Adaptive mutation is an induced response to environmental stress in which mutation rates rise, producing permanent genetic changes that can adapt cells to stress. This contrasts with neo-Darwinian views of genetic change rates blind to environmental conditions. DNA amplification is a flexible, reversible genomic change that has long been postulated to be adaptive. We report the discovery of adaptive amplification at the lac operon in Escherichia coli. Additionally, we find that adaptive amplification is separate from, and does not lead to, adaptive point mutation. This contradicts a prevailing alternative hypothesis whereby adaptive mutation is normal mutability in amplified DNA. Instead, adaptive mutation and amplification are parallel routes of inducible genetic instability allowing rapid evolution under stress, and escape from growth inhibition.

Several microbial systems have been shown to yield advantageous mutations in slowly growing or nongrowing cultures. In one assay system, the stationary-phase mutation mechanism differs from growth-dependent mutation, demonstrating that the two are different processes. This system assays reversion of a lac frameshift allele on an F plasmid in Escherichia coli. The stationary-phase mutation mechanism at lac requires recombination proteins of the RecBCD double-strand-break repair system and the inducible errorprone DNA polymerase IV, and the mutations are mostly ؊1 deletions in small mononucleotide repeats. This mutation mechanism is proposed to occur by DNA polymerase errors made during replication primed by recombinational double-strand-break repair. It has been suggested that this mechanism is confined to the F plasmid. However, the cells that acquire the adaptive mutations show hypermutation of unrelated chromosomal genes, suggesting that chromosomal sites also might experience recombination protein-dependent stationary-phase mutation. Here we test directly whether the stationary-phase mutations in the bacterial chromosome also occur via a recombination protein-and pol IV-dependent mechanism. We describe an assay for chromosomal mutation in cells carrying the F lac. We show that the chromosomal mutation is recombination protein-and pol IV-dependent and also is associated with general hypermutation. The data indicate that, at least in these male cells, recombination protein-dependent stationaryphase mutation is a mechanism of general inducible genetic change capable of affecting genes in the bacterial chromosome.Escherichia coli ͉ adaptive mutation ͉ SOS response ͉ DNA repair

Antibodies are required to control blood-stage forms of African trypanosomes in humans and animals. Here, we report that intradermal infections by low numbers of African trypanosomes are controlled by innate resistance but prime the adaptive immune response to increase susceptibility to a subsequent challenge. Mice were found 100 times more resistant to intradermal infections by Trypanosoma congolense or Trypanosoma brucei than to intraperitoneal infections. B cell–deficient and RAG2−/− mice are as resistant as wild-type mice to intradermal infections, whereas inducible nitric oxide synthase (iNOS)−/− mice and wild-type mice treated with antibody to tumor necrosis factor (TNF) α are more susceptible. We conclude that primary intradermal infections with low numbers of parasites are controlled by innate defense mediated by induced nitric oxide (NO). CD1d−/− and major histocompatibility complex (MHC) class II−/− mice are more resistant than wild-type mice to primary intradermal infections. Trypanosome-specific spleen cells, as shown by cytokine production, are primed as early as 24 h after intradermal infection. Infecting mice intradermally with low numbers of parasites, or injecting them intradermally with a trypanosomal lysate, makes mice more susceptible to an intradermal challenge. We suggest that intradermal infections with low numbers of trypanosomes or injections with trypanosomal lysates prime the adaptive immune system to suppress protective immunity to an intradermal challenge.

SUMMARYThe results of 193 transjugular liver biopsies performed with a modified needle are described. An adequate specimen was obtained in 97%, and complications were rare, although puncture of the liver capsule does occur and caused bleeding in two patients. Fever after the procedure was reduced by ultrasonic cleaning of the needle. Although not easy, this technique is safe and preferable in the management of selected patients, but in most patients percutaneous biopsy is to be preferred.Liver biopsy via the internal transjugular vein was first described in 1973' as an extension of transjugular cholangiography. It was advocated in patients with abnormal blood clotting in whom the risk of percutaneous biopsy was unacceptable. The rigidity of the initial modified Ross transeptal needle, however, made biopsy difficult, and histological specimens from patients with cirrhosis often fragmented during aspiration. 1-3 We overcame these disadvantages with a modified Vim-Silverman (Trucut) needle mounted on flexible coaxial cable;4 this resulted in less technical difficulties in obtaining a biopsy, and larger, less fragmented specimens. More recently a modified Menghini needle has been developed with bevelling of the tip to prevent damage to the catheter and reduce compression artefacts of the biopsy specimen.5 We now report our experience of transjugular liver biopsy using our St Thomas' modified needle in 193 patients biopsied during the last three years. Methods PATIENTSThe indications for the transjugular approach are shown in Table 1. Over half were performed because of abnormal blood clotting, usually due to liver disease, but sometimes because patients were receiving anticoagulant drugs.The method has been previously described in detail.

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