Abstract
The gastrointestinal tract is continuously in contact with commensal bacteria that are composed of more than 500 different species, and has an important role in human nutrition and health, by promoting nutrient supply, preventing pathogen colonization and shaping and maintaining normal mucosal immunity. The present review demonstrates the distribution of the intestinal commensal bacteria Enterococcus spp. and the prevalence of Escherichia coli phylogenetic groups in animals and humans in Portugal. The enterococcal population described in this review includes 1,909 enterococcal isolates recovered from a series of fecal samples of different animals (horses, swine, ostriches, partridges, mullet fish, garden dormice, seagulls, pets, poultry, wild boars, birds of prey, and wild rabbits) and healthy and clinical humans. We also compared the phylogenetic groups of Escherichia coli isolates (n = 203) recovered from healthy humans and animals (poultry, ostriches, seagulls, wild boars, birds of prey, and pigs). Phenotypic and molecular analysis allowed the identifying of Enterococcus faecium as the predominant species followed by Enterococcus faecalis. In addition, the Escherichia coli data from different studies showed that isolates of the A and B1 phylogenetic groups are predominant in the gut flora of animal origin and the phylogenetic group B2 isolates were the most common in healthy human samples.
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Anderson MA, Whitlock JE, Harwood VJ (2006) Diversity and distribution of Escherichia coli genotypes and antibiotic resistance phenotypes in feces of humans, cattle, and horses. Appl Environ Microbiol 72:6914–6922
Araújo C, Torres C, Gonçalves A, Carneiro C, López M, Radhouani H, Pardal M, Igrejas G, Poeta P (2011) Genetic detection and MLST typing of vanA-containing Enterococcus strains from mullets fish (Liza Ramada). Microb Drug Resist (in press)
Barreto A, Guimaraes B, Radhouani H, Araujo C, Goncalves A, Gaspar E, Rodrigues J, Igrejas G, Poeta P (2009) Detection of antibiotic resistant E. coli and Enterococcus spp. in stool of healthy growing children in Portugal. J Basic Microbiol 49:503–512
Bingen E, Picard B, Brahimi N, Mathy S, Desjardins P, Elion J, Denamur E (1998) Phylogenetic analysis of Escherichia coli strains causing neonatal meningitis suggests horizontal gene transfer from a predominant pool of highly virulent B2 group strains. J Infect Dis 177:642–650
Carneiro C, Araujo C, Goncalves A, Vinue L, Somalo S, Ruiz E, Uliyakina I, Rodrigues J, Igrejas G, Poeta P, Torres C (2010) Detection of CTX-M-14 and TEM-52 extended-spectrum beta-lactamases in fecal Escherichia coli isolates of captive ostrich in Portugal. Foodborne Pathog Dis 7:991–994
Chenoweth C, Schaberg D (1990) The epidemiology of enterococci. Eur J Clin Microbiol Infect Dis 9:80–89
Ciftci A, Findik A, Ica T, Bas B, Onuk EE, Gungordu S (2009) Slime production and antibiotic resistance of Enterococcus faecalis isolated from arthritis in chickens. J Vet Med Sci 71:849–853
Clermont O, Bonacorsi S, Bingen E (2000) Rapid and simple determination of the Escherichia coli phylogenetic group. Appl Environ Microbiol 66:4555–4558
Cortes P, Blanc V, Mora A, Dahbi G, Blanco JE, Blanco M, Lopez C, Andreu A, Navarro F, Alonso MP, Bou G, Blanco J, Llagostera M (2010) Isolation and characterization of potentially pathogenic antimicrobial-resistant Escherichia coli strains from chicken and pig farms in Spain. Appl Environ Microbiol 76:2799–2805
Costa D, Vinue L, Poeta P, Coelho AC, Matos M, Saenz Y, Somalo S, Zarazaga M, Rodrigues J, Torres C (2009) Prevalence of extended-spectrum beta-lactamase-producing Escherichia coli isolates in faecal samples of broilers. Vet Microbiol 138:339–344
