Arch Microbiol (2011) 193:179–185 DOI 10.1007/s00203-010-0656-7
ORIGINAL PAPER
Arginine-dependent acid-resistance pathway in Shigella boydii Kelvin Goh • Darren Chua • Brian Beck Marian L. McKee • Arvind A. Bhagwat
•
Received: 1 September 2010 / Revised: 12 October 2010 / Accepted: 29 October 2010 / Published online: 7 December 2010 Ó Springer-Verlag (outside the USA) 2010
Abstract Ability to survive the low pH of the human stomach is considered be an important virulent determinant. It was suggested that the unique acid tolerance of Shigella boydii 18 CDPH, the strain implicated in a 1998 outbreak, may have played an important role in surviving the acidic food (bean salad). The strain was capable of inducing arginine-dependent acid-resistance (ADAR) pathway. This pathway was assumed to be absent in Shigella sp. Here, we have examined occurrence and efficacy of ADAR pathway in 21 S. boydii strains obtained from the American Type Culture Collection (ATCC) along with strains of S. flexneri (n = 7), S. sonnei (n = 4), and S. dysenteriae (n = 2). The eight S. boydii strains were able to induce ADAR to survive the acid challenge at pH 2.0; additional 8 strains could tolerate acid challenge at pH 2.5 but not at pH 2.0. The remaining five S. boydii strains were not able to
Communicated by Erko Stackebrandt. K. Goh D. Chua A. A. Bhagwat (&) Environmental Microbial & Food Safety Laboratory, Henry A. Wallace Beltsville Agricultural Research Center, Agricultural Research Service, USDA, 10300 Baltimore Avenue, Bldg. 002, Room 117, BARC-W, Beltsville, MD 20705-2350, USA e-mail:
[email protected] B. Beck M. L. McKee ATCC, 1080 University Boulevard, Manassas, Virginia 20110-2209, USA Present Address: K. Goh D. Chua School of Life Sciences & Chemical Technology, Ngee Ann Polytechnic, 535 Clementi Road, Singapore 599489, Singapore Present Address: M. L. McKee BioReliance Corporation, 14920 Broschart Road, Rockville, MD 20850, USA
induce ADAR pathway and could not survive acid challenge even at pH 2.5. ADAR pathway also appears to be present in all four Shigella sp. Shigella ADAR pathway was induced when cells were grown under partial oxygen pressure while its expression in E. coli required mere fermentative growth on glucose. Keywords Microbial food safety Acid resistance Traveler’s diarrhea
Introduction Shigella boydii is one of four Shigella species namely Shigella dysenteriae, Shigella sonnei, and Shigella flexneri (Bopp et al. 1999; Lampel and Maurelli 2001). Shigella has no human reservoir and is normally transmitted from person to person via the fecal–oral route. All four species and especially S. dysenteriae continue to be the significant cause of human gastroenteritis. The incidences of shigellosis are more common among European travelers to tropical and developing countries (Vargas et al. 1999; Chart et al. 2009). The risk of contracting shigellosis is much high due to the fact that as few as 10–500 ingested Shigella cells can cause illness (Kotloff et al. 1995; Lampel and Maurelli 2001). The low infection dose (ID) associated with Shigella species is attributed to the organism’s acid-resistant nature. The importance of acid survival in pathogenesis is underscored by the fact that Vibrio cholera, non-typhi Salmonella sp., and Shigella flexneri have oral infection doses of 109, 105, and 102 respectively (Abe et al. 2004). These infection doses correlate with the level of acid resistance demonstrated by each organism, with V. cholera being the most acid sensitive and S. flexneri being most acid resistant (Abe et al. 2004; Allos et al. 2004).
