667 [
11]);
orf2 encodes an ABC transporter
identical to
mel in the mega of
S. pneumoniae. The open reading
frames
orf1 to -
5 of the
S. salivarius mega are homologous to
orf4 to -
8 from Tn
1207.1, a
mef(A)-carrying defective transposon
of
S. pneumoniae (AF227520 [
10]).
orf6 is not present in Tn
1207.1.
Upstream of
mef(A) of the
S. salivarius mega is a 16-bp gap
that eliminates a direct repeat in the mega of
S. pneumoniae (AF274302). This gap is also present in Tn
1207.1 and in megas
in Italian pneumococcal strains (
2). Contrary to Orf4 in the
mega of
S. pneumoniae, Orf4 in the
S. salivarius sequence is
19 amino acids larger due to a single base pair deletion but
is the same size as Orf7 in Tn
1207.1. Another deletion in Orf5
of the mega in
S. salivarius causes a shortened open reading
frame.
The mega in
S. salivarius was located on the chromosome. Downstream
of the
S. salivarius mega, three open reading frames with homologies
to a regulator protein (orfb), an ABC transporter (orfc), and
a transmembrane protein (orfd) were found.
The mega from S. salivarius Sp6 was transformed in two independent experiments with low but significant frequency (two transformants per 107 recipient cells and 100 ng of S. salivarius DNA) into erythromycin-susceptible S. pneumoniae strain R800 by in vitro transformation using accepted protocols and control DNA (1, 5). No spontaneous mutation of the recipient cells was obtained. The MIC for the transformants was comparable to that for the donor strains. SmaI-digested genomic DNA of the recipient strain and of transformants of S. pneumoniae R800 was analyzed by pulsed-field gel electrophoresis. The mega integrated into a 291-kbp fragment to yield a 388-kbp fragment detected by Southern hybridization (Fig. 1B).
The mega was detected by PCR with specific primers for nucleotides 1 to 2723 and 2291 to 5510 in oral streptococci from six healthy persons, including S. salivarius (10 strains), Streptococcus mitis (six strains), Streptococcus parasanguis (five strains), and Streptococcus cristatus (three strains).
This is the first report of the pneumococcal mega in S. salivarius and of a successful transformation of S. pneumoniae with S. salivarius DNA. The mechanism of worldwide dissemination of the mega in oral streptococci remains to be established.

REFERENCES
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Christina Stadler Michael Teuber*
Laboratory of Food Microbiology ETH Zurich CH-8092 Zurich, Switzerland
|
| | | | | |
* Phone: 41-1-632-33-33 Fax: 41-1-632-12-66 E-mail: teuber{at}ilw.agrl.ethz.ch. |
Antimicrobial Agents and Chemotherapy, November 2002, p. 3690-3691, Vol. 46, No. 11
0066-4804/02/$04.00+0 DOI: 10.1128/AAC.46.11.3690-3691.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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