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Antimicrobial Agents and Chemotherapy, July 2005, p. 3046-3049, Vol. 49, No. 7
0066-4804/05/$08.00+0 doi:10.1128/AAC.49.7.3046-3049.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Novel Spectinomycin/Streptomycin Resistance Gene, aadA14, from Pasteurella multocida
Corinna Kehrenberg,1
Boudewijn Catry,2
Freddy Haesebrouck,3
Aart de Kruif,2 and
Stefan Schwarz1*
Institut für Tierzucht, Bundesforschungsanstalt für Landwirtschaft (FAL), Höltystr. 10, 31535 Neustadt-Mariensee, Germany,1
Department of Obstetrics, Reproduction and Herd Health,2
Department of Pathology, Bacteriology and Poultry Diseases, Faculty of Veterinary Medicine, Ghent University, B-9820 Merelbeke, Belgium3
Received 28 February 2005/
Returned for modification 3 April 2005/
Accepted 21 April 2005

ABSTRACT
A novel spectinomycin/streptomycin resistance gene, designated
aadA14, was detected on the mobilizable 5,198-bp plasmid pCCK647
from
Pasteurella multocida. The
aadA14 gene encodes an aminoglycoside
adenylyltransferase of 261 amino acids. Sequence comparisons
revealed that the AadA14 protein showed less than 60% identity
to the AadA proteins known so far.

TEXT
Spectinomycin is an aminocyclitol antibiotic which inhibits
bacterial protein biosynthesis by reversibly binding to the
30S ribosomal subunit. Resistance to spectinomycin is commonly
due to enzymes which inactivate the drug by adenylylation. At
least two major groups of adenylyltransferases (AAD)also
known as nucleotidyltransferases (ANT)involved in spectinomycin
resistance can be differentiated. One group consists of enzymes
[referred to as AAD(3")(9) or ANT(3")(9)] which adenylylate
spectinomycin at the 9-OH position of the spectinomycin actinamine
ring but also adenylylate the aminoglycoside antibiotic streptomycin
at the 3"-OH position of the streptomycin glucosamine ring and
thereby mediate combined resistance to spectinomycin and streptomycin
(
39). Such enzymes, of which a considerable number of variants
have been described, are known to occur in a wide variety of
gram-negative bacteria and occasionally also in gram-positive
bacteria, such as
Enterococcus faecalis (
4). The corresponding
genes, which are commonly referred to as
aadA or
ant(
3")
-I,
have been detected on plasmids and in the chromosomal DNA, with
many of them being located on gene cassettes in class 1 integrons
(
1,
5,
15,
17,
20,
22,
23,
25,
27-
30,
32,
35,
36,
40). A second
group of adenylylating enzymes, including those encoded by the
genes
spc from transposon Tn
554 (
19) and
aad9 from the
E. faecalis plasmid pDL55 (
14), exhibits only AAD(9) [or ANT(9)] activity
and hence confers only resistance to spectinomycin.
In veterinary medicine, spectinomycin is commonly used to control bovine respiratory tract infections due to Pasteurella multocida, Mannheimia haemolytica, or Histophilus somni. Although P. multocida and M. haemolytica isolates which exhibit high-level resistance to spectinomycin, with MICs of
256 µg/ml, have recently been reported from Germany, attempts to identify aadA, spc, or aad9 genes in these isolates failed, as did experimental approaches to horizontally transfer the potential spectinomycin resistance genes (31). In the present study, we identified a first aadA gene on a small plasmid from a bovine P. multocida isolate from Belgium.
The ca. 5.2-kb plasmid pCCK647 was identified in a previously reported P. multocida capsular type F strain which was obtained from a case of fatal peritonitis in calves (3). The plasmid was transferred by electrotransformation into the recipient strains P. multocida P4000 (18) and Escherichia coli JM109 (Stratagene, Amsterdam, The Netherlands), where it mediated resistance to spectinomycin (MIC
512 µg/ml) and streptomycin (MIC = 256 µg/ml). Since PCR detection for the known spectinomycin/streptomycin or spectinomycin resistance genes (31) yielded negative results, it was assumed that plasmid pCCK647 harbored a so-far-undescribed type of spectinomycin/streptomycin resistance gene. To identify the resistance gene located on this plasmid, pCCK647 was subjected to restriction mapping (Fig. 1), and ClaI-EcoRI fragments of ca. 0.8 and 4.4 kb were cloned into pBluescript II SK+ (Stratagene). Both fragments were sequenced completely on both strands by primer walking starting with the M13 forward and reverse primers (MWG, Ebersberg, Germany).
Sequence analysis identified five open reading frames, with
one reading frame exhibiting similarity to a plasmid replication
gene, three reading frames resembling plasmid mobilization genes,
and the remaining reading frame coding for an adenylyltransferase
(Fig.
1). The putative
rep gene of plasmid pCCK647 coded for
a protein of 108 amino acids which showed 57% identity to a
61-amino-acid segment of the 94-amino-acid replication protein
RepB from
Rhodococcus erythropolis (accession no.
AAG29855).
A 2,680-bp region of pCCK647 comprising the three reading frames
for mobilization proteins showed 86.6% and 86.4% similarity
to the corresponding regions of the recently described tetracycline
resistance plasmid pHS-Tet from
Haemophilus parasuis (
13) and
the ß-lactamase-encoding plasmid pAB2 from
Mannheimia haemolytica (
37), respectively (Fig.
1). The smallest of the
three reading frames, coding for a 102-amino-acid MobC protein,
overlapped the
mobA reading frame by 3 bp. MobC from pCCK647
exhibited 88% identity to the 101-amino-acid MobC proteins from
plasmids pHS-Tet and pAB2. The 160-amino-acid MobB protein showed
89% identity to the 160-amino-acid MobB protein from pHS-Tet
and 91% identity to the N-terminal 84 amino acids of the 90-amino-acid
MobB protein from pAB2. The largest reading frame in pCCK647
coded for the 474-amino-acid MobA protein. This protein exhibited
79% identity to the 468-amino-acid MobA protein from pHS-Tet
and 86% identity to the N-terminal 313 amino acids of the 376-amino-acid
MobA protein from pAB2. Since the
mob genes of pCCK647 differed
from the ones previously described, mobilization of plasmid
pCCK647 was experimentally confirmed. The conjugative
tet(A)-carrying
tetracycline resistance plasmid pEC1591 originally isolated
from
E. coli and obtained from the strain collection of our
institute was chosen to provide the transfer apparatus for the
mobilization of plasmid pCCK647. For this, plasmid pCCK647 was
first transformed into
E. coli JM109 cells which carried the
conjugative plasmid pEC1591. Conjugation experiments into the
rifampin-resistant
E. coli strain HK225 (
21) by filter mating
followed a previously described protocol (
8). Transconjugants
were selected on triple-selective Luria-Bertani agar plates
supplemented with rifampin (100 µg/ml), tetracycline (15
µg/ml), and spectinomycin (50 µg/ml). Plasmid analysis
and determination of the resistance phenotype of the transconjugants
confirmed that the transconjugants carried both plasmids, pEC1591
and pCCK647, and were resistant to rifampin, tetracycline, streptomycin,
and spectinomycin. This observation suggests that the mobilization
system of plasmid pCCK647 is functionally active.
The fifth reading frame in pCCK647 coded for a (3")(9) adenylyltransferase of 261 amino acids, designated AadA14. Comparisons with other AadA proteins on the basis of a multisequence alignment revealed an overall low degree of 51.4% to 56.5% identity to the currently known AadA proteins, with the best matches to the AadA23 protein from Salmonella enterica serovar Agona (17) and its close relative Aad23b from E. coli (accession no. BAD38865). The corresponding homology tree shown in Fig. 2 confirms that AadA14 is only distantly related to the other AadA proteins and clusters with them at 57% identity. In the sequences flanking the aadA14 gene, neither relics of integron sequences nor sequences resembling a 59-base element or parts of the 3' conserved segments of class 1 or class 2 integrons (27) were detectable. Thus, it is unlikely that the aadA14 gene is a cassette-borne aadA gene.
To determine whether the gene
aadA14 also occurs in other epidemiologically
unrelated high-level spectinomycin/streptomycin-resistant
Pasteurella and
Mannheimia isolates, an
aadA14-specific PCR assay was developed.
The primers
aadA14-fw (5'-TCACTTGTTTGGTTCCGCAGT-3') and
aadA14-rev
(5'-TCTTTCGGATAAGCTGCCAGA-3') (annealing temperature, 60°C)
were used to amplify an internal 642-bp fragment of the
aadA14 gene. Moreover, this amplicon was cloned into pCR-Blunt II Topo
(Invitrogen, Groningen, The Netherlands), cut off from the vector
by EcoRI digestion, labeled with the Dig-High Prime DNA labeling
and detection starter kit I (Boehringer, Mannheim, Germany),
and used as a gene probe for Southern blot hybridization of
HindIII-digested whole cell DNA (
10,
11). Three
P. multocida and two
M. haemolytica isolates from Germany (
31), all exhibiting
MICs of spectinomycin of