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Comparative analysis of extreme acid survival in Salmonella enterica Serovar Typhimurium, Shigella flexneri, and E. coli revealed two amino acid–dependent acid-resistance systems in E. coli versus one in Shigella spp. and none in Salmonella spp. (Foster and Spector 1995; Bhagwat and Bhagwat 2004; Lin et al. 1995). More specifically, an arginine decarboxylase–based acid resistance (ADAR) was reported to be absent in Shigella and Salmonella spp. (Foster 2000; Zhao and Houry 2010). ADAR system is comprised of arginine decarboxylase AdiA and an antiporter, AdiC (YjdB) to transport out the arginine decarboxylation product agmatine (Richard and Foster 2004; Gong et al. 2003). E. coli cells grown under fermentative growth conditions synthesize the arginine decarboxylase (encoded by adiA) and arginine decarboxylation product agmatine antiporter (encoded by yjdE/adiC) (Foster 2004). This pathway when operative is able to increase the intracellular pH of E. coli to approximately 4.5 compared to a pH 3.5 in the absence of these genes under acid stress of pH 2.5 (Foster 2004; Richard and Foster 2004). A strain of S. boydii was implicated in a food-borne outbreak in 1998 at a restaurant in Chicago, IL. The Chicago Department of Public Health (CDPH) investigated the incident and found that the pathogen was associated with and survived in the bean salad (pH 4.0–5.0) for several days (Gong et al. 2003). Further analysis indicated that the outbreak strain was capable of inducing the ADAR (Chan and Blaschek 2005). The main aim of this study was to examine how widespread is the occurrence of ADAR pathway in various species of Shigella and more specifically among S. boydii isolates. We examined 21 isolates of S. boydii available at the collection of American Type Culture Collection (ATCC) along with 16 other strains of S. flexneri, S. sonnei, and S. dysenteriae for the occurrence of ADAR pathway and analyzed its potential to protect cells against acid challenge at pH 2.0 and pH 2.5. We observed that the ADAR pathway was present in several Shigella spp; however, it was induced only when cells were grown under partial oxygen pressure while its expression in E. coli required mere fermentative growth on glucose.
Materials and methods Bacterial strains, growth conditions and media Shigella strains were kept as frozen stocks in Luria–Bertani broth containing 20% glycerol, and they were routinely grown in LB broth or on LB agar plates at 37°C. A single colony was inoculated in either LB broth or LB broth buffered with 100 mM MES (morpholineethanesulfonic acid, pH 5.5) and grown under aeration (220 rpm, 37°C). For fermentative growth under anaerobic conditions, single
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colonies were inoculated in 3 mL of brain heart infusion broth (Becton–Dickinson) containing 0.4% glucose (BHIG) in 13 9 100 test tubes and were incubated without shaking for 18–20 h in anaerobic bags which achieve \2% O2, and [4% CO2 within 2 h of incubation at 37°C (Becton–Dickinson, Sparks, MD) (Ling et al. 2008). Acid-challenge assays The ADAR (acid challenge at pH 2 or pH 2.5) was analyzed as described previously (Fratamico et al. 2008). Briefly, cells were diluted from the growth medium (1:200) into minimal E medium containing 0.4% glucose (EG medium) (Lin et al. 1995), adjusted to pH 2.0 or 2.5 with 5 N HCl and supplemented with 1.5 mM arginine. The cells were challenged for 2 h at 37°C and then diluted in 50 mM phosphate-buffered saline (pH 7.2) and plated onto LB agar medium to determine the viable cell count. As per the previous classification schemes (Bhagwat et al. 2005; Fratamico et al. 2008), if the surviving population after the acid challenge was[1.0%, the isolate was considered resistant. The acid-resistance phenotype was tested at least three times, and each experiment had two replicates. Less than 0.001% cells survived acid challenge at pH 2.0 without addition of arginine. Statistical analysis For all statistical analyses, SigmaStat 3.0 software (Ashburn, VA) was used. Data were analyzed by one-way ANOVA test to determine statistical differences between means of treatments. Detection of glycogen synthesis Glycogen synthesis was used as an indirect and nonselective measure of functional RpoS (Bhagwat et al. 2006; Preiss and Romeo 1994) and was monitored as described earlier (Govons et al. 1969) with minor modifications (Wei et al. 2000). Briefly, overnight cultures were plated onto Kornberg agar medium (Wei et al. 2000), and after 24-h incubation at 37°C, plates were further incubated at 10°C for 24 h and then stained with an iodine solution for 1–2 min. Dark brown colonies indicated the synthesis of glycogen, while pale brown or white colonies indicated partial or lack of glycogen synthesis.