de Vaux A, Laguerre G, Diviès C, Prévost H (1998) Enterococcus asini sp. nov. isolated from the caecum of donkeys (Equus asinus). Int J Syst Bacteriol 2:383–387
Delgado M, Neto I, Correia JH, Pomba C (2007) Antimicrobial resistance and evaluation of susceptibility testing among pathogenic enterococci isolated from dogs and cats. Int J Antimicrob Agents 30:98–100
Desai PJ, Pandit D, Mathur M, Gogate A (2001) Prevalence, identification and distribution of various species of enterococci isolated from clinical specimens with special reference to urinary tract infection in catheterized patients. Indian J Med Microbiol 19:132–137
Devriese LA, Van de Kerckhove A, Kilpper-Bälz R, Schleifer KH, Phillips BA (1987) Characterization of and identification of Enterococcus species isolated from animals. Int J Syst Bacteriol 37:257–259
Devriese LA, Ceyssens K, Rodrigues UM, Collins MD (1990) Enterococcus columbae, a species from pigeon intestines. FEMS Microbiol Lett 59:247–251
Devriese LA, Ducatelle R, Uyttebroek E, Haesebrouck F (1991) Enterococcus hirae infection and focal necrosis of the brain of chicks. Vet Rec 129:316
Devriese LA, Cruz Colque JI, De Herdt P, Haesebrouck F (1992a) Identification and composition of the tonsillar and anal enterococcal and streptococcal flora of dogs and cats. J Appl Bacteriol 73:421–425
Devriese LA, Laurier L, Herdt PD, Haesebrouck F (1992b) Enterococcal and streptococcal species isolated from faeces of calves, young cattle and dairy cows. J Appl Microbiol 72:29–31
Devriese LA, Hommez J, Pot B, Haesebrouck F (1994) Identification and composition of the streptococcal and enterococcal flora of tonsils, intestines and faeces of pigs. J Appl Microbiol 77:31–36
Devriese LA, Baele M, Butaye P (2006) The genus Enterococcus. In: Dworkin M, Falkow S, Rosenberg E, Schliefer KH, Stackebrandt E (eds) The Prokaryotes, 3rd edn. Springer, New York, pp 163–174
Duriez P, Clermont O, Bonacorsi S, Bingen E, Chaventre A, Elion J, Picard B, Denamur E (2001) Commensal Escherichia coli isolates are phylogenetically distributed among geographically distinct human populations. Microbiology 147:1671–1676
Dutka-Malen S, Evers S, Courvalin P (1995) Detectionof glycopeptide resistance genotypes and identificationto the species level of clinically relevant enterococciby PCR. J Clin Microbiol 33:24–27
Eaton TJ, Gasson MJ (2001) Molecular screening of Enterococcus virulence determinants and potential for genetic exchange between food and medical isolates. Appl Environ Microbiol 67:1628–1635
Firmesse O, Rabot S, Bermúdez-Humarán LG, Corthier G, Furet JP (2007) Consumption of Camembert cheese stimulates commensal enterococci in healthy human intestinal microbiota. FEMS Microbiol Lett 276:189–192
Foulquie Moreno MR, Sarantinopoulos P, Tsakalidou E, De Vuyst L (2006) The role and application of enterococci in food and health. Int J Food Microbiol 106:1–24
Franz CM, Holzapfel WH, Stiles ME (1999) Enterococci at the crossroads of food safety? Int J Food Microbiol 47:1–24
Franz CM, Muscholl-Silberhorn AB, Yousif NM, Vancanneyt M, Swings J, Holzapfel WH (2001) Incidence of virulence factors and antibiotic resistance among Enterococci isolated from food. Appl Environ Microbiol 67:4385–4389
Franz CM, Stiles ME, Schleifer KH, Holzapfel WH (2003) Enterococci in foods–a conundrum for food safety. Int J Food Microbiol 88:105–122
Franz CM, van Belkum MJ, Holzapfel WH, Abriouel H, Galvez A (2007) Diversity of enterococcal bacteriocins and their grouping in a new classification scheme. FEMS Microbiol Rev 31:293–310
Gin AS, Zhanel GG (1996) Vancomycin-resistant enterococci. Ann Pharmacother 30:615–624
Giuffra E, Kijas JM, Amarger V, Carlborg O, Jeon JT, Andersson L (2000) The origin of the domestic pig: independent domestication and subsequent introgression. Genetics 154:1785–1791
