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Journal of Bacteriology, April 2006, p. 2726-2729, Vol. 188, No. 7
0021-9193/06/$08.00+0 doi:10.1128/JB.188.7.2726-2729.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
ß-Lactam Antibiotics Induce the SOS Response and Horizontal Transfer of Virulence Factors in Staphylococcus aureus
Elisa Maiques,1
Carles Úbeda,1
Susana Campoy,2
Noelia Salvador,3
Íñigo Lasa,4
Richard P. Novick,5
Jordi Barbé,3 and
José R. Penadés1,6*
Departamento de Química, Bioquímica y Biología Molecular, Universidad Cardenal Herrera-CEU, 46113 Moncada, Valencia,1
Centre de Recerca en Sanitat Animal (CReSA),2
Departament de Genètica i Microbiologia, Universitat Autónoma de Barcelona, 08193 Bellaterra,3
Instituto de Agrobiotecnología y Recursos Naturales, CSICUniversidad Pública de Navarra, 31006 Pamplona, Navarra,4
Centro de Investigación y Tecnología Animal, Instituto Valenciano de Investigaciones Agrarias (CITA-IVIA), Apdo. 187, 12.400 Segorbe, Castellón, Spain,6
Skirball Institute, New York University Medical Center, 540 First Avenue, New York, New York 100165
Received 18 October 2005/
Accepted 17 January 2006

ABSTRACT
Antibiotics that interfere with DNA replication and cell viability
activate the SOS response. In
Staphylococcus aureus, the antibiotic-induced
SOS response promotes replication and high-frequency horizontal
transfer of pathogenicity island-encoded virulence factors.
Here we report that ß-lactams induce a bona fide SOS
response in
S. aureus, characterized by the activation of the
RecA and LexA proteins, the two master regulators of the SOS
response. Moreover, we show that ß-lactams are capable
of triggering staphylococcal prophage induction in
S. aureus lysogens. Consequently, and as previously described for SOS
induction by commonly used fluoroquinolone antibiotics, ß-lactam-mediated
phage induction also resulted in replication and high-frequency
transfer of the staphylococcal pathogenicity islands, showing
that such antibiotics may have the unintended consequence of
promoting the spread of bacterial virulence factors.

TEXT
Previous studies have revealed that certain genes of
Staphylococcus aureus, including those encoding proteins involved in cell wall
metabolism and stress responses, are upregulated after treatment
with ß-lactam antibiotics (
5,
15,
18), suggesting
the existence of a cell wall stimulon induced in response to
cell wall-active agents. In addition, it is well documented
that in
Escherichia coli, the SOS response is induced by antibiotics
that interfere with cell wall synthesis (
8,
12) as well as DNA
replication (
13). The SOS system represents a global response
to DNA damage that upregulates genes involved in DNA repair
and cell survival (
2,
4). The SOS response is governed by the
LexA and RecA proteins. The LexA protein binds to operator sites
of SOS-regulated genes, effectively repressing their expression.
Conversely, the presence of DNA lesions activates RecA, which
promotes the autocatalytic cleavage of LexA at a specific Ala-Gly
bond (
7). Cleaved LexA is unable to bind DNA, leading to the
derepression of SOS genes. Once DNA damage has been addressed,
newly synthesized RecA and LexA restore repression to the system.
Furthermore, the SOS response has been shown to induce the lateral
transfer of antibiotic resistance encoded by the
Vibrio cholerae integrating conjugative element SXT (
1), of pathogenicity island-encoded
virulence factors in staphylococci (
16), and of prophage-encoded
Shiga toxin in
E. coli (
19).
In this study we investigated the influence of subinhibitory concentrations of different antibiotics, including ß-lactams (ampicillin, penicillin, ceftriaxone, and cloxacillin), macrolide-lincosamide-streptogramin B antibiotics (erythromycin), aminoglycosides (kanamycin), chloramphenicol, and tetracycline, on the replication and transfer of superantigen-carrying staphylococcal pathogenicity islands (SaPIs). In S. aureus, several related pathogenicity islands have been described, including SaPI1 to SaPI4, SaPbov1, SaPIbov2, and SaPIn1 to SaPIn3 (for reviews see references 10 and 11). These elements most commonly encode TSST-1 plus two or more other superantigen toxins, with the exception of SaPIbov2, which encodes the biofilm-associated protein Bap (17). Phage-assisted replication, transduction, and site-specific integration in a recA mutant strain demonstrated the mobility of SaPI1 (6, 14) and SaPIbov1 (16). Additionally, we demonstrated that the fluoroquinolone-induced SOS response is fully effective for the mobilization of SaPIbov1 and SaPI1 and, by implication, for that of all other SOS-induced SaPIs (16). In view of these results and of the results of Cohen and coworkers (8), we analyzed the possibility that other antibiotics used in clinical practice could also induce the SOS response, resulting in the dissemination of virulence factors in staphylococci. The results of our study show that ß-lactams induce SaPI replication and transfer in an SOS-dependent manner and imply that they, as well as other SOS-inducing antibiotics, could thus increase microbial strain diversification and promote the spread of temperate phages and phage-inducible pathogenicity islands, resulting in the dissemination of superantigens and other virulence factors.
In the experiments presented here, we employed S. aureus strains RN27 and RN451, carrying SOS-inducible prophages 80
and
11, respectively (9). Induction of the SOS response by any of the antibiotics analyzed would be expected to induce phage replication in these strains. Bacteria grown in Trypticase soy broth to an optical density at 540 nm of 0.4 were tested for prophage induction by the addition of subinhibitory concentrations (ranging from 0.05 µg/ml to 10 µg/ml) of ampicillin, penicillin, erythromycin, chloramphenicol, tetracycline, or kanamycin. Cultures were grown at 32°C with slow shaking (80 rpm). After 16 h, phage titers were determined by plating suitable dilutions on RN4220. Phage replication was stimulated by exposure of bacteria to ampicillin and penicillin (Table 1), although the phage titers were lower than those with mitomycin C induction (Table 1). In contrast, none of the non-ß-lactam antibiotics tested induced phage replication (data not shown). Since the ß-lactam antibiotics (ampicillin and penicillin) utilized have low clinical relevance in the treatment of staphylococcal infections, we repeated the phage inductions with the ß-lactam antibiotics ceftriaxone and cloxacillin, which are extensively used in the treatment of staphylococcal infections. As shown in Table 1, exposure of bacteria to ceftriaxone and cloxacillin also increased the phage titers.
To determine whether ampicillin-mediated phage induction was
SOS dependent, we measured the phage titers after antibiotic
treatment of strain RN1030, a
recA-defective strain lysogenic
for

