Streptococcus faecalis mutants defective in regulation of cytoplasmic pH

Cytoplasmic pH measurement and homeostasis in bacteria and archaea

Of all the molecular determinants for growth, the hydronium and hydroxide ions are found naturally in the widest concentration range, from acid mine drainage below pH 0 to soda lakes above pH 13. Most bacteria and archaea have mechanisms that maintain their internal, cytoplasmic pH within a narrower range than the pH outside the cell, termed "pH homeostasis." Some mechanisms of pH homeostasis are specific to particular species or groups of microorganisms while some common principles apply across the pH spectrum. The measurement of internal pH of microbes presents challenges, which are addressed by a range of techniques under varying growth conditions. This review compares and contrasts cytoplasmic pH homeostasis in acidophilic, neutralophilic, and alkaliphilic bacteria and archaea under conditions of growth, non-growth survival, and biofilms. We present diverse mechanisms of pH homeostasis including cell buffering, adaptations of membrane structure, active ion transport, and metabolic consumption of acids and bases.

Assessment of viability of microorganisms employing fluorescence techniques

Viability assessment of microorganisms is relevant for a wide variety of applications in industry, including evaluation of inactivation treatments and quality assessment of starter cultures for beer, wine, and yoghurt production. Usually, the ability of microbial cells to reproduce is considered as the benchmark method for determination of cell viability, and this is most commonly determined by the plate count method. The time needed to form visible colonies, however, is relatively long. Therefore, there is an increasing interest in rapid methods which exploit criteria other than reproduction. In this review the applications of fluorescent probes for, e.g., determination of membrane integrity, enzyme activities, respiration, membrane potential and intracellular pH, are discussed in detail.

Isolation and properties of Enterococcus hirae mutants defective in the potassium/proton antiport system

A K+/H+ antiporter regulates cytoplasmic pH in Enterococcus hirae growing at alkaline pH. Mutants defective in this antiport activity were alkaline pH sensitive. One mutant, Pop1, lacked both K+/methylamine exchange at pH 9.5 and concomitant acidification of cytoplasmic pH. Pop1 grew well at pHs below 8 but did not at pHs above 9, conditions under which cytoplasmic pH was not fully acidified.

Inorganic cation transport and energy transduction in Enterococcus hirae and other streptococci

Energy metabolism by bacteria is well understood from the chemiosmotic viewpoint. We know that bacteria extrude protons across the plasma membrane, establishing an electrochemical potential that provides the driving force for various kinds of physiological work. Among these are the uptake of sugars, amino acids, and other nutrients with the aid of secondary porters and the regulation of the cytoplasmic pH and of the cytoplasmic concentration of potassium and other ions. Bacteria live in diverse habitats and are often exposed to severe conditions. In some circumstances, a proton circulation cannot satisfy their requirements and must be supplemented with a complement of primary transport systems. This review is concerned with cation transport in the fermentative streptococci, particularly Enterococcus hirae. Streptococci lack respiratory chains, relying on glycolysis or arginine fermentation for the production of ATP. One of the major findings with E. hirae and other streptococci is that ATP plays a much more important role in transmembrane transport than it does in nonfermentative organisms, probably due to the inability of this organism to generate a large proton potential. The movements of cations in streptococci illustrate the interplay between a variety of primary and secondary modes of transport.

The Na+-responsive ntp operon is indispensable for homeostatis of K+ and Na+ in Enterococcus hirae at limited proton potential

Enterococcus hirae ATCC 9790 grew well in Na+-deficient, low-K+ medium, but growth was inhibited by carbonylcyanide m-chlorophenylhydrazone (CCCP). Growth inhibition and decrease of cellular K+ levels in the presence of CCCP were relieved by the addition of Na+ and a high concentration of K+. In contrast, in the mutant defective in Na+-ATPase or the NtpJ component of the KtrII K+ uptake system, CCCP-induced growth inhibition was rescued by a high concentration of K+ but not of Na+. These transporters are thus indispensable for homeostatis of K+ and Na+ at low proton potential.

