John W. Foster

Bio: John W. Foster is an academic researcher from University of South Alabama. The author has contributed to research in topics: rpoS & Escherichia coli. The author has an hindex of 55, co-authored 91 publications receiving 12124 citations.

Escherichia coli acid resistance: tales of an amateur acidophile

Abstract: Gastrointestinal pathogens are faced with an extremely acidic environment. Within moments, a pathogen such as Escherichia coli O157:H7 can move from the nurturing pH 7 environment of a hamburger to the harsh pH 2 milieu of the stomach. Surprisingly, certain microorganisms that grow at neutral pH have elegantly regulated systems that enable survival during excursions into acidic environments. The best-characterized acid-resistance system is found in E. coli.

Control of acid resistance in Escherichia coli.

Abstract: Acid resistance (AR) in Escherichia coli is defined as the ability to withstand an acid challenge of pH 2.5 or less and is a trait generally restricted to stationary-phase cells. Earlier reports described three AR systems in E. coli. In the present study, the genetics and control of these three systems have been more clearly defined. Expression of the first AR system (designated the oxidative or glucose-repressed AR system) was previously shown to require the alternative sigma factor RpoS. Consistent with glucose repression, this system also proved to be dependent in many situations on the cyclic AMP receptor protein. The second AR system required the addition of arginine during pH 2.5 acid challenge, the structural gene for arginine decarboxylase (adiA), and the regulator cysB, confirming earlier reports. The third AR system required glutamate for protection at pH 2.5, one of two genes encoding glutamate decarboxylase (gadA or gadB), and the gene encoding the putative glutamate:γ-aminobutyric acid antiporter (gadC). Only one of the two glutamate decarboxylases was needed for protection at pH 2.5. However, survival at pH 2 required both glutamate decarboxylase isozymes. Stationary phase and acid pH regulation of the gad genes proved separable. Stationary-phase induction of gadA and gadB required the alternative sigma factor ςS encoded by rpoS. However, acid induction of these enzymes, which was demonstrated to occur in exponential- and stationary-phase cells, proved to be ςS independent. Neither gad gene required the presence of volatile fatty acids for induction. The data also indicate that AR via the amino acid decarboxylase systems requires more than an inducible decarboxylase and antiporter. Another surprising finding was that the ςS-dependent oxidative system, originally thought to be acid induced, actually proved to be induced following entry into stationary phase regardless of the pH. However, an inhibitor produced at pH 8 somehow interferes with the activity of this system, giving the illusion of acid induction. The results also revealed that the AR system affording the most effective protection at pH 2 in complex medium (either Luria-Bertani broth or brain heart infusion broth plus 0.4% glucose) is the glutamate-dependent GAD system. Thus, E. coli possesses three overlapping acid survival systems whose various levels of control and differing requirements for activity ensure that at least one system will be available to protect the stationary-phase cell under naturally occurring acidic environments.

Mechanisms of acid resistance in enterohemorrhagic Escherichia coli.

Abstract: Enterohemorrhagic strains of Escherichia coli must pass through the acidic gastric barrier to cause gastrointestinal disease. Taking into account the apparent low infectious dose of enterohemorrhagic E. coli, 11 O157:H7 strains and 4 commensal strains of E. coli were tested for their abilities to survive extreme acid exposures (pH 3). Three previously characterized acid resistance systems were tested. These included an acid-induced oxidative system, an acid-induced arginine-dependent system, and a glutamate-dependent system. When challenged at pH 2.0, the arginine-dependent system provided more protection in the EHEC strains than in commensal strains. However, the glutamate-dependent system provided better protection than the arginine system and appeared equally effective in all strains. Because E. coli must also endure acid stress imposed by the presence of weak acids in intestinal contents at a pH less acidic than that of the stomach, the ability of specific acid resistance systems to protect against weak acids was examined. The arginine- and glutamate-dependent systems were both effective in protecting E. coli against the bactericidal effects of a variety of weak acids. The acids tested include benzoic acid (20 mM; pH 4.0) and a volatile fatty acid cocktail composed of acetic, propionic, and butyric acids at levels approximating those present in the intestine. The oxidative system was much less effective. Several genetic aspects of E. coli acid resistance were also characterized. The alternate sigma factor RpoS was shown to be required for oxidative acid resistance but was only partially involved with the arginine- and glutamate-dependent acid resistance systems. The arginine decarboxylase system (including adi and its regulators cysB and adiY) was responsible for arginine-dependent acid resistance. The results suggest that several acid resistance systems potentially contribute to the survival of pathogenic E. coli in the different acid stress environments of the stomach (pH 1 to 3) and the intestine (pH 4.5 to 7 with high concentrations of volatile fatty acids). Of particular importance to the food industry was the finding that once induced, the acid resistance systems will remain active for prolonged periods of cold storage at 4 degrees C.

