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Antibiotic Resistance Conferred by a Class I Integron and SXT Constin in Vibrio cholerae O1 Strains Isolated in Laos. Masaaki Iwanaga, 2004.Changes in the drug susceptibility pattern were observed in Vibrio cholerae O1 isolated in the Lao People's Democratic Republic during 1993 to 2000 . In this study, 50 V . cholerae O1 strains were selected during this period for studying the presence of class I integron and SXT constin . Twenty-four streptomycin-resistant strains out of 26 isolated before 1997 contained a class I integron harboring the aadA1 gene cassette . Twenty-four strains isolated after 1997 contained an SXT constin (a large conjugative element) . Twenty of the strains were resistant to chloramphenicol, tetracycline, streptomycin, and trimethoprim-sulfamethoxazole, while four strains were susceptible to the antibiotic tested . The resistance genes included in the SXT constins were floR, tetA, strAB, and sulII, which encode resistance to chloramphenicol, tetracycline, streptomycin, and sulfamethoxazole, respectively . The antibiotic resistance gene cluster was found to be deleted in the four susceptible strains . SXTLAOS did not contain dfrA1 or dfr18, which confer resistance to trimethoprim in SXTET and SXTMO10, respectively . A hot spot region of SXTLAOS was sequenced, and we identified two novel open reading frames showing homology to sO24 (exonuclease) and sO23 (helicase) of the genomic island associated with the multidrug resistance region of Salmonella enterica serovar Typhimurium DT104 . Analysis of SXTLAOS showed that there is a continuous flux of genes among V . cholerae SXT constins which should be carefully monitored . Molecular Genetics of Bacteria and Phages, 2001. Ry Young, 2002. Photobactin: a Catechol Siderophore Produced by Photorhabdus luminescens, an Entomopathogen Mutually Associated with Heterorhabditis bacteriophora NC1 Nematodes. Todd A. Ciche, 2003.The nematode Heterorhabditis bacteriophora transmits a monoculture of Photorhabdus luminescens bacteria to insect hosts, where it requires the bacteria for efficient insect pathogenicity and as a substrate for growth and reproduction . Siderophore production was implicated as being involved in the symbiosis because an ngrA mutant inadequate for supporting nematode growth and reproduction was also deficient in producing siderophore activity and ngrA is homologous to a siderophore biosynthetic gene, entD . The role of the siderophore in the symbiosis with the nematode was determined by isolating and characterizing a mini-Tn5-induced mutant, NS414, producing no detectable siderophore activity . This mutant, being defective for growth in iron-depleted medium, was normal in supporting nematode growth and reproduction, in transmission by the dauer juvenile nematode, and in insect pathogenicity . The mini-Tn5 transposon was inserted into phbH; whose protein product is a putative peptidyl carrier protein homologous to the nonribosomal peptide synthetase VibF of Vibrio cholerae . Other putative siderophore biosynthetic and transport genes flanking phbH were characterized . The catecholate siderophore was purified, its structure was determined to be 2-(2,3-dihydroxyphenyl)-5-methyl-4,5-dihydro-oxazole-4-carboxylic acid [4-(2,3-dihydroxybenzoylamino)-butyl]-amide, and it was given the generic name photobactin . Antibiotic activity was detected with purified photobactin, indicating that the siderophore may contribute to antibiosis of the insect cadaver . These results eliminate the lack of siderophore activity as the cause for the inadequacy of the ngrA mutant in supporting nematode growth and reproduction .
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