Gonçalves A, Poeta P, Silva N, Araujo C, Lopez M, Ruiz E, Uliyakina I, Direitinho J, Igrejas G, Torres C (2010a) Characterization of vancomycin-resistant enterococci isolated from fecal samples of ostriches by molecular methods. Foodborne Pathog Dis 7:1133–1136
Gonçalves A, Torres C, Silva N, Carneiro C, Radhouani H, Coelho C, Araujo C, Rodrigues J, Vinue L, Somalo S, Poeta P, Igrejas G (2010b) Genetic characterization of extended-spectrum beta-lactamases in Escherichia coli isolates of pigs from a Portuguese intensive swine farm. Foodborne Pathog Dis (in press)
Gordon DM (1997) The genetic structure of Escherichia coli populations in feral house mice. Microbiology 143:2039–2046
Graves A, Weaver RW, Entry J (2009) Characterization of enterococci populations in livestock manure using BIOLOG. Microbiol Res 164:260–266
Guimaraes B, Barreto A, Radhouani H, Figueiredo N, Gaspar E, Rodrigues J, Torres C, Igrejas G, Poeta P (2009) Genetic detection of extended-spectrum beta-lactamase-containing Escherichia coli isolates and vancomycin-resistant enterococci in fecal samples of healthy children. Microb Drug Resist 15:211–216
Hamilton-Miller JM, Shah S (1999) Identification of clinically isolated vancomycin-resistant enterococci: comparison of API and BBL Crystal systems. J Med Microbiol 48:695–696
Hancock V, Dahl M, Klemm P (2010) Probiotic Escherichia coli strain Nissle 1917 outcompetes intestinal pathogens during biofilm formation. J Med Microbiol 59:392–399
Herzer PJ, Inouye S, Inouye M, Whittam TS (1990) Phylogenetic distribution of branched RNA-linked multicopy single-stranded DNA among natural isolates of Escherichia coli. J Bacteriol 172:6175–6181
Hilali F, Ruimy R, Saulnier P, Barnabe C, Lebouguenec C, Tibayrenc M, Andremont A (2000) Prevalence of virulence genes and clonality in Escherichia coli strains that cause bacteremia in cancer patients. Infect Immun 68:3983–3989
Jenkins MB, Hartel PG, Olexa TJ, Stuedemann JA (2003) Putative temporal variability of Escherichia coli ribotypes from yearling steers. J Environ Qual 32:305–309
Johnson JR, Goullet P, Picard B, Moseley SL, Roberts PL, Stamm WE (1991) Association of carboxylesterase B electrophoretic pattern with presence and expression of urovirulence factor determinants and antimicrobial resistance among strains of Escherichia coli that cause urosepsis. Infect Immun 59:2311–2315
Kamada N, Inoue N, Hisamatsu T, Okamoto S, Matsuoka K, Sato T, Chinen H, Hong KS, Yamada T, Suzuki Y, Suzuki T, Watanabe N, Tsuchimoto K, Hibi T (2005) Nonpathogenic Escherichia coli strain Nissle 1917 prevents murine acute and chronic colitis. Inflamm Bowel Dis 11:455–463
Klein G (2003) Taxonomy, ecology and antibiotic resistance of enterococci from food and the gastro-intestinal tract. Int J Food Microbiol 88:123–131
Kruis W, Schutz E, Fric P, Fixa B, Judmaier G, Stolte M (1997) Double-blind comparison of an oral Escherichia coli preparation and mesalazine in maintaining remission of ulcerative colitis. Aliment Pharmacol Ther 11:853–858
Kühn I, Iversen A, Burman LG, Olsson-Liljequist B, Franklin A, Finn M, Aarestrup F, Seyfarth AM, Blanch AR, Taylor H, Caplin J, Moreno MA, Dominguez L, Mollby R (2000) Epidemiology and ecology of enterococci, with special reference to antibiotic resistant strains, in animals, humans and the environment. Example of an ongoing project within the European research programme. Int J Antimicrob Agents 14:337–342
Kühn I, Iversen A, Burman LG, Olsson-Liljequist B, Franklin A, Finn M, Aarestrup F, Seyfarth AM, Blanch AR, Vilanova X, Taylor H, Caplin J, Moreno MA, Dominguez L, Herrero IA, Mollby R (2003) Comparison of enterococcal populations in animals, humans, and the environment–a European study. Int J Food Microbiol 88:133–145
