Use of polymyxin B, levallorphan, and tetracaine to isolate novel envelope mutants of Escherichia coli

Outer membrane permeability and antibiotic resistance

To date most antibiotics are targeted at intracellular processes, and must be able to penetrate the bacterial cell envelope. In particular, the outer membrane of gram-negative bacteria provides a formidable barrier that must be overcome. There are essentially two pathways that antibiotics can take through the outer membrane: a lipid-mediated pathway for hydrophobic antibiotics, and general diffusion porins for hydrophilic antibiotics. The lipid and protein compositions of the outer membrane have a strong impact on the sensitivity of bacteria to many types of antibiotics, and drug resistance involving modifications of these macromolecules is common. This review will describe the molecular mechanisms for permeation of antibiotics through the outer membrane, and the strategies that bacteria have deployed to resist antibiotics by modifications of these pathways.

A pmrA constitutive mutant sensitizes Escherichia coli to deoxycholic acid

An Escherichia coli mutant was isolated and shown to be polymyxin B resistant. Mapping and sequence analysis revealed a missense mutation at codon 53 within the pmrA (basR) gene that results in a G-to-V substitution. Fusions of promoters from the pmrC, yibD, and pmrH genes with the lacZ reporter showed that they were constitutively expressed in pmrA53 cells. In pmrA+ strains, these promoters were induced by iron and zinc, while a DeltapmrA mutation blocked induction. The PmrA regulon regulates genes whose products remodel the composition and charge of lipid A and hence the barrier properties of the outer membrane. Along these lines, the pmrA53 mutant was also found to be hypersensitive to the anionic bile detergent deoxycholic acid.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Isolation and characterization of the Escherichia coli msbB gene, a multicopy suppressor of null mutations in the high-temperature requirement gene htrB

Previous work established that the htrB gene of Escherichia coli is required for growth in rich media at temperatures above 32.5 degrees C but not at lower temperatures. In an effort to determine the functional role of the htrB gene product, we have isolated a multicopy suppressor of htrB, called msbB. The msbB gene has been mapped to 40.5 min on the E. coli genetic map, in a 12- to 15-kb gap of the genomic library made by Kohara et al. (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987). Mapping data show that the order of genes in the region is eda-edd-zwf-pykA-msbB. The msbB gene codes for a protein of 37,410 Da whose amino acid sequence is similar to that of HtrB and, like HtrB, the protein is very basic in nature. The similarity of the HtrB and MsbB proteins could indicate that they play functionally similar roles. Mutational analysis of msbB shows that the gene is not essential for E. coli growth; however, the htrB msbB double mutant exhibits a unique morphological phenotype at 30 degrees C not seen with either of the single mutants. Analysis of both msbB and htrB mutants shows that these bacteria are resistant to four times more deoxycholate than wild-type bacteria but not to other hydrophobic substances. The addition of quaternary ammonium compounds rescues the temperature-sensitive phenotype of htrB bacteria, and this rescue is abolished by the simultaneous addition of Mg2+ or Ca2+. These results suggest that MsbB and HtrB play an important role in outer membrane structure and/or function.

Increased outer membrane resistance to ethylenediaminetetraacetate and cations in novel lipid A mutants

Polymyxin-resistant pmrA mutants of Salmonella typhimurium differed from their parents in that they were resistant to tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetate-lysozyme, tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetate-deoxycholate, and tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetate-bacitracin. Tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetate released about 50% less lipopolysaccharide from the pmrA strains than from the parental strains when the bacteria were grown in L-broth containing 2 mM Ca2+. Protamine, polylysine, octapeptin, benzalkonium chloride, cold NaCl, cold MgCl2, or cold tris(hydroxymethyl)aminomethane hydrochloride (pH 7.2) caused no leakage or markedly less leakage of periplasmic beta-lactamase from a pmrA mutant than from its parent strain. pmrA mutants were more resistant than their parent strains to protamine and polylysine but not to octapeptin or benzalkonium chloride, as measured by the ability of these agents to kill the bacteria or to sensitize them to deoxycholate-induced lysis. The pmrA strains did not differ from their parent strains in sensitivity to several antibiotics, in porin function (as measured by cephaloridine diffusion across the outer membrane), or in outer membrane-associated phospholipase A activity.

Sodium dodecyl sulfate-sensitive septation in a mitomycin C-sensitive, mtc, mutant of Escherichia coli

A mitomycin C-sensitive, mtc, mutant of Escherichia coli has an altered cell surface and is sensitive to sodium dodecyl sulfate (SDS). The mutant, M27, formed multinucleate nonseptated filaments in the presence of a low concentration of SDS (50 microgram/ml). When the culture grown at that concentration of SDS was diluted with an SDS-free medium, the filaments began to divide at a very rapid rate after a lag of about 20 min. Chloramphenicol inhibited this recovery division when added within 10 min after SDS dilution but did not inhibit the division when added 20 min after dilution. Penicillin G at a low concentration, which is enough to cause filamentation, had virtually no effect on the recovery division of SDS-induced filaments. The division of penicillin G-induced filaments was inhibited by SDS.

Decreased binding of polymyxin by polymyxin-resistant mutants of Salmonella typhimurium

Polymyxin-resistant pmrA strains were shown to absorb only about 25% of the amount of polymyxin absorbed by the corresponding polymyxin-sensitive parent strains. The lipopolysaccharide from the pmrA strains bound less polymyxin than the lipopolysaccharide from the parent strains.

Levallorphan-tolerant mutants of Excherichia coli with altered morphologies

The penetration of levallorphan, a synthetic morphinan known to interact with the cytoplasmic membrane of E. coli, is seriously limited by the outer membrane. To select target-resistant mutants rather than outer membrane mutants, a two-step procedure was developed, which involved the selection by penicillin of an "intermediate" parental strain with a decreased penetration barrier and a subsequent positive selection of levallorphan-tolerant pseudo-revertant clones. Unlike the direct selection, this technique yielded various types of mutants in which the morphology, the septation ability, and the growth rate were greatly affected.

Lipid and lipopolysaccharide composition of Escherichia coli surface-altered mutants selected for resistance to levallorphan, tetracaine, and polymyxin

Certain mutants of Escherichia coli with an altered permeability barrier have an essentially normal lipopolysaccharide, fatty acid, and phospholipid content, with a slight increase in the membrane protein:lipid ratio. The phospholipid metabolism of the lev and tec strains shows an abnormal response to growth in the selective agents levallorphan and tetracaine, respectively.