Characterization of a mutant of Lactococcus lactis with reduced membrane-bound ATPase activity under acidic conditions

A mutant of Lactococcus lactis subsp. lactis C2 with reduced membrane-bound ATPase activity was characterized to clarify its acid sensitivity. The cytoplasmic pH of the mutant was measured in reference to the parental strain under various pH conditions. At low pH, the mutant could not maintain its cytoplasmic pH near neutral, and lost its viability faster than the parental strain. The ATPase activities of cells cultured under neutral and acidic conditions using pH-controlled jar fermentors were measured. The relative ATPase activity of the mutant at pH 7.0 was 42% of the parental strain. At pH 4.5, the parental strain showed an ATPase activity 2.8-fold higher than that at pH 7.0 while the level of increase in the mutant was only 1.6. Northern and Western blot analyses found that at pH 7.0 the transcriptional level and the amount of F1 beta subunit were similar in both strains, suggesting that the mutant has a defective ATPase structural gene. On the other hand, at pH 4.5 the transcriptional level and the amount of F1 beta subunit were found to be significantly higher in both strains than those at pH 7.0. From these results, it was suggested that the mutant has a normal regulation system for ATPase gene expression. It was concluded that the mutant is acid sensitive due to its inability to extrude protons out of the cell with defective ATPase under acidic conditions.

Electrogenic Na+ transport by Enterococcus hirae Na(+)-ATPase

Energy-dependent generation of a membrane potential (delta psi) (-45 mV, interior negative) was observed in the F0F1, H(+)-ATPase-defective mutant of Enterococcus hirae. The generation of delta psi was found at high pH (but not at low pH), for which intracellular Na+ was required but not extracellular K+. The delta psi-generating activity was induced in cells cultured in media containing high concentrations of Na+, and was not observed in the Na(+)-ATPase mutants. These results suggest that E. hirae Na(+)-ATPase is responsible for the electrogenic sodium pump.

Complementation of an Enterococcus hirae (Streptococcus faecalis) mutant in the alpha subunit of the H(+)-ATPase by cloned genes from the same and different species

We isolated an Enterococcus hirae (formerly Streptococcus faecalis) mutant, designated MS117, in which 'G' at position 301 of the alpha-subunit gene of the F1F0 type of H(+)-ATPase was deleted. MS117 had low H(+)-ATPase activity, was deficient in the regulatory system of cytoplasmic pH, and was unable to grow at pH 6.0. When the alpha-subunit gene of E. hirae H(+)-ATPase was ligated with the shuttle vector pHY300PLK at the downstream region of the tet gene of the vector, it was expressed without its own promoter in MS117, and the mutation of MS117 was complemented; the mutant harbouring the plasmid had the ability to maintain a neutral cytoplasm and grew at pH 6.0. We next transformed MS117 with pHY300PLK containing the alpha-subunit gene of Bacillus megaterium F1F0-ATPase constructed in the same way. The transformant grew at pH 6.0, and the ATP hydrolysis activity was recovered. These results suggested that an active hybrid H(+)-ATPase containing the B. megaterium alpha subunit was produced, and that the hybrid enzyme regulated the enterococcal cytoplasmic pH, although the function of the B. megaterium enzyme did not include pH regulation. Thus, our present results support the previous proposal that the enterococcal cytoplasmic pH is regulated by the F1F0 type of H(+)-ATPase.

Studies of the Physiological and Genetic Basis of Acid Tolerance in Rhizobium leguminosarum biovar trifolii

Acid-tolerant Rhizobium leguminosarum biovar trifolii ANU1173 was able to grow on laboratory media at a pH as low as 4.5. Transposon Tn 5 mutagenesis was used to isolate mutants of strain ANU1173, which were unable to grow on media at a pH of less than 4.8. The acid-tolerant strain ANU1173 maintained a near-neutral intracellular pH when the external pH was as low as 4.5. In contrast, the acid-sensitive mutants AS25 and AS28 derived from ANU1173 had an acidic intracellular pH when the external pH was less than 5.5. The acid-sensitive R. leguminosarum biovar trifolii ANU794, which was comparatively more sensitive to low pH than mutants AS25 and AS28, showed a more acidic internal pH than the two mutants when the three strains were exposed to medium buffered at a pH of less than 5.5. The two acid-sensitive mutants had an increased membrane permeability to protons but did not change their proton extrusion activities. However, the acid-sensitive strain ANU794 exhibited both a higher membrane permeability to protons and a lower proton extrusion activity compared with the acid-tolerant strain ANU1173. DNA hybridization analysis showed that mutants AS25 and AS28 carried a single copy of Tn 5 located in 13.7-kb (AS25) and 10.0-kb (AS28) Eco RI DNA fragments. The wild-type DNA sequences spanning the mutation sites of mutants AS25 and AS28 were cloned from genomic DNA of strain ANU1173. Transfer of these wild-type DNA sequences into corresponding Tn 5 -induced acid-sensitive mutants, respectively, restored the mutants to their acid tolerance phenotypes. Mapping studies showed that the AS25 locus was mapped to a 5.6-kb Eco RI- Bam HI megaplasmid DNA fragment, whilst the AS28 locus was located in an 8.7-kb Bgl II chromosomal DNA fragment.

