RESEARCH HIGHLIGHTS

BACTERIAL PHYSIOLOGY IN BRIEF

Ever-decreasing circles E NVIRONMENTAL MICROBIOLOGY Role of metal-reducing bacteria in arsenic release from It’s generally accepted that the single degraded — proving that the method Bengal delta sediments bacterial chromosome of Escherichia works. Expression of individual SSGs Islam, F. S. et al. Nature 430, 68–71 (2004) coli is organized into approximately was quantitated using RNase protec- The contamination of groundwater by the release of arsenic from 100 equal-sized domains of about tion. The results were compared with sediments poses a major threat to human health in some parts of 50–100 kb. However, Postow et al.now Monte Carlo simulations of different Asia, particularly Bangladesh and West Bengal. The mechanisms report in Genes and Development that models of domain organization and responsible for this release have been hotly debated. Now, Islam the chromosome is packed into small, closely matched a model in which the and colleagues report in Nature that their research using a variable domains, with an average chromosome is packed into randomly laboratory microcosm approach has provided direct evidence that size of just 10 kb, that are randomly placed domains with an average anaerobic metal-reducing bacteria are involved in the formation positioned around the chromosome. size of 9–11 kb. Gentle chromosome of toxic As in sediments collected in West Bengal. They also Bacterial chromosomes are con- preparation and analysis with electron (III) obtained data to support the theory that the introduction of densed about 1,000-fold, in part by microscopy confirmed these experi- carbon, for example by irrigation techniques, could be enhancing DNA gyrase which introduces nega- mental observations — domains are the mobility of arsenic in shallow groundwater in this area. tive supercoils, so that they fit inside much smaller than was previously the cell. Supercoiled DNA is arranged thought. into domains and stabilized by pro- Domain barriers could be formed BACTERIAL TOXINS teins and possibly RNAs, so that one by association with the cell membrane, strand break doesn’t unwind the other parts of the chromosome or A new family of potent AB5 cytotoxins produced by whole chromosome, which would kill with macromolecular complexes such Shiga toxigenic Escherichia coli the cell. as DNA or RNA polymerases. As the Paton, A. W. et al. J. Exp. Med. (June 28 2004) doi: 10.1084/jem.20040392 By inhibiting DNA gyrase, more domain size is already known to than 300 supercoiling-sensitive genes increase when the cell enters station- Toxins belonging to the AB5 toxin family are potent virulence (SSGs) were identified in E. coli. ary phase and global transcript levels factors produced by some bacterial pathogens such as Shiga Postow et al. realized that monitoring decrease, formation of transient toxigenic E. coli and Shigella dysenteriae (Shiga toxin), and the expression of the SSG gene-set domains dependent on local DNA Bordetella pertussis (pertussis toxin). AB5 toxins comprise an might allow chromosome domain activity is an appealing model. enzymatic A subunit and a pentameric B subunit that is size to be accurately delineated. They The domain size determines chro- responsible for docking the toxin onto target eukaryotic host cells. used an inducible restriction enzyme mosome compaction, which affects In this work, Paton et al.characterize a new member of the AB5 to randomly cut the chromosome, DNA repair, transcription and repli- toxin family from a highly virulent Shiga toxigenic E. coli strain. then monitored levels of SSG tran- cation. Small domains are more easily The A subunit of this new toxin has a subtilase-like activity, the scripts using microarrays. The dis- repaired because loose ends from first time that a cytotoxic effect has been associated with this tance from random cut sites to the breaks aren’t too far apart. As domains serine protease family. The authors speculate that this new toxin domain boundary will affect the form the basis of chromosome orga- could work synergistically with Shiga toxin in the pathogenesis of extent of local unwinding (relaxation) nization from bacteria to man, under- Shiga toxigenic E. coli. of the supercoils, so that the level of an standing the organization of the E. coli SSG transcript will only change if the chromosome is relevant to all of us. E NVIRONMENTAL MICROBIOLOGY cut site is in the same domain as the Susan Jones References and links SSG. Differences in SSG transcript Molecular analysis of shower curtain biofilm microbes abundance were detected well before ORIGINAL RESEARCH PAPER Postow, L. et al. Topological domain structure of the Escherichia coli Kelly, S. T. et al. Appl. Environ. Microbiol. 70, 4187–4192 (2004) the chromosome was completely chromosome. Genes Dev. 18, 1766–1779 (2004) When did you last clean your shower curtain? Over time, vinyl shower curtains accumulate a deposit commonly known as soap scum. In a new study published in Applied and Environmental Microbiology,Kelly et al. used a variety of techniques to analyse this material from four shower curtains that had been in use for more than 6 months. Epifluorescence microscopy demonstrated that soap scum contains a rich variety of microorganisms within a biofilm matrix. DNA from the shower curtain communities was extracted and sequenced. rRNA analysis revealed that the composition of each microbial community was complex, with Sphingomonas spp. and Methylobacterium spp. among the most prominent members. The results of this study indicate that shower curtains could be a source of opportunistic pathogens and so should be cleaned regularly, particularly in households with immunosuppressed individuals.

NATURE REVIEWS | MICROBIOLOGY VOLUME 2 | SEPTEMBER 2004 | 687