Energetic basis of cadmium toxicity in Staphylococcus aureus

Cd(2+) extrusion by P-type Cd(2+)-ATPase of Staphylococcus aureus 17810R via energy-dependent Cd(2+)/H(+) exchange mechanism

Cd²⁺ is highly toxic to Staphylococcus aureus since it blocks dithiols in cytoplasmic 2-oxoglutarate dehydrogenase complex (ODHC) participating in energy conservation process. However, S. aureus 17810R is Cd²⁺-resistant due to possession of cadA-coded Cd²⁺ efflux system, recognized here as P-type Cd²⁺-ATPase. This Cd²⁺ pump utilizing cellular energy—ATP, ∆μ_H⁺ (electrochemical proton potential) and respiratory protons, extrudes Cd²⁺ from cytoplasm to protect dithiols in ODHC, but the mechanism of Cd²⁺ extrusion remains unknown. Here we propose that two Cd²⁺ taken up by strain 17810R via Mn²⁺ uniporter down membrane potential (∆ψ) generated during glutamate oxidation in 100 mM phosphate buffer (high P_iB) are trapped probably by high affinity sites in cytoplasmic domain of Cd²⁺-ATPase, forming SCdS. This stops Cd²⁺ transport towards dithiols in ODHC, allowing undisturbed NADH production, its oxidation and energy conservation, while ATP could change orientation of SCdS towards facing transmembrane channel. Now, increased number of P_i-dependent protons pumped electrogenically via respiratory chain and countertransported through the channel down ∆ψ, extrude two trapped cytoplasmic Cd²⁺, which move to low affinity sites, being then extruded into extracellular space via ∆ψ-dependent Cd²⁺/H⁺ exchange. In 1 mM phosphate buffer (low P_iB), external Cd²⁺ competing with decreased number of P_i-dependent protons, binds to ψ_s of Cd²⁺-ATPase channel, enters cytoplasm through the channel down ∆ψ via Cd²⁺/Cd²⁺ exchange and blocks dithiols in ODHC. However, Mg²⁺ pretreatment preventing external Cd²⁺ countertransport through the channel down ∆ψ, allowed undisturbed NADH production, its oxidation and extrusion of two cytoplasmic Cd²⁺ via Cd²⁺/H⁺ exchange, despite low P_iB.

Resistance index of penicillin-resistant bacteria to various physicochemical agents

Widespread use of various antimicrobial agents resulted in the emergence of bacterial resistance. Mechanisms like direct efflux, formation, and sequestration of metals and drugs in complexes and antiporter pumps are some examples. This investigation aims to investigate the resistance pattern of penicillin-resistant bacterial strains to some physicochemical agents. Sensitivity/resistance pattern of common bacterial strains to antimicrobial agents were evaluated by disk diffusion assay. Broth and agar dilution method were used for determination of minimum inhibitory concentration and minimal bactericidal concentration. The impact of UV ray on the bacterial growth under laminar flow hood was measured using photonmeter. Our data demonstrates that the most prevalent metal resistance was against arsenate (95.92%), followed by cadmium (52.04%) and mercury (36.73%). There was significant difference between cetrimide resistances among studied microbial strains especially for P. aeruginosa (P < 0.05). High rate of pathogen resistance to various antibacterial agents in our study supports previously published data. This great rate of bacterial resistance is attributed to the emergence of defense mechanisms developed in pathogens. The higher general bacterial resistance rate among Staphylococcus strains rather than E. coli and P. aeruginosa strains draws attention towards focusing on designing newer therapeutic compounds for Staphylococcus strains.

Presynaptic malfunction: the neurotoxic effects of cadmium and lead on the proton gradient of synaptic vesicles and glutamate transport

Exposure to Cd(2+) and Pb(2+) has neurotoxic consequences for human health and may cause neurodegeneration. The study focused on the analysis of the presynaptic mechanisms underlying the neurotoxic effects of non-essential heavy metals Cd(2+) and Pb(2+). It was shown that the preincubation of rat brain nerve terminals with Cd(2+) (200 μM) or Pb(2+) (200 μM) resulted in the attenuation of synaptic vesicles acidification, which was assessed by the steady state level of the fluorescence of pH-sensitive dye acridine orange. A decrease in L-[(14)C]glutamate accumulation in digitonin-permeabilized synaptosomes after the addition of the metals, which reflected lowered L-[(14)C]glutamate accumulation by synaptic vesicles inside of synaptosomes, may be considered in the support of the above data. Using isolated rat brain synaptic vesicles, it was found that 50 μM Cd(2+) or Pb(2+) caused dissipation of their proton gradient, whereas the application of essential heavy metal Mn(2+) did not do it within the range of the concentration of 50-500 μM. Thus, synaptic malfunction associated with the influence of Cd(2+) and Pb(2+) may result from partial dissipation of the synaptic vesicle proton gradient that leads to: (1) a decrease in stimulated exocytosis, which is associated not only with the blockage of voltage-gated Ca(2+) channels, but also with incomplete filling of synaptic vesicles; (2) an attenuation of Na(+)-dependent glutamate uptake.

