Shigella flexneri serotype 1c derived from serotype 1a by acquisition of gtrIC gene cluster via a bacteriophage.

BACKGROUND: Shigella spp. are the primary causative agents of bacillary dysentery. Since its emergence in the late 1980s, the S. flexneri serotype 1c remains poorly understood, particularly with regard to its origin and genetic evolution. This article provides a molecular insight into this novel serotype and the gtrIC gene cluster that determines its unique immune recognition. RESULTS: A PCR of the gtrIC cluster showed that serotype 1c isolates from different geographical origins were genetically conserved. An analysis of sequences flanking the gtrIC cluster revealed remnants of a prophage genome, in particular integrase and tRNA(Pro) genes. Meanwhile, Southern blot analyses on serotype 1c, 1a and 1b strains indicated that all the tested serotype 1c strains may have had a common origin that has since remained distinct from the closely related 1a and 1b serotypes. The identification of prophage genes upstream of the gtrIC cluster is consistent with the notion of bacteriophage-mediated integration of the gtrIC cluster into a pre-existing serotype. CONCLUSIONS: This is the first study to show that serotype 1c isolates from different geographical origins share an identical pattern of genetic arrangement, suggesting that serotype 1c strains may have originated from a single parental strain. Analysis of the sequence around the gtrIC cluster revealed a new site for the integration of the serotype converting phages of S. flexneri. Understanding the origin of new pathogenic serotypes and the molecular basis of serotype conversion in S. flexneri would provide information for developing cross-reactive Shigella vaccines.

HIV-1 mother-to-child transmission, post-test counselling, and antiretroviral prophylaxis in Northern Viet Nam: a prospective observational study.

BACKGROUND: Pregnant women fear being identified as HIV-1-infected and this has hampered prevention programmes and the calculation of transmission rates in Viet Nam. We introduced post-test counselling, antiretroviral prophylaxis, and formula feeding, and determined the vertical transmission rate in parts of Northern Viet Nam. METHODS: HIV infection was identified in 234 pregnant women; 182 (77.8%) accepted follow-up of their children. Counselling was given on 3-7 occasions for altogether approximately 6 h on antiretroviral prophylaxis and formula feeding to avoid transmission, and on the importance of surveillance of the child. All children were formula-fed. A polymerase chain reaction (PCR) was used for the diagnosis of HIV-1 in the children. One hundred and thirty-five of the 182 mothers allowed ≥ 3 blood samples to be taken from birth to ≥ 1 y of age, 32/182 provided a birth sample only, and 15/182 provided a sample later only. Nevirapine was given at delivery to 93/135 (69%) women, and to 128/135 (95%) children. Additionally, combination therapy was given to 15/135 (11%) who entered the study before delivery, and azidothymidine to their children for 1 week. RESULTS: Nine of 135 (6.7%) children became infected and 2/15 of the others, giving altogether 11/150 infected (7.3%). Intrauterine transmission was identified in 7/167 (4.2%) children by a positive PCR test at birth. PCR was negative at birth but positive at 1 month in 2/135 (1.5%), pointing to delivery-associated transmission. Thus, intrauterine transmission accounted for 78% (7/9). None of the uninfected children died, but 3/11 (p =0.004) of the HIV-1-infected died (in AIDS). CONCLUSION: Post-test confidential counselling, formula feeding, and antiretroviral prophylaxis resulted in low rates of delivery-associated and late HIV-1 transmissions.

A novel glucosyltransferase involved in O-antigen modification of Shigella flexneri serotype 1c.

The O antigen of serotype 1c differs from the unmodified O antigen of serotype Y by the addition of a disaccharide (two glucosyl groups) to the tetrasaccharide repeating unit. It was shown here that addition of the first glucosyl group is mediated by the previously characterized gtrI cluster, which is found within a cryptic prophage at the proA locus in the bacterial chromosome. Transposon mutagenesis was performed to disrupt the gene responsible for addition of the second glucosyl group, causing reversion to serotype 1a. Colony immunoblotting was used to identify the desired revertants, and subsequent sequencing, cloning, and functional expression successfully identified the gene encoding serotype 1c-specific O-antigen modification. This gene (designated gtrIC) was present as part of a three-gene cluster, similar to other S. flexneri glucosyltransferase genes. Relative to the other S. flexneri gtr clusters, the gtrIC cluster is more distantly related and appears to have arrived in S. flexneri from outside the species. Analysis of surrounding sequence suggests that the gtrIC cluster arrived via a novel bacteriophage that was subsequently rendered nonfunctional by a series of insertion events.