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Journal of Bacteriology, August 2006, p. 5650-5653, Vol. 188, No. 15
0021-9193/06/$08.00+0     doi:10.1128/JB.00323-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Arginine-Dependent Acid Resistance in Salmonella enterica Serovar Typhimurium

Jasper Kieboom1,2,{dagger} and Tjakko Abee1*

Laboratory of Food Microbiology, Wageningen University Agrotechnology and Food Sciences Group, P.O. Box 8129, 6700 EV Wageningen, The Netherlands,1 RIKILT Institute of Food Safety, P.O. Box 230, 6700 AE Wageningen, The Netherlands2

Received 6 March 2006/ Accepted 15 May 2006


   ABSTRACT
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Salmonella enterica serovar Typhimurium does not survive a pH 2.5 acid challenge under conditions similar to those used for Escherichia coli (J. W. Foster, Nat. Rev. Microbiol. 2:898-907, 2004). Here, we provide evidence that S. enterica serovar Typhimurium can display arginine-dependent acid resistance (AR) provided the cells are grown under anoxic conditions and not under the microaerobic conditions used for assessment of AR in E. coli. The role of the arginine decarboxylase pathway in Salmonella AR was shown by the loss of AR in mutants lacking adiA, which encodes arginine decarboxylase; adiC, which encodes the arginine-agmatine antiporter; or adiY, which encodes an AraC-like regulator. Transcription of adiA and adiC was found to be dependent on AdiY, anaerobiosis, and acidic pH.


   TEXT
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Acid resistance (AR) in the Enterobacteriaceae requires the presence of amino acids in the challenge medium; while Escherichia coli and Shigella flexneri are classified as AR, it has repeatedly been found that S. enterica serovar Typhimurium does not survive a pH 2.5 acid challenge under conditions similar to those used for E. coli (8, 9, 12, 15). The AR mechanisms in E. coli have been demonstrated by several researchers; they enable this organism to survive at pHs as low as 2.5 (15, 16, 20). The first AR system in E. coli (AR 1) is apparent when cells are grown to stationary phase in rich medium and subsequently challenged in EG mineral medium at pH 2.5 (4). The alternative sigma factor RpoS and the global regulatory protein cyclic AMP receptor protein regulate AR 1. Repression of AR 1 by glucose permitted the discovery of the amino acid-dependent AR systems in E. coli (15). Cells that have been grown in rich medium, pH 5.5, to stationary phase in the presence of glucose survive dilution in mineral medium, pH 2.5, only in the presence of glutamate (AR 2), arginine (AR 3), and to a lesser extent lysine (AR 4) (4, 13). AR 2 is the most extensively studied mechanism of the three amino acid decarboxylase systems in E. coli (5, 9), but it seems to be absent from S. enterica serovar Typhimurium (15). The genes for the AR 3 system are present in E. coli and S. enterica serovar Typhimurium but seem to contribute to AR only in E. coli. In E. coli, the arginine decarboxylase (adiA) and the arginine-agmatine antiporter (adiC) are responsible for acid resistance (11). CysB is a positive regulator of adi transcription, and, when overexpressed, the AraC-like regulator AdiY is also involved in regulation of adiA (19, 22). The AR 4 system is not present in E. coli, but in S. enterica serovar Typhimurium the lysine decarboxylase (cadA) and the lysine-cadaverine antiporter (cadB) work in concert to protect this organism at pHs as low as 3.0 (18).

It has generally been recognized that S. enterica serovar Typhimurium is not able to survive pH values as low as 2.5 (9, 12, 15). Recently, however, de Jonge et al. (7) showed that S. enterica serovar Typhimurium was able to survive in mineral medium at pH 2.5 for a prolonged period of time, and this triggered us to investigate the role of L-arginine in AR 3 in S. enterica serovar Typhimurium in more detail.

Arginine-dependent AR in S. enterica serovar Typhimurium. AR 3 in stationary-phase S. enterica serovar Typhimurium cells grown in LBG was determined as described previously (3). In short, 10 µl of stationary-phase culture was transferred to 10 ml of fresh EG mineral medium at pH 2.5. EG mineral medium consisted of Vogel-Bonner mineral medium E (24), 100 mM MES (morpholineethanesulfonic acid), 10 mM L-arginine, and 4 g/liter of glucose at pH 2.5. Growth of the cells in a 10-ml filled screw-cap culture tube without shaking created anoxic conditions. Without the addition of L-arginine, the number of surviving bacteria decreased by more than 4 log after 1 h at pH 2.5. The amounts of S. enterica serovar Typhimurium DT104 strain BAA-188 (2) that survived in challenge medium supplemented with L-arginine were approximately 59.4% and 12.5% after 1 and 2 h, respectively, at pH 2.5 (Fig. 1). Similar results were obtained for the other serovar DT104 strains. The survival rates of S. enterica serovar Typhimurium LT2 (10) and UK1 (6) at pH 2.5 were slightly higher, at approximately 83% and 38% after 1 and 2 h, respectively.