256 µg/ml and MICs of streptomycin
of

128 µg/ml, were investigated for the presence of the
gene
aadA14. Another 11 bovine
P. multocida isolates which exhibited
only spectinomycin resistance, 7 from Germany and 4 from Belgium,
were also included. However, negative results were obtained
with both methods for all 16 isolates tested. The PCR-based
observation that the four Belgian isolates also did not carry
so-far-known
aadA genes or the genes
spc and
aad9 is in agreement
with previously published findings on the German isolates (
31).
Attempts to detect the spectinomycin adenylyltransferase gene
aadA from
Legionella longbeachae (accession no.
AF288536) and
the aminocyclitol/aminoglycoside phosphotransferase gene
aph(9)-
Ia from
Legionella pneumophila (accession no.
U94857) also yielded
negative results for all 16 isolates. These results strongly
suggest that so-far-undescribed genes are responsible for spectinomycin
and spectinomycin/streptomycin resistance in
Pasteurella and
Mannheimia organisms. Moreover, the results of this study and
another recently published study (
12) show that
Pasteurella isolates carry certain resistance genes that are distantly related
to genes from other bacteria which mediate the same resistance
phenotype.
Nucleotide sequence accession number.
The sequence of the 5,198-bp plasmid pCCK641 has been deposited in the EMBL database under accession number AJ884726.

ACKNOWLEDGMENTS
We thank Vera Nöding and Roswitha Becker for excellent
technical assistance.

FOOTNOTES
* Corresponding author. Mailing address: Institut für Tierzucht, Bundesforschungsanstalt für Landwirtschaft (FAL), Höltystr. 10, 31535 Neustadt-Mariensee, Germany. Phone: 49-5034-871-241. Fax: 49-5034-871-246. E-mail:
stefan.schwarz{at}fal.de.


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Antimicrobial Agents and Chemotherapy, July 2005, p. 3046-3049, Vol. 49, No. 7
0066-4804/05/$08.00+0 doi:10.1128/AAC.49.7.3046-3049.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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