Comparative analysis of extreme acid survival in Salmonella typhimurium, Shigella flexneri, and Escherichia coli

Abstract: Several members of the family Enterobacteriaceae were examined for differences in extreme acid survival strategies. A surprising degree of variety was found between three related genera. The minimum growth pH of Salmonella typhimurium was shown to be significantly lower (pH 4.0) than that of either Escherichia coli (pH 4.4) or Shigella flexneri (pH 4.8), yet E. coli and S. flexneri both survive exposure to lower pH levels (2 to 2.5) than S. typhimurium (pH 3.0) in complex medium. S. typhimurium and E. coli but not S. flexneri expressed low-pH-inducible log-phase and stationary-phase acid tolerance response (ATR) systems that function in minimal or complex medium to protect cells to pH 3.0. All of the organisms also expressed a pH-independent general stress resistance system that contributed to acid survival during stationary phase. E. coli and S. flexneri possessed several acid survival systems (termed acid resistance [AR]) that were not demonstrable in S. typhimurium. These additional AR systems protected cells to pH 2.5 and below but required supplementation of minimal medium for either induction or function. One acid-inducible AR system required oxidative growth in complex medium for expression but successfully protected cells to pH 2.5 in unsupplemented minimal medium, while two other AR systems important for fermentatively grown cells required the addition of either glutamate or arginine during pH 2.5 acid challenge. The arginine AR system was only observed in E. coli and required stationary-phase induction in acidified complex medium. The product of the adi locus, arginine decarboxylase, was responsible for arginine-based acid survival.

Adaptive acidification tolerance response of Salmonella typhimurium.

Abstract: Salmonella typhimurium can encounter a wide variety of environments during its life cycle. One component of the environment which will fluctuate widely is pH. In nature, S. typhimurium can experience and survive dramatic acid stresses that occur in diverse ecological niches ranging from pond water to phagolysosomes. However, in vitro the organism is very sensitive to acid. To provide an explanation for how this organism survives acid in natural environments, the adaptive ability of S. typhimurium to become acid tolerant was tested. Logarithmically grown cells (pH 7.6) shifted to mild acid (pH 5.8) for one doubling as an adaptive procedure were 100 to 1,000 times more resistant to subsequent strong acid challenge (pH 3.3) than were unadapted cells shifted directly from pH 7.6 to 3.3. This acidification tolerance response required protein synthesis and appears to be a specific defense mechanism for acid. No cross protection was noted for hydrogen peroxide, SOS, or heat shock. Two-dimensional polyacrylamide gel electrophoretic analysis of acid-regulated polypeptides revealed 18 proteins with altered expression, 6 of which were repressed while 12 were induced by mild acid shifts. An avirulent phoP mutant was 1,000-fold more sensitive to acid than its virulent phoP+ parent, suggesting a correlation between acid tolerance and virulence. The Mg2(+)-dependent proton-translocating ATPase was also found to play an important role in acid tolerance. Mutants (unc) lacking this activity were unable to mount an acid tolerance response and were extremely acid sensitive. In contrast to these acid-sensitive mutants, a constitutively acid-tolerant mutant (atr) was isolated from wild-type LT2 after prolonged acid exposure. This mutant overexpressed several acidification tolerance response polypeptides. The data presented reveal an important acidification defense modulon with broad significance toward survival in biologically hostile environments.

Pathogenic Escherichia coli

Abstract: Few microorganisms are as versatile as Escherichia coli. An important member of the normal intestinal microflora of humans and other mammals, E. coli has also been widely exploited as a cloning host in recombinant DNA technology. But E. coli is more than just a laboratory workhorse or harmless intestinal inhabitant; it can also be a highly versatile, and frequently deadly, pathogen. Several different E. coli strains cause diverse intestinal and extraintestinal diseases by means of virulence factors that affect a wide range of cellular processes.

Molecular Basis of Bacterial Outer Membrane Permeability Revisited

Abstract: Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Bacterial iron homeostasis

Abstract: Iron is essential to virtually all organisms, but poses problems of toxicity and poor solubility. Bacteria have evolved various mechanisms to counter the problems imposed by their iron dependence, allowing them to achieve effective iron homeostasis under a range of iron regimes. Highly efficient iron acquisition systems are used to scavenge iron from the environment under iron-restricted conditions. In many cases, this involves the secretion and internalisation of extracellular ferric chelators called siderophores. Ferrous iron can also be directly imported by the G protein-like transporter, FeoB. For pathogens, host–iron complexes (transferrin, lactoferrin, haem, haemoglobin) are directly used as iron sources. Bacterial iron storage proteins (ferritin, bacterioferritin) provide intracellular iron reserves for use when external supplies are restricted, and iron detoxification proteins (Dps) are employed to protect the chromosome from iron-induced free radical damage. There is evidence that bacteria control their iron requirements in response to iron availability by down-regulating the expression of iron proteins during iron-restricted growth. And finally, the expression of the iron homeostatic machinery is subject to iron-dependent global control ensuring that iron acquisition, storage and consumption are geared to iron availability and that intracellular levels of free iron do not reach toxic levels.

Pathways of Oxidative Damage

Abstract: The phenomenon of oxygen toxicity is universal, but only recently have we begun to understand its basis in molecular terms. Redox enzymes are notoriously nonspecific, transferring electrons to any good acceptor with which they make electronic contact. This poses a problem for aerobic organisms, since molecular oxygen is small enough to penetrate all but the most shielded active sites of redox enzymes. Adventitious electron transfers to oxygen create superoxide and hydrogen peroxide, which are partially reduced species that can oxidize biomolecules with which oxygen itself reacts poorly. This review attempts to present our still-incomplete understanding of how reactive oxygen species are formed inside cells and the mechanisms by which they damage specific target molecules. The vulnerability of cells to oxidation lies at the root of obligate anaerobiosis, spontaneous mutagenesis, and the use of oxidative stress as a biological weapon.