JAC Advance Access originally published online on September 15, 2006
Journal of Antimicrobial Chemotherapy 2006 58(5):1095-1097; doi:10.1093/jac/dkl352
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Correspondence |
Prevalence of aadA1 and dfrA15 class 1 integron cassettes and SXT circulation in Vibrio cholerae O1 isolates from Africa
1 Dip. Biologia Cellulare e dello Sviluppo, Univ. La Sapienza Via dei Sardi 70, 00185 Rome, Italy 2 Department of Biomedical Sciences, Division of Clinical and Experimental Microbiology, University of Sassari Viale San Pietro 42B, 07100 Sassari, Sardinia, Italy
*Corresponding author. Tel: +39-0649917585; Fax: +39-0649917594; E-mail: mauro.colombo{at}uniroma1.it
Keywords: mobile genetic elements , V. cholerae , resistance
Sir,
Mobile genetic elements able to transfer multiple drug resistance among Vibrio cholerae strains have been described in numerous African cholera epidemics since the early 1980s and are considered a major public health problem.1 In particular, integrons are an important group of genetic elements able to capture, integrate and express resistance gene cassettes, and are widely distributed in different bacterial species, including V. cholerae.2,3 Class 1 integrons have been found in V. cholerae isolates from Guinea-Bissau and South Africa, located on a plasmid and chromosome, respectively.2,3 We recently identified a common plasmid carrying three different class 1 integrons containing, respectively, dfrA15, blaP1 and qacH-aadA8 cassettes in clinical and environmental isolates of V. cholerae from Angola.4
Integrative and conjugative elements (ICEs), such as SXTMO10 from V. cholerae O139 and SXTET (ICEVchInd1), have been discovered in India. They also contribute to the resistance profile of V. cholerae, through a cluster of several antibiotic resistance genes that exhibits a certain degree of polymorphism.5,6 Recently all ICEs bearing the highly related SXT integrase gene (intSXT), and able to integrate themselves into the prfC chromosomal gene, have been grouped in the SXT/R391 family.6 To date in Africa, the SXT element and the susceptible unclassified intSXT+ ICEs (ICEVchAng1 and 2) have only been detected in South Africa and Angola, respectively.3,4,6
A total of 19 V. cholerae O1 clinical strains, isolated between 1985 and 1998 in five different African countries and described previously, formed the basis of the study.1,4,7 To characterize the resistance profiles of the isolates, the following concentrations of drugs (in mg/L) were used in susceptibility and conjugation experiments: ampicillin (30), chloramphenicol (2.5), erythromycin (10), kanamycin (30), nalidixic acid (40), penicillin (20), rifampicin (100), streptomycin (100), spectinomycin (20), sulfamethoxazole (160), tetracycline (10) and trimethoprim (10). Detection of class 1 integrons and resistance cassette content was performed by PCR, as described previously.3,4 SXT investigation involved detection of the integrase gene (int) and integration site into the prfC (chromosomal SXT/prfC right junction).4,5 To ascertain the contribution of SXT to strain resistance profile, we tested for the presence of floR, strA, strB, sul2, and dfrA18 or dfrA1, typical clustered resistance genes, able to discriminate among SXT variants.4
All V. cholerae strains studied showed multiple drug resistances (four to seven agents, see Table 1). The high prevalence of resistance to sulfamethoxazole, trimethoprim, spectinomycin and streptomycin is explained by the circulation of integrons, SXT elements and conjugative plasmids. All but four isolates were able to transfer variable multiple resistance profiles in mating experiments. Interestingly, two integron resistance cassettes, dfrA15 and aadA1, were found in all geographic areas, and thus seem to be a general feature of V. cholerae O1 African strains. The dfrA15 trimethoprim resistance cassette was associated with sulfamethoxazole resistance, coded by integron 3' conserved sequence. Its circulation may have been positively selected by widespread use of co-trimoxazole. The dfrA15 cassette was also found in a conjugative plasmid in Angola.4 The aadA1 cassette has already been described in Guinea-Bissau and is the most frequently found resistance gene in the variable region of integrons.2 This determinant was plasmid located in isolate CZM7 from Somalia, along with dfrA15. Spectinomycin resistance may be directly associated with the aadA1 cassette, as shown by transfer of both the resistance and gene cassette into exconjugants. The aadA1 cassette was absent in CZM36, at the time the dominant epidemic strain in Somalia, which is characterized by a tetracycline resistance bearing plasmid.1
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The aadA1 cassette was present in all isolates from Mozambique. Dalsgaard et al.3 reported the presence of aadA2 in V. cholerae O1 isolated in migrant workers from Mozambique in South Africa. They deduced that the aadA2 cassette was circulating in Mozambique, but no further evidence was produced and this conclusion is not supported by our results. They also reported for the first time the presence of SXT in V. cholerae O1 clinical isolates in Africa. We now report the ubiquitous presence of ICEVchInd1-like in strains isolated in Zimbabwe, Swaziland and Mozambique in 1997–98. Most of the positive strains (13/15) were floR (CHL), strA and strB (STR), sul2 and dfrA1 positive, a pattern shared with ICEVchInd1 (see Table 1).