Results Induction of ADAR in Shigella isolates Initially Shigella isolates were examined for ADAR under the induction conditions similar to E. coli O157:H7
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Table 1 Arginine-dependent acid-resistance phenotypes of S. boydii strains Group Strain description
I
% Survival after acid challenge of 2 h in the presence of 0.6 mM arginine at pH 2.0
pH 2.5
S. boydii strains ATCC 8702, 12027, 12028, 12030, 29928, 29929, BAA-1247, 9207 (n = 8)
39.1 ± 9.3 (A)
81.8 ± 11.5 (A)
24.2 ± 2.3 (A)
61.8 ± 11.5 (A)
S. sonnei ATCC 29030
20.1 ± 5.6 (A)
78.2 ± 9.8 (A)
S. flexneri WRAIR collection #1771-62 and 1771-62Tet II
III
S. boydii strains ATCC 8700, 9905, 25930, 35965, 49348, 49812, 12033, 12035 (n = 8)
0.39 ± 0.5 (B)
23.5 ± 17.2 (B)
\0.01
26.3 ± 5.5 (B)
S. sonnei ATCC 29029; WRAIR collection # Mosley and #30
\0.01
32.3 ± 8.5 (B)
S. flexneri ATCC 12023, 700930; WRAIR collection # Lou Baron, M90TW \0.01 S. dysenteriae WRAIR collection # Uban 12, C924-M3
18.9 ± 5.2 (B)
S. boydii strains ATCC 8704, 12029, 12031, 35964, 12034 (n = 5)
\0.01
0.11 ± 0.1 (C)
S. flexneri WRAIR collection #M76-39
\0.01
0.25 ± 0.2 (C)
Mean values (n = 3) in each column that are not followed by the same letter in the parenthesis indicate significant (P \ 0.05) differences
a 100
% Survival
10 Group I, ATCC 12027 Group I, ATCC 12028 Group II, ATCC 8700 Group II, ATCC 25930 Group III, ATCC 8704 Group III, ATCC 12031
1
0.1
0.01 0
20
40
60
80
100
120
80
100
120
Time (min)
b 100
10
% Survival
(Bhagwat and Bhagwat 2004). However, acid induction of ADAR (growth under mild acidic conditions at pH 5.5) or induction under semi-aerobic condition was weak, and variable when cells were challenged at pH 2.5 for 2 h in the presence of 1.5 mM arginine (data not shown). Next, we examined whether ADAR pathway could be induced upon growth under anaerobic conditions (\2% O2, and [4% CO2) and cell survival was monitored at two acid-challenge conditions, pH 2.0 and pH 2.5 (for 2 h at 37°C and in the presence of 1.5 mM arginine) (Table 1). Based on cell survival responses, 21 S. boydii isolates could be classified into three groups. Group I strains could utilize ADAR to protect the acid challenge as low as pH 2.0. Eight S. boydii ATCC isolates belonged to this group. S. sonnei ATCC 29030 and two S. flexneri isolates (WRAIR 1771-62 and 1771-62-Tet) also showed significantly robust ADAR compared to Group II strains at pH 2.0 (P \ 0.05). In the absence of arginine, cells did not survive the acid challenge either at pH 2.0 or at pH 2.5 (\0.01% survival). When acid challenged at pH 2.5, 8 more S. boydii isolates could utilize ADAR to survive the acid shock along with several other isolates of S. sonnei (n = 2), S. flexneri (n = 4) and S. dysenteriae (n = 2). These groups of Shigella isolates were classified as Group II, and their ADAR response at pH 2.5 was significantly weaker compared to Group I strains (P \ 0.05). Remainder of the S. boydii isolates (n = 5) and one S. flexneri isolate could not induce ADAR, and the cells failed to survive the acid challenge even at pH 2.5. Increasing the arginine concentration beyond 1.2 mM did not influence the outcome of ADAR survival assay during the 2-h challenge at pH 2.0 or 2.5 across the three groups (Fig. 1) (data not shown).