Figure 1
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  		FIG. 1. Arginine-dependent AR of stationary-phase S. enterica serovar Typhimurium cells. Stationary-phase cells grown in LBG at pH 5 were challenged in EG mineral medium, pH 2.5, at an initial concentration of approximately 5 x 105 CFU/ml. Percent survival is depicted as follows: open bars, survival after 1 h without L-arginine; solid bars, survival with L-arginine after 1 h; gray bars, survival with L-arginine after 2 h. All experiments were performed twice in triplicate. The detection limit of the experiment was 5 x 102 CFU/ml.

 
The results presented describe for the first time arginine-dependent stationary-phase AR in S. enterica serovar Typhimurium under conditions similar to those used for E. coli. All seven S. enterica serovar Typhimurium strains tested displayed this phenotype, suggesting a common property of S. enterica serovar Typhimurium strains. Omission of L-arginine from the challenge medium resulted in an acid-sensitive phenotype in all S. enterica serovar Typhimurium strains tested. Our results show that the LT2 strain displays AR similar to those of the S. enterica serovar Typhimurium DT104 strains, which were shown in a previous study by Berk et al. (3) to contain wild-type rpoS. This indicates, as suggested by Park et al. (17), that RpoS has no major role, if any role at all, in AR 3 in S. enterica serovar Typhimurium.

Role of oxygen in arginine-dependent AR S. enterica serovar Typhimurium DT104. Previous reports stated that S. enterica serovar Typhimurium does not survive pH 2.5 acid challenge under conditions similar to those used for E. coli (9, 12, 15). Notably, in those studies cells grown under microaerobic conditions were used. Therefore, we assessed the role of oxygen in more detail in the development of arginine-dependent AR in Salmonella during culturing. S. enterica serovar Typhimurium DT104 strain BAA-188 was grown to stationary phase in LBG, pH 5, under anoxic, microaerobic, and aerobic conditions and subsequently challenged in mineral medium supplemented with 20 mM L-arginine at pH 2.5 under different oxygen conditions (Table 1). Aerobic cultures (10 ml) were grown in 250-ml Erlenmeyer flasks with shaking at 225 rpm. Cultures grown under microaerobic conditions, the standard conditions used for assessment of E. coli AR (15), were obtained by growing the cells in 3 ml of medium in 15- by 150-mm test tubes with shaking at 225 rpm. Analysis of stationary-phase AR under different oxygen conditions revealed that both aerobically and microaerobically grown stationary-phase S. enterica serovar Typhimurium cells were unable to survive at pH 2.5, whereas anaerobically grown cells again showed excellent survival. Moreover, the presence of oxygen during the AR test at pH 2.5 resulted in a decreased survival capacity. Under microaerobic challenge conditions, approximately 18.4% and 1.2% of the anaerobically grown cells survived after 1 and 2 h, respectively. Under aerobic conditions, the survival of these cells decreased even further, to approximately 7.8% and 0.3% after 1 and 2 h, respectively.


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  		TABLE 1. Impact of oxygen conditions on arginine-dependent AR of stationary-phase S. enterica serovar Typhimurium DT104 strain BAA-188 cellsa

 
Obviously, the oxygen condition during growth is a critical determinant of the arginine-dependent AR phenotype in Salmonella; i.e., only anaerobically grown stationary-phase cells display the AR phenotype. Our results offer an explanation for the lack of AR noted by other researchers, since in their experiments microaerobic growth conditions were used for testing AR at pH 2.5 (9, 12, 15). Culture conditions that resulted in an AR phenotype in E. coli did not result in an AR phenotype in S. enterica serovar Typhimurium. Apparently, development of arginine-dependent AR in Salmonella is strictly dependent on anoxic growth conditions.

Role of the arginine decarboxylase pathway in S. enterica serovar Typhimurium DT104 AR. To assess the role of the arginine decarboxylase system in S. enterica serovar Typhimurium DT104 AR at pH 2.5, several mutants in the adi gene cluster were constructed. Suicide plasmids were constructed by PCR amplification of an internal fragment of the target gene with Pwo polymerase (Roche Diagnostics, Almere, The Netherlands) and subsequent insertion in the {lambda} pir-dependent plasmid pERFORM-Z (1) by methods described previously (14). Single-crossover transconjugants were selected with zeocin (25 µg/ml ml) and tetracycline (12.5 µg/ml) for counterselection against E. coli upon conjugation. PCR and Southern analysis confirmed that the selected strains harbored the inserted antibiotic resistance cassette or plasmid (data not shown). In this way, S. enterica serovar Typhimurium DT104 strain BAA-188 adiA (arginine decarboxylase), adiY (regulator of adiA), yjdB (putative antiporter in the adi gene cluster), and adiC (arginine-agmatine antiporter) deletion mutants were obtained.