Cd bioaccumulation by a freshwater bacterium, Rhodospirillum rubrum

Cd bioaccumulation by Rhodospirillum rubrum, a Gram-negative freshwater bacterium, was studied in a synthetic medium. The free ion (Cd2+) was the best predictor of the Cd internalization fluxes. Representation of the short-term uptake fluxes as a function of [Cd2+] in the medium demonstrated a linear relationship, as would be expected for a rate-limiting, first-order internalization with a single transporter. Nonetheless, several different accumulation profiles were observed, depending on the Cd concentration. Cd uptake was regulated differently for concentrations above and below 10(-6) M (or was regulated only above [Cd2+] = 10(-6) M). Short-and long-term studies revealed that regulation was rapidly initiated for the highest Cd concentrations examined, effectively decreasing both adsorbed and internalized Cd. Anodic stripping voltammetry demonstrated that a Cd complexing ligand was produced within minutes upon exposure to 5 x 10(-6) M Cd2+ and that an extracellular sequestration of Cd was one mechanism regulating Cd uptake. Competition studies with other cations revealed a competitive inhibition of Cd uptake by Zn and an uptake enhancement in the presence of Mn and Cu.

Cadmium-dependent enzyme activity alteration is not imputable to lipid peroxidation

The effect of cadmium on the liver-specific activities of NADPH-cytochrome P450 reductase (CPR), malic dehydrogenase (MDH), glyceraldehyde-3-phosphate dehydrogenase (GADPH), and sorbitol dehydrogenase (SDH) was assessed 6, 24, and 48 h after administration of the metal to rats (2.5 mg/kg of body weight, as CdCl2, single ip injection). CPR specific activity increased after 6 h and afterward decreased significantly, while MDH specific activity increased up to 24 h and then remained unchanged. Both SDH and GADPH specific activities reduced after 6 h, the former only a little but the latter much more, and after 24 and 48 h were strongly inhibited. In vitro experiments, by incubating rat liver microsomes, mitochondria, or cytosol with CdCl2 in the pH range 6.0-8.0, excluded cadmium-induced lipid peroxidation as the cause of the reduction in enzyme activity. In addition, from these experiments, we obtained indications on the type of interactions between cadmium and the enzymes studied. In the case of CPR, the inhibitory effect is probably due to Cd2+ binding to the histidine residue of the apoenzyme, which, at physiological pH, acts as a nucleophilic group. In vitro, mitochondrial MDH was not significantly affected by cadmium at any pH, indicating that this enzyme is probably not involved in the decrease in mitochondrial respiration caused by this metal. As for GADPH specific activity, its inhibition at pH 7.4 and above is imputable to the binding of cadmium to the SH groups present in the enzyme active site, since in the presence of dithiothreitol this inhibition was removed. SDH was subjected to a dual effect when cytosol was exposed to cadmium. At pH 6.0 and 6.5, its activity was strongly stimulated up to 75 microM CdCl2 while at higher metal concentrations it was reduced. At pH 7.4 and 8.0, a stimulation up to 50 microM CdCl2 occurred but above this concentration, a reduction was found. These data seem to indicate that cadmium can bind to different enzyme sites. One, at low cadmium concentration, stimulates the SDH activity while the other, at higher metal concentrations, substitutes for zinc, thus causing inhibition. This last possibility seems to occur in vivo essentially at least 24 h after intoxication. The cadmium-induced alterations of the investigated enzymes are discussed in terms of the metabolic disorders produced which are responsible for several pathological conditions.