Involvement of Genes on a Megaplasmid in the Acid-Tolerant Phenotype of Rhizobium leguminosarum Biovar Trifolii

The acid-tolerant Rhizobium leguminosarum biovar trifolii strain ANU1173 exhibited several new phenotypes when cured of its symbiotic (Sym) plasmid and the second largest megaplasmid. Strain P22, which has lost these two plasmids, had reduced exopolysaccharide production and cell mobility on TY medium. The parent strain ANU1173 was able to grow easily in laboratory media at pH 4.5, whereas the derivative strain P22 was unable to grow in media at a pH of <4.7. The intracellular pH of strain ANU1173 was 6.8 when the external pH was 4.5. In contrast, strain P22 had an acidic intracellular pH of <6.4 when the external pH was <5.5. Strain P22 had a dramatically increased membrane permeability to protons and decreased proton extrusion activity. Analysis with sodium dodecyl sulfate-polyacrylamide gels showed that strain P22 lacked a slow-migrating lipopolysaccharide (LPS) banding group which was present in the parent strain. Mobilization of the second largest megaplasmid of strain ANU1173 back into strain P22 restored the altered LPS structure and physiological characteristics of strain P22. Mobilization of the Sym plasmid of strain ANU1173 into strain P22 showed that the second largest megaplasmid of strain ANU1173 was required for the establishment of nitrogen-fixing nodules on Trifolium repens and Trifolium subterraneum . Furthermore, an examination of a large number of specific exopolysaccharide- or LPS-deficient Rhizobium mutants did not show a positive correlation between exopolysaccharide or LPS synthesis and acid tolerance.

Permeation of bacterial cells, permeation of cytoplasmic and artificial membrane vesicles, and channel formation on lipid bilayers by peptide antibiotic AS-48

Peptide AS-48 induces ion permeation, which is accompanied by the collapse of the cytoplasmic membrane potential, in sensitive bacteria. Active transport by cytoplasmic membrane vesicles is also impaired by AS-48. At low concentrations, this peptide also causes permeability of liposomes to low-molecular-weight compounds without a requirement for a membrane potential. Higher antibiotic concentrations induce severe disorganization, which is visualized under electron microscopy as aggregation and formation of multilamellar structures. Electrical measurements suggest that AS-48 can form channels in lipid bilayers.

Release of the component of Streptococcus faecalis Na(+)-ATPase from the membranes

The Na(+)-stimulated ATPase activity of Streptococcus faecalis was lost by washing the membranes with ethylenediaminetetraacetic acid (EDTA). ATPase activities of both the EDTA extract and the stripped membranes did not show any stimulation by Na+ ions. However, the Na(+)-stimulated ATPase was readily reconstituted by an incubation of these fractions combined. It was only reconstituted from the fractions prepared under the condition that the Na(+)-ATPase is amplified, and not from those boiled or digested by trypsin. Thus, the component of Na(+)-ATPase of this organism is capable of being released from the membranes.