1
Group I, ATCC 12027 Group I, ATCC 12028 Group II, ATCC 8700 Group II, ATCC 25930 Group III, ATCC 8704 Group III, ATCC 12031
0.1
0.01 0
20
40
60
Time (min)
Fig. 1 Survival response of ADAR phenotype Groups I, II, and III strains to acid challenge at pH 2.0 (a) and 2.5 (b) in the presence of 0.6 mM Arginine
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Fig. 2 RpoS expression status of representative ADAR phenotype Groups I, II, and III strains of S. boydii as measured by glycogen staining
ADAR responses measured as surviving cell fraction at various time intervals at pH 2.0 (Fig. 2a) and pH 2.5 (Fig. 2b) are shown for the representative isolates of Group I (ATCC 12027 and 12028), Group II (ATCC 8700 and 25930), and Group III (ATCC 8704 and 12031). RpoS status and ADAR induction Regulation of ADAR is not extensively studied. In E. coli, induction of ADAR is independent of RpoS regulation (Cui et al. 2001; Lin et al. 1996; Foster 2004). The ability to synthesize glycogen has been effectively used as an indicator of the rpoS status of cell cultures in the chemostat as well as in clinical isolates of E. coli (King et al. 2004; Bhagwat et al. 2006). Both S. boydii ATCC 8704 (Fig. 2c) and ATCC 12031 (Fig 2d) were unable to induce ADAR (Table 1, Group III strains) and had very divergent glycogen synthesis phenotypes. Based on glycogen staining pattern, it appears that ATCC 12031 may have
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dysfunctional RpoS compared to the functional RpoS in ATCC 8704. S. boydii ATCC 35965 (Fig. 2a) and ATCC 29929 (Fig. 2b) were able to induce ADAR (Table 1, Group II and Group I strains, respectively), but they too differed in their glycogen synthesis phenotypes. Likewise, other strains had variable glycogen synthesis phenotypes (data not shown), and no correlation was evident to indicate that functional RpoS may be required for the induction of ADAR phenotype. Genome comparison at the adiA locus among E. coli and Shigella strains We compared genomes of 6 Shigella strains (S. flexneri, n = 3; S. sonnei, S. dysenteriae, and S. boydii) and E. coli O157:H7 EDL933 strain using multigenome browser tool (Keseler et al. 2009) and *8-kbp adiA region with annotated genes, and their location in the respective genomes is depicted in Fig. 3. All Shigella strains had the three genes
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Fig. 3 Alignment of chromosomal regions flanking adiA (arginine decarboxylase) in E. coli O157:H7 and Shigella sp
in identical order which constitute the ADAR system namely, adiA (biodegradative arginine decarboxylase), adiY (AraC-type regulator), and yjdE/aniC (arginine:agmatine transporter). S. dysenteriae strain Sd197 had two insertion elements insB and insA downstream of adiA. Biological significance of this insertion with reference to ADAR is not clear at this time.
Discussion Although E. coli O157:H7 is the most commonly examined food-borne pathogen for studying acid resistance, it is important to also study other pathogens to understand the differences in their acid-resistance systems (King et al. 2004; Zhao and Houry 2010). Three acid-resistance systems have been well characterized in E. coli (Foster 2004). The first acid-resistance system (AR1) is referred to as the glucose-repressible oxidative pathway and protects cells from acidic stress above pH 3.0 (Slonczewski and Foster 1996; Lin et al. 1996, 1995). The structural components of
this acid-resistance system (other than RpoS) as well as the mechanisms by which it protects the cells are still unknown (Audia et al. 2001; Castanie-Cornet et al. 1999). The second acid-resistance system (AR2) is glutamate dependent (GDAR) and can protect cells from acidic stress below pH 3 (Hersh et al. 1996; Slonczewski and Foster 1996). Several notable differences in the induction and regulation of AR2 of S. flexneri from that of E. coli have been observed (Bhagwat and Bhagwat 2004; Waterman and Small 2003; Zhao and Houry 2010). Shigella species were previously assumed to be unable to induce ADAR (AR3) pathway (Foster 2004, 2000; Lin et al. 1995); acid tolerance analysis of the 1999 outbreak strain of Shigella boydii revealed its capacity to induce ADAR pathway (Chan and Blaschek 2005). NCBI’s Protein Clusters database groups the proteins annotated on the complete prokaryote genome, plasmid, and organelle RefSeq sequences based on the function and sequence similarity (Pruitt et al. 2007). Based on computational analysis of the 20-kbp region flanking the biodegradative arginine decarboxylase locus from Protein Cluster CLS 998383