Laukova A, Simonova M, Strompfova V, Styriak I, Ouwehand AC, Varady M (2008) Potential of enterococci isolated from horses. Anaerobe 14:234–236
Layton BA, Walters SP, Lam LH, Boehm AB (2010) Enterococcus Species Distribution among Human and Animal Hosts Using Multiplex Pcr. J Appl Microbiol 109:539–547
Leatham MP, Banerjee S, Autieri SM, Mercado-Lubo R, Conway T, Cohen PS (2009) Precolonized human commensal Escherichia coli strains serve as a barrier to E. coli O157:H7 growth in the streptomycin-treated mouse intestine. Infect Immun 77:2876–2886
Leclerc H, Mossel DA, Edberg SC, Struijk CB (2001) Advances in the bacteriology of the coliform group: their suitability as markers of microbial water safety. Annu Rev Microbiol 55:201–234
Lee S, Yu JK, Park K, Oh EJ, Kim SY, Park YJ (2010) Phylogenetic groups and virulence factors in pathogenic and commensal strains of Escherichia coli and their association with blaCTX-M. Ann Clin Lab Sci 40:361–367
Linaje R, Coloma MD, Perez-Martinez G, Zuniga M (2004) Characterization of faecal enterococci from rabbits for the selection of probiotic strains. J Appl Microbiol 96:761–771
Machado E, Coque TM, Canton R, Sousa JC, Peixe L (2008) Antibiotic resistance integrons and extended-spectrum {beta}-lactamases among Enterobacteriaceae isolates recovered from chickens and swine in Portugal. J Antimicrob Chemother 62:296–302
Malchow HA (1997) Crohn's disease and Escherichia coli. A new approach in therapy to maintain remission of colonic Crohn's disease? J Clin Gastroenterol 25:653–658
Miele A, Bandera M, Goldstein BP (1995) Use of primersselective for vancomycin resistance genes to determine vangenotype in enterococci and to study gene organization in VanAisolates. Antimicrob Agents Chemother 39:1772–1778
Molina M, Cyterski M, Maimes J, Fisher J, Johnson B (2007) Comparison of the temporal variability of enterococcal clusters in impacted streams using a multiplex polymerase chain reaction procedure. In proceedings, Georgia Water Resources Conference, Athens GA. Georgia institute of technology, Atlanta, GA 1–4
Moura I, Radhouani H, Torres C, Poeta P, Igrejas G (2010) Detection and genetic characterisation of vanA-containing Enterococcus strains in healthy Lusitano horses. Equine Vet J 42:181–183
Murray BE (1990) The life and times of the Enterococcus. Clin Microbiol Rev 3:46–65
Noble CJ (1978) Carriage of group D streptococci in the human bowel. J Clin Pathol 31:1182–1186
Novais C, Sousa JC, Coque TM, Peixe LV (2003) First report of the activity of linezolid against Portuguese enterococci from human, animal and environmental sources. J Antimicrob Chemother 51:1314–1315
Novais C, Coque TM, Sousa JC, Peixe LV (2006) Antimicrobial resistance among faecal enterococci from healthy individuals in Portugal. Clin Microbiol Infect 12:1131–1134
Novais C, Freitas AR, Sousa JC, Baquero F, Coque TM, Peixe LV (2008) Diversity of Tn1546 and its role in the dissemination of vancomycin-resistant enterococci in Portugal. Antimicrob Agents Chemother 52:1001–1008
Patterson JE, Sweeney AH, Simms M, Carley N, Mangi R, Sabetta J, Lyons RW (1995) An analysis of 110 serious enterococcal infections. Epidemiology, antibiotic susceptibility, and outcome. Medicine (Baltimore) 74:191–200
Picard B, Garcia JS, Gouriou S, Duriez P, Brahimi N, Bingen E, Elion J, Denamur E (1999) The link between phylogeny and virulence in Escherichia coli extraintestinal infection. Infect Immun 67:546–553
Pinto L, Radhouani H, Coelho C, Martins da Costa P, Simoes R, Brandao RM, Torres C, Igrejas G, Poeta P (2010) Genetic detection of extended-spectrum beta-lactamase-containing Escherichia coli isolates from birds of prey from Serra da Estrela Natural Reserve in Portugal. Appl Environ Microbiol 76:4118–4120