Maintenance of Intracellular pH and Acid Tolerance in Rhizobium meliloti

The development and function of the Rhizobium meliloti-Medicago sp. symbiosis are sensitive to soil acidity. Physiological criteria that can be measured in culture which serve to predict acid tolerance in soil would be valuable. The intracellular pH of R. meliloti was measured using either radioactively labeled weak acids (5,5-dimethyloxazolidine-2,4-dione and butyric acid) or pH-sensitive fluorescent compounds; both methods gave similar values. Six acid-tolerant strains (WSM419, WSM533, WSM539, WSM540, WSM852, and WSM870) maintained an alkaline intracellular pH when the external pH was between 5.6 and 7.2. In contrast, two Australian commercial inoculant strains (CC169 and U45) and four acid-sensitive strains from alkaline soils in Iraq (WSM244, WSM301, WSM365, and WSM367) maintained an alkaline intracellular pH when the external pH was ≥6.5, but had intracellular pH values of ≤6.8 when the external pH was ≤6.0. Four transposon Tn 5 -induced mutants of acid-tolerant strain WSM419, impaired in their ability to grow at pH 5.6, showed limited control over the intracellular pH. The ability to generate a large pH gradient under acid conditions may be a better indicator of acid tolerance in R. meliloti under field conditions than is growth on acidic agar plates.

Sodium/proton antiporter in Streptococcus faecalis

Streptococcus faecalis, like other bacteria, accumulates potassium ions and expels sodium ions. This paper is concerned with the pathway of sodium extrusion. Earlier studies (D.L. Heefner and F.M. Harold, Proc. Natl. Acad. Sci. USA 79:2798-2802, 1982) showed that sodium extrusion is effected by a primary, ATP-linked sodium pump. I report here that cells grown under conditions in which sodium ATPase is not induced can still expel sodium ions. This finding suggested the existence of an alternate pathway. Sodium extrusion by the alternate pathway requires the cells to generate a proton motive force. This conclusion rests on the following observations. (i) Sodium extrusion required glucose. (ii) Sodium extrusion was observed at neutral pH, which allows the cells to generate a proton motive force, but not at alkaline pH, which reduces the proton motive force to zero. (iii) Sodium extrusion was inhibited by the addition of dicyclohexylcarbodiimide and of proton-conducting ionophores. (iv) In response to an artificial pH gradient (with the exterior acid), energy-depleted cells exhibited a transient sodium extrusion which was unaffected by treatments that dissipated the membrane potential and which was blocked by proton conductors. I propose that streptococci have two independent systems for sodium extrusion: an inducible sodium ATPase and a constitutive sodium/proton antiporter.

Lowering of cytoplasmic pH is essential for growth of Streptococcus faecalis at high pH

The growth of Streptococcus faecalis at high pH was significantly stimulated by carbonate. In the absence of added carbonate the cells were unable to grow at a pH above 9.5, but in media containing 50 mM HCO3- they grew even at pH 10.5. Both rate and yield of growth at pH 9.5 were significantly stimulated by as little as 5 mM carbonate. The cytoplasmic pH in growing cells was maintained at about 7.8 to 8.2, whereas the medium pH ranged from 8.4 to 9.5. Nigericin and gramicidin D, ionophores which conduct protons, blocked growth at pH 9.5 but not at pH 7.5. These results indicate that lowering of the cytoplasmic pH is essential for the growth of this organism at high pH.

Proton motive force is not obligatory for growth of Escherichia coli

When 50 microM carbonyl cyanide-m-chlorophenyl hydrazone (CCCP), a protonophore, was added to growth medium containing glucose at pH 7.5, Escherichia coli TK1001 (trkD1 kdpABC5) started exponential growth after 30 min; the generation time was 70 min at 37 degrees C. Strain AS1 (acrA), another strain derived from E. coli K-12, also grew in the presence of 50 microM CCCP under the same conditions, except that the lag period was ca. 3 h. When this strain was grown in the presence of 50 microM CCCP and then transferred to fresh medium containing 50 microM CCCP, cells grew without any lag. Neither a membrane potential nor a pH gradient was detected in strain AS1 cells growing in the presence of CCCP. When either succinate or lactate was substituted for glucose, these strains did not grow in the presence of 50 microM CCCP. Thus, it is suggested that E. coli can grow in the absence of a proton motive force when glucose is used as an energy source at pH 7.5.