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(Altschul et al. 1990; Wheeler and Bhagwat 2007), we predicted that the ADAR pathway could be also functional in several Shigella species (Bhagwat and Bhagwat 2008). Data presented in this manuscript are indicative of the fact that the S. boydii outbreak-associated strain was not unique to possess ADAR pathway (Gong et al. 2003; Chan and Blaschek 2005). Lack of ADAR in five strains of S. boydii and one strain of S. flexneri (Table 1) will require more investigation since PCR amplification of adiA locus could not differentiate ADAR positive Shigella isolates (data not shown). However, based on comparative genome map of 6 Shigella strains, the ADAR pathway appears to be present across all four species of Shigella (Fig. 3). It may be noted that Shigella strains required stronger environmental signal, such as anaerobiosis and growth on glucose to stimulate the ADAR. Similar differences among E. coli and Shigella species with respect to AR2 induction pathway have been reported earlier (Bhagwat and Bhagwat 2004). This may explain why ADAR pathway was presumed to be lacking in Shigella sp. using conditions similar to those used for E. coli (Foster and Spector 1995; Lin et al. 1995; Foster 2004). We found no evidence for the role of RpoS in induction of ADAR in Shigella strains as demonstrated by the fact that glycogen-synthesizing strains (indicative of functional RpoS) failed to induce ADAR, while glycogen defective strains (indicative of dysfunctional RpoS) were able to induce ADAR (compare ATCC 8704 and ATCC 35965; Fig 2c vs. a, respectively). AdiY, which is a AraC-like regulator, flanked by adiA (arginine decarboxylase) and yjdE (arginine-agmatine transporter) genes (Fig. 3), although can enhance the expression, is not essential for the ADAR system in E. coli (Foster 2004). When grown under anaerobic conditions, CysB protein of E. coli was shown to act as an activator of adiA/C genes at low pH (Shi and Bennett 1994). Historically S. sonnei and S. flexneri are more commonly reported Shigella species associated with food-borne outbreaks (Lewis et al. 2009; Gaynor et al. 2009; Faruque et al. 2002). In contrast, outbreaks associated with S. boydii appear to be few or infrequent (Smith et al. 2009). S. boydii isolates can be difficult to distinguish from S. flexneri and enteroinvasive E. coli and misidentification could result in under reporting. Indeed, several enteroinvasive E. coli isolates and previously unidentified Shigella species have now been characterized as S. boydii (Grimont et al. 2007; Navarro et al. 2010). These findings may indicate that Shigella species and E. coli despite some biochemical and antigenic differences are much closely related and were further confirmed based on the study analyzing housekeeping genes (Lan and Reeves 2002; Day and Maurelli 2002). Presence of ADAR or AR3 in all four Shigella species may underscore their close relationship with E. coli.
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References Abe K, Saito N, Kasuga F, Yamamoto S (2004) Prolonged incubation period of salmonellosis associated with low bacterial doses. J Food Prot 67:2735–2740 Agle ME, Martin SE, Blaschek HP (2005) Survival of Shigella boydii 18 in bean salad. J Food Prot 68:838–840 Allos BM, Moore MR, Griffin PM, Tauxe RV (2004) Surveillance for sporadic foodborne disease in the 21 st century: the foodnet perspective. Clin Infet Dis 38(Suppl 3):S115–S120 Altschul SF, Gish W, Miller W, Myers EW, Lipman DJ (1990) Basic local alignment search tool. J Mol Biol 215:403–410 Audia JP, Webb CC, Foster JW (2001) Breaking through the acid barrier: an orchestrated response to proton stress by enteric bacteria. Int J Med Microbiol 291:97–106 Bhagwat AA, Bhagwat MA (2004) Comparative analysis of transcriptional