Poeta P, Costa D, Rodrigues J, Torres C (2005a) Study of faecal colonization by vanA-containing Enterococcus strains in healthy humans, pets, poultry and wild animals in Portugal. J Antimicrob Chemother 55:278–280
Poeta P, Costa D, Saenz Y, Klibi N, Ruiz-Larrea F, Rodrigues J, Torres C (2005b) Characterization of antibiotic resistance genes and virulence factors in faecal enterococci of wild animals in Portugal. J Vet Med B Infect Dis Vet Public Health 52:396–402
Poeta P, Costa D, Rodrigues J, Torres C (2006a) Antimicrobial resistance and the mechanisms implicated in faecal enterococci from healthy humans, poultry and pets in Portugal. Int J Antimicrob Agents 27:131–137
Poeta P, Costa D, Rodrigues J, Torres C (2006b) Detection of genes encoding virulence factors and bacteriocins in fecal enterococci of poultry in Portugal. Avian Dis 50:64–68
Poeta P, Costa D, Igrejas G, Rodrigues J, Torres C (2007a) Phenotypic and genotypic characterization of antimicrobial resistance in faecal enterococci from wild boars (Sus scrofa). Vet Microbiol 125:368–374
Poeta P, Costa D, Rojo-Bezares B, Zarazaga M, Klibi N, Rodrigues J, Torres C (2007b) Detection of antimicrobial activities and bacteriocin structural genes in faecal enterococci of wild animals. Microbiol Res 162:257–263
Poeta P, Radhouani H, Igrejas G, Goncalves A, Carvalho C, Rodrigues J, Vinue L, Somalo S, Torres C (2008) Seagulls of the Berlengas natural reserve of Portugal as carriers of fecal Escherichia coli harboring CTX-M and TEM extended-spectrum beta-lactamases. Appl Environ Microbiol 74:7439–7441
Poeta P, Radhouani H, Pinto L, Martinho A, Rego V, Rodrigues R, Goncalves A, Rodrigues J, Estepa V, Torres C, Igrejas G (2009) Wild boars as reservoirs of extended-spectrum beta-lactamase (ESBL) producing Escherichia coli of different phylogenetic groups. J Basic Microbiol 49:584–588
Pupo GM, Karaolis DK, Lan R, Reeves PR (1997) Evolutionary relationships among pathogenic and nonpathogenic Escherichia coli strains inferred from multilocus enzyme electrophoresis and mdh sequence studies. Infect Immun 65:2685–2692
Radhouani H, Poeta P, Igrejas G, Goncalves A, Vinue L, Torres C (2009) Antimicrobial resistance and phylogenetic groups in isolates of Escherichia coli from seagulls at the Berlengas nature reserve. Vet Rec 165:138–142
Radhouani H, Pinto L, Coelho C, Goncalves A, Sargo R, Torres C, Igrejas G, Poeta P (2010a) Detection of Escherichia coli harbouring extended-spectrum {beta}-lactamases of the CTX-M classes in faecal samples of common buzzards (Buteo buteo). J Antimicrob Chemother 65:171–173
Radhouani H, Poeta P, Pinto L, Miranda J, Coelho C, Carvalho C, Rodrigues J, Lopez M, Torres C, Vitorino R, Domingues P, Igrejas G (2010b) Proteomic characterization of vanA-containing Enterococcus recovered from Seagulls at the Berlengas Natural Reserve. W Portugal Proteome Sci 8:48
Rembacken BJ, Snelling AM, Hawkey PM, Chalmers DM, Axon AT (1999) Non-pathogenic Escherichia coli versus mesalazine for the treatment of ulcerative colitis: a randomised trial. Lancet 354:635–639
Robredo B, Singh KV, Baquero F, Murray BE, Torres C(1999) From vanA Enterococcus hirae to vanA Enterococcusfaecium: a study of feed supplementation with avoparcin andtylosin in young chickens. Antimicrob Agents Chemother 43:1137–1143
Rodrigues J, Poeta P, Martins A, Costa D (2002) The importance of pets as reservoirs of resistant Enterococcus strains, with special reference to vancomycin. J Vet Med B Infect Dis Vet Public Health 49:278–280
Ruoff KL, de la Maza L, Murtagh MJ, Spargo JD, Ferraro MJ (1990) Species identities of enterococci isolated from clinical specimens. J Clin Microbiol 28:435–437