Among Mozambican strains, two showed a peculiar resistance gene profile: isolate 5594 contained only the floR and dfrA1 resistance genes and isolate 5556 contained the dfrA1 gene only. SXT ability to conjugate and conserve its genetic profile in the corresponding E. coli recipient was observed in 10 strains (see Table 1). The wide circulation of these elements in Africa, at least in the southern region, is also indicated by the discovery of a member of the R391 family in South Africa in environmental Providencia spp. in 1967.5,6 All these findings are consistent with the autonomous emergence and spread of the SXT/R391 family in Africa.6 This picture is intriguing, but is still a hypothesis. Further molecular evidence is needed to clarify the African ICE phylogeny and its correlation with the Indian phylogeny.
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None to declare.
Acknowledgements
We are grateful to V. Burrus for the valuable advice and comments on the manuscript, to A. Salvia for technical help and to M. J. Brian and J. Cliff for manuscript revision. This work was supported by MURST and by MAE-DGCS, Italy.
References
1
Coppo A, Colombo M, Pazzani C, et al. (1995) Vibrio cholerae in the horn of Africa: epidemiology, plasmids, tetracycline resistance gene amplification, and comparison between O1 and non-O1 strains. Am J Trop Med Hyg 53:351–9.
2
Dalsgaard A, Forslund A, Petersen A, et al. (2000) Class 1 integron-borne, multiple-antibiotic resistance encoded by a 150-kilobase conjugative plasmid in epidemic Vibrio cholerae O1 strains isolated in Guinea-Bissau. J Clin Microbiol 38:3774–9.
3
Dalsgaard A, Forslund A, Sandvang D, et al. (2001) Vibrio cholerae O1 outbreak isolates in Mozambique and South Africa in 1998 are multiple-drug resistant, contain the SXT element and the aadA2 gene located on class 1 integrons. J Antimicrob Chemother 48:827–38.
4
Ceccarelli D, Salvia AM, Sami J, et al. (2006) A new cluster of plasmid-located class 1 integrons in V. cholerae O1 and dfrA15 cassette-containing integron in V. parahaemolyticus isolated in Angola. Antimicrob Agents Chemother 50:2493–9.
5 Hochhut B and Waldor MK. (1999) Site-specific integration of the conjugal Vibrio cholerae SXT element into prfC. Mol Microbiol 32:99–110.[CrossRef][ISI][Medline]
6 Burrus V, Marrero J, Waldor MK. (2006) The current ICE age: biology and evolution of SXT-related integrative conjugative elements. Plasmid 55:173–83.[CrossRef][ISI][Medline]
7 Folgosa E, Mastrandrea S, Cappuccinelli P, et al. (2001) Molecular identification of pathogenicity genes and ERIC types in Vibrio cholerae O1 epidemic strains from Mozambique. Epidemiol Infect 127:17–25.[Medline]
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