regulatory elements of glutamate-dependent acidresistance systems of Shigella flexneri and Escherichia coli o157:H7. FEMS Microbiol Lett 234:139–147 Bhagwat AA, Bhagwat M (2008) Methods and tools for comparative genomics of foodborne pathogens. Foodborne Pathog Dis 5(4):487–497 Bhagwat AA, Chan L, Han R, Tan J, Kothary M, Jean-Gilles J, Tall BD (2005) Characterization of enterohemorrhagic Escherichia coli strains based on acid resistance phenotypes. Infect Immun 73(8):4993–5003 Bhagwat AA, Tan J, Sharma M, Kothary M, Low S, Tall BD, Bhagwat M (2006) Functional heterogeneity of rpos in stress tolerance of enterohemorrhagic Escherichia coli strains. Appl Environ Microbiol 72(7):4978–4986 Bopp CA, Brenner FW, Wells JG, Strockbine NA (1999) Escherichia, Shigella and Salmonella. In: Murray P, Baron EJ, Pfaller MA, Tenover FC, Yolken RH (eds) Manual of clinical microbiology, 7th edn. American Society for Microbiology, Washington, pp 459–474 Castanie-Cornet MP, Penfound TA, Smith D, Elliott JF, Foster JW (1999) Control of acid resistance in Escherichia coli. J Bacteriol 181:3525–3535 Chan YC, Blaschek HP (2005) Comparative analysis of Shigella boydii 18 foodborne outbreak isolate and related enteric bacteria: role of rpos and adia in acid stress response. J Food Prot 68:521–527 Chart H, Daniel R, Cheasy T (2009) The expression of lipopolysaccharide by strains of Shigella dysenteriae, Shigella flexneri and Shigella boydii and their cross-reacting strains of Eschrichia coli. FEMS Microbiol Lett 292:21–26 Cui S, Meng J, Bhagwat AA (2001) Availability of glutamate and arginine during acid challenge determines cell density-dependent survival phenotype of Escherichia coli strains. Appl Environ Microbiol 67:4914–4918 Day WA, Maurelli AT (2002) Shigella and enteroinvasive Escherichia coli: paradigms for pathogen evolution and host-parasite interactions. In: Donnenberg MS (ed) Escherichia coli––virulence mechanisms of a versatile pathogen. Academic Press, San Diego, pp 209–237 Faruque SM, Khan R, Kamruzzaman M, Yamasaki S, Ahmad QS, Azim T, Nair GB, Takeda Y, Sack DA (2002) Isolation of Shigella dysenteriae type 1 and S. Flexneri strains from surface waters in Bangladesh: comparative molecular analysis of environmental Shigella isolates versus clinical strains. Appl Environ Microbiol 68(8):3908–3913 Foster JW (2000) Microbial responses to acid stress. In: Storz G, Hengge-Aronis R (eds) Bacterial stress responses. ASM Press, Washington, pp 99–115 Foster JW (2004) Escherichia coli acid resistance: tales of an amateur acidophile. Nat Rev Microbiol 2(11):898–907
Arch Microbiol (2011) 193:179–185 Foster JW, Spector MP (1995) How Salmonella survive against the odds. Annu Rev Microbiol 49:145–174 Fratamico PM, Bhagwat AA, Injaian L, Fedorka-Cray PJ (2008) Characterization of shiga toxin-producing Escherichia coli strains isolated from swine feces. Foodborne Pathog Dis 5(6):827–838 Gaynor K, Park SY, Kanenaka R, Colindres R, Mintz E, Ram PK, Kitsutani P, Nakata M, Wedel S, Boxrud D, Jennings D, Yoshida H, Tosaka N, He H, Ching-Lee M, Effler PV (2009) International foodborne outbreak of Shigella sonnei infection in airline passengers. Epidemiol Infect 137(3):335–341 Gong S, Richard H, Foster JW (2003) Yjde (adic) is the arginine:Agmatine antiporter essential for arginine-dependent acid resistance in Escherichia coli. J Bacteriol 185(15):4402–4409. doi:10.1128/jb.185.15.4402-4409.2003 Govons S, Vinopal R, Ingraham J, Preiss J (1969) Isolation of mutants of Escherichia coli b altered in their ability to synthesize glycogen. J Bacteriol 97:970–972 Grimont F, Lejay-Collin M, Talukder KA, Carle I, Issenhuth S, Le Roux K, Grimont PAD (2007) Identification of a group of shigella-like isolates as Shigella boydii 20. J Med Microbiol 56(6):749–754 Hersh BM, Farooq FT, Barstad DN, Blankenhorn DL, Slonczewski JL (1996) A glutamate-dependent acid resistance gene in Escherichia coli. J Bacteriol 178:3978–3981 Keseler IM, Bonavides-Martinez C, Collado-Vides J, Gama-Castro S, Gunsalus RP, Johnson DA, Krummenacker M, Nolan LM, Paley S, Paulsen IT, Peralta-Gil M, Santos-Zavaleta A, Shearer AG, Karp PD (2009) Ecocyc: a comprehensive view of Escherichia coli biology. Nucl