Saavedra JM (2001) Clinical applications of probiotic agents. Am J Clin Nutr 73:1147S–1151S
Saenz Y, Brinas L, Dominguez E, Ruiz J, Zarazaga M, Vila J, Torres C (2004) Mechanisms of resistance in multiple-antibiotic-resistant Escherichia coli strains of human, animal, and food origins. Antimicrob Agents Chemother 48:3996–4001
Sartor RB (2005) Probiotic therapy of intestinal inflammation and infections. Curr Opin Gastroenterol 21:44–50
Schaberg DR, Culver DH, Gaynes RP (1991) Major trends in the microbial etiology of nosocomial infection. Am J Med 91:72S–75S
Schouten MA, Voss A, Hoogkamp-Korstanje JA (1999) Antimicrobial susceptibility patterns of enterococci causing infections in Europe. The European VRE Study Group. Antimicrob Agents Chemother 43:2542–2546
Shepard BD, Gilmore MS (2002) Antibiotic-resistant enterococci: the mechanisms and dynamics of drug introduction and resistance. Microbes Infect 4:215–224
Silva N, Igrejas G, Figueiredo N, Goncalves A, Radhouani H, Rodrigues J, Poeta P (2010) Molecular characterization of antimicrobial resistance in enterococci and Escherichia coli isolates from European wild rabbit (Oryctolagus cuniculus). Sci Total Environ 408:4871–4876
Silva N, Igrejas G, Vaz V, Araújo C, Cardoso L, Rodrigues J, Torres C, Poeta P (2011) Virulence factors in enterococci from partridges (Alectoris rufa) representing a food safety problem. Foodborne Pathog Dis 8:831–833
Sorum H, Sunde M (2001) Resistance to antibiotics in the normal flora of animals. Vet Res 32:227–241
Stark PL, Lee A (1982) The microbial ecology of the large bowel of breast-fed and formula-fed infants during the first year of life. J Med Microbiol 15:198–203
Takahashi K (2010) Interaction between the intestinal immune system and commensal bacteria and its effect on the regulation of allergic reactions. Biosci Biotechnol Biochem 74:691–695
Tannock GW (1997) Modification of the normal microbiota by diet, stress, antimicrobial agents and probiotics. In: Mackie RI, White BA, Isaacson RE (eds) Gastrointestinal microbiology, vol 2, Gastrointestinal microbes and host interactions. Chapman and Hall, New York, pp 434–465
Todorov SD, Wachsman M, Tome E, Dousset X, Destro MT, Dicks LM, Franco BD, Vaz-Velho M, Drider D (2010) Characterisation of an Antiviral Pediocin-Like Bacteriocin Produced by Enterococcus faecium. Food Microbiol 27:869–879
Tyrrell GJ, Bethune RN, Willey B, Low DE (1997) Species identification of enterococci via intergenic ribosomal PCR. J Clin Microbiol 35:1054–1060
van den Bogaard AE, Stobberingh EE (2000) Epidemiology of resistance to antibiotics. Links between animals and humans. Int J Antimicrob Agents 14:327–335
Vankerckhoven V, Van Autgaerden T, Vael C, Lammens C, Chapelle S, Rossi R, Jabes D, Goossens H (2004) Development of a multiplex PCR for the detection of asa1, gelE, cylA, esp, and hyl genes in enterococci and survey for virulence determinants among European hospital isolates of Enterococcus faecium. J Clin Microbiol 42:4473–4479
Wachsman MB, Farias ME, Takeda E, Sesma F, De Ruiz Holgado AP, De Torres RA, Coto CE (1999) Antiviral activity of enterocin CRL35 against herpesviruses. Int J Antimicrob Agents 12:293–299
Xu J, Bjursell MK, Himrod J, Deng S, Carmichael LK, Chiang HC, Hooper LV, Gordon JI (2003) A genomic view of the human-Bacteroides thetaiotaomicron symbiosis. Science 299:2074–2076
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Silva, N., Igrejas, G., Gonçalves, A. et al. Commensal gut bacteria: distribution of Enterococcus species and prevalence of Escherichia coli phylogenetic groups in animals and humans in Portugal. Ann Microbiol 62, 449–459 (2012). https://doi.org/10.1007/s13213-011-0308-4
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DOI: https://doi.org/10.1007/s13213-011-0308-4