11 (
9). As is evident in Table
1, no induction was observed
in the presence of the antibiotic.
To resolve the basis of ampicillin-mediated SOS induction further, we investigated the effect of ampicillin on expression of lexA. Thus, reverse transciption-PCR analysis using internal oligonucleotides specific to the S. aureus lexA gene were carried out as previously reported (3). As shown in Fig. 1, our experiments showed that the presence of ampicillin produces an increase of approximately 19-fold in S. aureus lexA expression from strain RN450 (9), while expression of the gene in its derivative recA mutant strain RN981 (9) was not affected, indicating that this ß-lactam induces the SOS response.
This conclusion was additionally confirmed by mutational inactivation
of the LexA protein and testing for phage induction. The substitution
of glutamate for glycine at the Ala-Gly RecA cleavage site in
the LexA protein results in a noncleavable repressor that was
predicted to be less sensitive to SOS induction (
7). The noncleavability
mutation was introduced into
lexA in strains RN27 and RN451,
using oligonucleotides lexA-1mB (5'-CGCGGATCCGGCTGTTTGCTCCTTTGCTTCTTC-3'),
lexA-2c (5'-CTCAGCCATTAATGAATTCTATTGGTC-3'), lexA-3m (5'-GGTAAAGTCACAGCTGAGGTTCCTATTACCGC-3'),
and lexA-4c (5'-GCGGTAATAGGAACCTCAGCTGTGACTTTACC-3'), as previously
described (
16). The resulting strains, JP83 and JP84, respectively,
each encoded noncleavable LexA [LexA (G94E)]. Strain JP83 [RN27
LexA (G94E)] showed diminished phage titers upon ampicillin
treatment in comparison to those induced by ampicillin in strain
RN27, confirming the role of LexA in the ampicillin-mediated
SOS response. Surprisingly, although ampicillin induced

11,
similar phage titers were obtained upon ampicillin treatment
of strains JP84 [RN451 LexA (G94E)] and RN451 (Table
1), suggesting
the existence of a LexA-independent pathway in ampicillin-mediated
SOS induction of