Acids Res 37(Suppl 1):464–470. doi: 10.1093/nar/gkn751 King T, Ishihama A, Kori A, Ferenci T (2004) A regulatory trade-off as a source of strain variation in the species Escherichia Coli. J Bacteriol 186(17):5614–5620 Kotloff KL, Nataro JP, Losonsky GA, Wasserman SS, Hale TL, Taylor DN, Sadoff JC, Levine MM (1995) A modified Shigella volunteer challenge model in which the inoculum is administered with bicarbonate buffer: clinical experience and implications for Shigella infectivity. Vaccine 13:1488–1494 Lampel KA, Maurelli AT (2001) Shigella species. In: Doyle MP, Beuchat LR, Montville T (eds) Food microbiology. ASM Press, Washington, pp 247–261 Lan R, Reeves PR (2002) Escherichia coli in disguise: molecular origins of shigella. Microbes Infect 4:1125–1132 Lewis HC, Etheberg S, Olsen KEP, Nielsen EM, Lisby M, Madsen SB, Boel J, Stafford R, Kirk M, Smith HV, Tikumrum S, Wisetrojana A, Bangtrakulnonth A, Vithyarungruangsri J, Siriarayaporn P, Uingchusak K, Bishop J, Molbak K (2009) Outbreaks of Shigella sonnei infections in Denmark and Australia linked to consumption of imported raw baby corn. Epidemiol Infect 137(3):326–334 Lin J, Lee IS, Frey J, Slonczewski JL, Foster JW (1995) Comparative analysis of extreme acid survival In Salmonella typhimurium,
185 Shigella flexneri, and Escherichia coli. J Bacteriol 177: 4097–4104 Lin J, Smith MP, Chapin KC, Baik HS, Bennett GN, Foster JW (1996) Mechanisms of acid resistance in enterohemorrhagic Escherichia coli. Appl Environ Microbiol 62:3094–3100 Ling J, Sharma M, Bhagwat AA (2008) The role of RNA polymerase sigma-factor (rpos) in induction of glutamate-dependent acidresistance of Escherichia albertii under anaerobic conditions. FEMS Microbiol Lett 283:75–82 Navarro A, Eslava C, Perea LM, Inzunza A, Delgado G, MoralesEspinosa R, Cheasty T, Cravioto A (2010) New enterovirulent Escherichia coli serogroup 64474 showing antigenic and genotypic relationships to Shigella boydii 16. J Med Microbiol 59:453–461 Preiss J, Romeo T (1994) Molecular biology and regulatory aspects of glycogen biosynthesis in bacteria. Prog Nucleic Acid Res Mol Biol 47:329–399 Pruitt KD, Tatusova T, Maglott DR (2007) Ncbi reference sequence (refseq): a curated non-redundant sequence database of genomes, transcripts and proteins. Nucleic Acids Res 35:61–65 Richard H, Foster JW (2004) Escherichia coli glutamate- and arginine-dependent acid resistance systems increase internal ph and reverse transmembrane potential. J Bacteriol 186(18): 6032–6041. doi:10.1128/jb.186.18.6032-6041.2004 Shi X, Bennett GN (1994) Effects of rpoa and cysb mutations on acid inductinof biodegradative arginine decarboxylase in Escherichia coli. J Bacteriol 176:7017–7023 Slonczewski JL, Foster JW (1996) Ph-regulated genes and survival at acidic ph. In: Neidhardt FC, Curtiss R III, Ingraham JL et al (eds) Escherichia coli and Salmonella cellular and molecular biology, vol 1, 2nd edn. ASM Press, Washington, pp 1539–1549 Smith A, Keddy KH, Sooka A, Ismail H, DeJong G, Hoyland G (2009) Analysis of a temporal cluster of Shigella boydii isolates in Mpumalanga, South Africa, November to December 2007. J Infect Dev Ctries 3:65–70 Vargas M, Gason J, Jimenez DeAnta M, Vila J (1999) Prevalence of Shigella enterotoxins 1 and 2 among Shigella strains isolated from patients with travelers diarrhea. J Clin Microbiol 37:3608–3611 Waterman SR, Small PLC (2003) Identification of the promoter regions and sigma-s-dependent regulation of the gada and gadbc genes associated with glutamate-dependent acid resistance in Shigella flexneri. FEMS Microbiol Lett 225:155–160 Wei B, Shin S, LaPorte D, Wolfe AJ, Romeo T (2000) Global regulatory mutations in csra and rpos cause severe central carbon stress in Escherichia coli in the presence of acetate. J Bacteriol 182:1632–1640 Wheeler D, Bhagwat M (2007) Blast quick start: Example-driven web-based blast. In: Bergman NH (ed) Methods in molecular biology: comparative genomics-i. Methods in molecular biology, vol 395. Humana Press, Totowan, pp 149–176 Zhao B, Houry WA (2010) Acid stress response in enteropathogenic gammaproteobacteria: an aptitude for survival. Biochem Cell Biol 88:301–314
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