Amplification of the Streptococcus faecalis proton-translocating ATPase by a decrease in cytoplasmic pH

When Streptococcus faecalis was grown in the presence of protonophores , an ATPase activity of the membrane was increased at a pH below 8.0 but not at a pH above 8.0. Characteristics of this increased ATPase were identical to those of a proton-translocating ATPase (H+-ATPase) located on the membrane of normal cells. The cytoplasmic pH was regulated at 7.6 to 7.8 but was not regulated in the presence of protonophores . The increase in the H+-ATPase was observed when the cytoplasmic pH was lowered to less than 7.6 by the addition of protonophores and was not related to the dissipation of the proton motive force. Thus, we suggest that the H+-ATPase of the membrane is amplified when the cytoplasmic pH is lowered below the pH at which it is regulated under normal conditions.

Sodium-stimulated ATPase in Streptococcus faecalis

We measured Na+-stimulated ATPase activity in a mutant of Streptococcus faecalis defective in the generation of proton motive force. The activity in membrane vesicles was 62.1 +/- 5.9 nmol of phosphate produced per min per mg of protein when cells were grown on medium containing 0.12 M Na+. Activity decreased as the concentration of Na+ in the growth medium decreased. The decrease in enzyme activity corresponded to the decrease in transport activity for Na+ in both whole cells and membrane vesicles. The effects of pH on both activities were identical. Thus, it is suggested that Na+ movement is mediated by this enzyme. Sodium extrusion and ATPase activity in the wild-type strain were markedly lower than those observed in the mutant strain. Elevated activities of both Na+ extrusion and Na+-stimulated ATPase could be detected in the wild-type strain when cells were grown in the absence of proton motive force. Thus, we propose that the level of ATPase is increased by dissipation of the proton motive force.

The effects of weak acids on potassium uptake by Escherichia coli K-12 inhibition by low cytoplasmic pH

The activity of the Escherichia coli K+ transport system TrkA was measured as a function of the cytoplasmic pH of the cell. For this purpose, pHin was decreased by the addition of the weak acids acetic acid, benzoic acid or salicylic acid to K+-depleted cells. Under these conditions, the initial rate of K+ uptake decreased strongly with pHin, and was almost independent of the acid used. This inhibition was due to a strong decrease in the Vmax for K+ uptake, which indicates that low cytoplasmic pH inactivates the TrkA K+ uptake system. The relevance of this inhibition for growth and metabolism at low pHin is discussed.

Second system for potassium transport in Streptococcus faecalis

It has been reported that the accumulation of K+ by Streptococcus faecalis is mediated by a transport system which required both ATP and the proton motive force (Bakker and Harold, J. Biol. Chem. 255:433-440, 1980). My results indicate that S. faecalis has a second transport system for K+. The features of this system are as follows: (i) the system is driven by ATP (or a derivative of ATP) and does not require the proton motive force; (ii) the system is normally absent in the wild-type strain but can be derepressed by lowering rhe intracellular concentration of K+; (iii) the pH optimum of this system is about 8.5, and no detectable K+ is accumulated at pH values below 6.5; and (iv) the rate of Rb+ accumulation by this system is very low. These properties are quite different from those of the transport system described by Bakker and Harold. Therefore, I propose that S. faecalis has two K+ transport systems.

Internal pH and ATP-ADP pools in the cyanobacterium Synechococcus sp. during exposure to growth-inhibiting low pH

Y-7c-s Synechococcus thermophilic strain grew at its maximum rate at pH 8 and above. The growth rate of this strain was inhibited at pH 7.0 and below, and at pH 6.0 there was no sustained growth. At a suboptimal pH, high light intensity further depressed the growth rate. The inhibition of growth resulted neither from pheophytinization nor from a low chlorophyll content. At pH 5.0 a loss of viability preceded the appearance of pheophytin. Cells exposed to low, growth-inhibiting external pH levels continued to maintain a high internal pH (pH 7.1 to 7.3, as determined at moderate light intensities by 31P nuclear magnetic resonance spectroscopy). Even during exposure to pH 4.8, cells retained a relatively high internal pH. Thus, it appeared that the inhibition of growth at low pH was not caused by acidification of the cytoplasm. Darkened cells maintained a slightly lower internal pH than irradiated cells. The ATP/(ATP + ADP) ratio decreased from 0.80 to 0.82 at pH 8.0 to about 0.6 when growth was limited by exposure to pH 6.0 or by low light intensity. It is possible, but not likely, that a limitation of the energy supply may slow or stop growth when the external pH is lowered.