Energy conservation in aerobically grown Staphylococcus aureus

The present studies provide new data on the involvement of menaquinol oxidases in substrate oxidation and energy conservation in aerobically grown, resting cells of Staphylococcus aureus 17810R, starved of endogenous energy reserves and supplemented with glutamate or L-lactate. These cells were energetically competent, since they oxidized both substrates, generated an electrochemical proton gradient (deltamuH+) and synthesized ATP via oxidative phosphorylation. Studies with KCN showed that: (i) L-lactate oxidation occurred via two terminal menaquinol oxidases - the ba3-type sensitive to low KCN and the bo-type insensitive to cyanide, (ii) glutamate oxidation proceeded via the bo-type oxidase, and (iii) ATP synthesis with glutamate or L-lactate was coupled only to the bo-type oxidase. Also in glucose-grown cells oxidizing L-lactate, ATP synthesis was coupled to the highly repressed bo-type oxidase. It is suggested that in the respiratory chain of strain 17810R two energy coupling sites may be present: in the complex of NADH-menaquinone oxidoreductase and in the complex of the bo-type menaquinol oxidase. The rate of ATP synthesis was similar with both substrates, but the rate of their oxidation differed significantly: the P/O ratios were 1.5 and 0.03 with glutamate and L-lactate, respectively. CCCP accelerated glutamate oxidation by 50% but was without effect on L-lactate oxidation. In cell lysates, the rates of NADH and L-lactate oxidation were equal. It is concluded that in whole cells of S. aureus 17810R oxidation of NADH derived from glutamate breakdown is tightly coupled to phosphorylation, while L-lactate oxidation seems to be rather loosely coupled.

Substrate-dependent cadmium toxicity affecting energy-linked K+/86Rb transport in Staphylococcus aureus

Bacteria accumulate high amounts of potassium in the cytoplasm. For studying transport of K+ (with 86Rb as a marker) in bacteria (Staphylococcus aureus 17810S), the cells were depleted of the internal K+ pool by a DNP treatment. Kinetics and energetics of 86Rb transport was assayed with glucose as an exogenous energy source. It was shown that 86Rb uptake proceeded via a low affinity K+ transport system with an apparent K(m) of 2.3 mmol/L Rb+. Studies with the lipophilic cation TPP+ (tetraphenylphosphonium), the protonophore CCCP (carbonyl cyanide 3-chlorophenylhydrazone) and inhibitors (HQNO--2-heptyl-4-hydroxyquinoline N-oxide; iodoacetate) indicated that 86Rb transport was driven by delta psi (membrane potential) generated via the respiratory chain. The effect of Cd2+ on 86Rb transport was assayed with two energy donors--glucose and L-lactate. It was found that Cd2+ strongly inhibited delta psi-dependent 86Rb transport energized by cadmium-sensitive glucose oxidation, but was not toxic when cadmium-insensitive L-lactate was used as an energy source. The mechanism of these differential, substrate-dependent effects of Cd2+ on 86Rb transport is discussed.

Cadmium-sensitive targets in the aerobic respiratory metabolism of Staphylococcus aureus

Studies on the effect of various Cd2+ concentrations on substrate oxidation by whole cells of cadmium-sensitive Staphylococcus aureus 17810S showed that oxidation of glutamate or pyruvate was highly sensitive to low Cd2+ concentrations (5 microM), whereas L-lactate oxidation was insensitive even to high Cd2+ concentrations (100 microM). Location of the cadmium-sensitive targets in the enzyme systems involved in oxidation of these substrates was studied in subcellular fractions prepared from cells pretreated with 5 or 100 microM Cd2+. Activities of the cytoplasmic 2-oxoglutarate dehydrogenase complex (ODHC)') and pyruvate dehydrogenase complex (PDHC) were strongly inhibited with 5 microM Cd2+, while with 100 microM Cd2+ the inhibition was almost complete. In contrast, activities of the cytoplasmic NAD-dependent glutamate dehydrogenase (NAD-GDH), the membrane-bound NADH dehydrogenase (NDH) and HQNO-sensitive NADH oxidase were not sensitive to 100 microM Cd2+. These data indicate that the accessible, cadmium-sensitive targets are located only in the cytoplasmic ODHC and PDHC. It is postulated that two vicinal dithiols present in ODHC and PDHC may be regarded as the primary cadmium-sensitive targets in the systems oxidizing glutamate or pyruvate. Since activities of the membrane-bound NAD-independent L-lactate dehydrogenase (iLDH) and HQNO-sensitive L-lactate oxidase were not affected by 100 microM Cd2+, this indicates that the L-lactate oxidizing system lacks the accessible, cadmium-sensitive targets. The mechanism of Cd2+ toxicity to energy conservation with glutamate, pyruvate or L-lactate in S. aureus is discussed.