As demonstrated in Fig. 2, disruption of the arginine decarboxylase gene adiA results in an AR of 0.014% after 2 h at pH 2.5, compared to 68.6% for the wild type. A dramatic loss of AR was also observed in the adiY (transcriptional regulator) and adiC (arginine-agmatine antiporter) deletion mutants, which showed 0.035% and 0.048% survival, respectively, after 2 h at pH 2.5 in challenge medium supplemented with 20 mM L-arginine. Disruption of yjdB, encoding a putative antiporter, downstream of adiC did not result in loss of the AR phenotype.


Figure 2
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  		FIG. 2. Arginine-dependent AR of stationary-phase S. enterica serovar Typhimurium DT104 strain BAA-188. Stationary-phase cells grown in LBG at pH 5 were challenged in EG mineral medium, pH 2.5, with 20 mM L-arginine at an initial concentration of approximately 5 x 105 CFU/ml. Survival after 1 h (open bars) and 2 h (black bars) is depicted. All experiments were performed twice in triplicate. WT, wild type.

 
The arginine decarboxylase system has been implicated in AR in E. coli and Shigella flexneri (15). From the results presented here, it is obvious that the arginine decarboxylase system, including the arginine decarboxylase (adiA), the AraC-like regulator (adiY), and the arginine-agmatine antiporter (adiC), confers AR in S. enterica serovar Typhimurium.

Expression of adi genes in S. enterica serovar Typhimurium DT104 strain BAA-188. RNA from mid-logarithmic-phase S. enterica serovar Typhimurium DT104 strain BAA-188 cultures grown in LBG was extracted with TRIzol reagent (Invitrogen, Carlsbad, CA) and treated with RQ1 RNase-free DNase (Promega, Leiden, The Netherlands), as recommended by the supplier. Subsequently the expression of adiA, adiC, and adiY was examined by quantitative PCR (qPCR) using the Bio-Rad MyiQ single-color real-time PCR detection system (Bio-Rad, Veenendaal, The Netherlands) and the iQ SYBR green Supermix kit (Bio-Rad), as recommended by the manufacturer. Primer sets were designed with Beacon Designer, version 4.02 (PREMIER Biosoft International, Palo Alto, CA). After optimization, the mean relative expression values were calculated. Expression of adiA and adiC was found to be induced by acid and anoxic growth conditions (Table 2). Similar results have been reported for transcription regulation of adiA in E. coli (11, 21) and adiC in Salmonella (17). The expression of adiY appears to be higher in Salmonella grown under aerobic conditions and to not be affected by the pH. It has been reported that the XylS/AraC-like transcriptional regulator AdiY, which is part adi gene cluster, acts as a positive regulator of adiA in E. coli (22). The data presented here reveal that adiY is essential for induction of arginine-dependent AR in Salmonella. Indeed, qPCR analysis revealed transcription of adiA and adiC to be reduced significantly in the adiY deletion mutant (Table 2). Recently, Gong et al. (11) suggested that AdiY acts as a conditional regulator for adiA, as they failed to shown a reduction in adiA transcription in an E. coli adiY deletion mutant. In E. coli, furthermore, the XylS/AraC-like transcriptional regulators EnvY and AppY were shown to stimulate adiA transcription (22). In S. enterica serovar Typhimurium, however, adiY has a unique role in that it is essential in the induction of arginine-dependent AR via direct or indirect transcriptional activation of the adiA and adiC genes.


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  		TABLE 2. Change in relative gene expression in S. enterica serovar Typhimurium DT104 strain BAA-188 as measured by qPCRa

 
In conclusion, this study clearly shows that S. enterica serovar Typhimurium has an active arginine-dependent AR mechanism (AR 3), permitting survival at pH 2.5. The adiA, adiY, and adiC gene products are critically important in Salmonella AR 3, with adiY playing a crucial role in the regulation of transcription of adiA and adiC. Foster recently stated that there was more to the AR 3 system of Salmonella than simply finding an active arginine decarboxylase and arginine-agmatine antiporter (9). From our results, however, it is now clear that the AR 3 system contributes to pH 2.5 acid survival only when Salmonella is grown anaerobically and not under conditions normally used for assessment of AR in E. coli.


   ACKNOWLEDGMENTS
 
This study was supported by the Netherlands Organization for Health Research and Development, program "Nutrition: Health, Safety and Sustainability."


   FOOTNOTES
 
* Corresponding author. Mailing address: Laboratory of Food Microbiology, Wageningen University Agrotechnology and Food Sciences Group, P.O. Box 8129, 6700 EV Wageningen, The Netherlands. Phone: 31 317 484981. Fax: 31 317 484978. E-mail: tjakko.abee{at}wur.nl. Back

{dagger} Present address: TNO Defence, Security and Safety, Business Unit, Biological and Chemical Protection, P.O. Box 45, 2280 AA Rijswijk, The Netherlands. Back


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Journal of Bacteriology, August 2006, p. 5650-5653, Vol. 188, No. 15
0021-9193/06/$08.00+0     doi:10.1128/JB.00323-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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