11.
We have previously shown that mitomycin C induction of the SOS response in lysogenic SaPI1- and SaPIbov1-containing S. aureus induced replication and high-frequency transduction of the island (6, 14, 16). Similarly, growth of lysogenic SaPI1 or SaPIbov1 donor cells in ciprofloxacin, a widely used fluoroquinolone antibiotic that activates the SOS response (13), also increased SaPI1 and SaPIbov1 replication and transfer (16). To determine whether the ß-lactam-mediated SOS response could induce SaPIbov1, strains JP44 (RN27 SaPIbov1 tst::tetM [16]) and JP47 (RN451 SaPIbov1 tst::tetM [16]) were grown with ampicillin, penicillin, ceftriaxone, or cloxacillin (10 µg/ml); as shown in Table 2, all the ß-lactam antibiotics analyzed also increased SaPIbov1 replication and transfer.
To confirm that the stimulation of SaPIbov1 transfer by ampicillin
was a consequence of SOS induction, we induced JP50 (a
recA mutant
S. aureus strain, a derivative of RN1030, lysogenic for

11, that carries SaPIbov1
tst::
tetM [
16]) and JP85 (a derivative
of JP83 that carries SaPIbov1
tst::
tetM). As expected, neither
replication nor significant transfer was observed upon ampicillin
induction of strain JP50 (Table
2), while a significant reduction
in SaPIbov1 transfer was observed after ampicillin induction
of strain JP85. Additionally, the role of ampicillin in the
SOS-mediated transfer of SaPIbov1 was confirmed by analysis
of strain JP52, a SaPIbov1-positive strain, lysogenic for

11,
that carries a mutation eliminating the

11 phage repressor (cI)
cleavage site (
16). Ampicillin did not increase SaPIbov1 transfer
in this strain (Table
2), suggesting that cI cleavage mediates
the SOS enhancement of SaPIbov1 transfer.
Finally, we addressed the question of whether SaPI1, the prototypical S. aureus pathogenicity island, could also be excised, replicated, and transferred at a high frequency after antibiotic treatment. For that purpose, RN8667 (SaPI1 positive, lysogenic for
80
[6]) was grown with ampicillin, penicillin, ceftriaxone, or cloxacillin, as described above. As expected, the ß-lactam-activated SOS response increased SaPI1 replication and transfer (Table 2). Thus, activation of the SOS response by ß-lactam antibiotics greatly stimulates the transfer of S. aureus pathogenicity islands.
Recently, it has been reported that ß-lactam antibiotics induce the SOS response in E. coli through the DpiBA two-component signal transduction system (8). This event, which requires the SOS-promoted cleavage of RecA and LexA as well as dpiA, transiently halts bacterial cell division, enabling the organisms to survive otherwise lethal antibiotic exposure. In addition, it has been reported that transcription of the SOS-regulated dinB gene, encoding DNA polymerase IV, is induced by inhibition of cell wall synthesis by ß-lactam antibiotics (12). Whether corresponding genes are involved in SOS induction by ß-lactams in S. aureus remains to be determined. Nevertheless, our findings indicate that ß-lactam antibiotics are extracellular stimuli of the SOS response in S. aureus as well as in E. coli and demonstrate a novel mechanism for horizontal dissemination of staphylococcal virulence factors. Thus, our results, even though anticipated, reinforce the need for great caution in the use of SOS response-inducing antibiotics. Such antibiotics not only promote the dissemination of antibiotic resistance genes and the production of toxins regulated by repressors sensitive to RecA cleavage but also promote the induction of prophages and SaPIs, staphylococcal elements that frequently encode virulence factors.

ACKNOWLEDGMENTS
This work was supported by grants BIO2002-04542-C02-01 and BIO2005-08399-C02-02
from the Comisión Interministerial de Ciencia y Tecnología
(C.I.C.Y.T.), grants from the Cardenal Herrera-CEU University,
the Conselleria de Agricultura, Pesca i Alimentació (CAPiA),
and the Generalitat Valenciana (CTIDIA/2002/62, CTESPP/2003/027,
and AE04-8) to J.R.P., and a grant from the MEC (AGL2005-03574/GAN)
to J.B. Fellowship support for C.U. from CAPiA and for E.M.
from the Cardenal Herrera-CEU University is gratefully acknowledged.

FOOTNOTES
* Corresponding author. Mailing address: Centro de Investigación y Tecnología Animal, Instituto Valenciano de Investigaciones Agrarias (CITA-IVIA), Apdo. 187, 12.400 Segorbe, Castellón, Spain. Phone: 34 964 71 21 15. Fax: 34 964 71 02 18. E-mail:
jpenades{at}ivia.es.


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Journal of Bacteriology, April 2006, p. 2726-2729, Vol. 188, No. 7
0021-9193/06/$08.00+0 doi:10.1128/JB.188.7.2726-2729.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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