Improved O2-tolerance in variants of a H2-evolving [NiFe]-hydrogenase from Klebsiella oxytoca HP1

In this study, we investigated the mechanism of O2 tolerance of Klebsiella oxytoca HP1 H2-evolving hydrogenase 3 (KHyd3) by mutational analysis and three-dimensional structure modeling. Results revealed that certain surface amino acid residues of KHyd3 large subunit, in particular those at the outer entrance of the gas channel, have a visible effect on its oxygen tolerance. Additionally, solution pH, immobilization and O2 partial pressure also affect KHyd3 O2-tolerance to some extent. We propose that the extent of KHyd3 O2-tolerance is determined by a balance between the rate of O2 access to the active center through gas channels and the deoxidation rate of the oxidized active center. Based on our findings, two higher O2-tolerant KHyd3 mutations G300E and G300M were developed.

Enzymatic saccharification of cassava residues and glucose inhibitory kinetics on β-glucosidase from Hypocrea orientalis

Cassava residues are byproducts of the starch industry containing abundant cellulose for bioproduction of green fuel. To obtain maximum sugar yields from cassava residues, the optimal conditions for hydrolyzing the residues were determined using cellulase prepared from a novel Hypocrea orientalis strain. The optimal pH value and optimal temperature for the cellulase hydrolysis were 5.0 and 50 °C, respectively. The concentration of NaOH was determined to be 1% for pretreatment of cassava residues to gain enough soluble sugars suitably. The yield of released sugars was 10 mg/mL in the optimal conditions after 24 h of reaction, which was similar to that of bagasse and wheat grass. Inhibition kinetics of H. orientalis β-glucosidase (BG) by glucose was first studied using the progress-of-substrate-reaction method as described by Tsou (Tsou, C. L. Adv. Enzymol. Related Areas Mol. Biol. 1988, 61, 381-436), and the microscopic inhibition rate constants of glucose were determined. The results showed that glucose could inhibit BG reversibly and competitively. The rate constants of forward (k(+0)) and reverse (k(-0)) reaction were measured to be 4.88 × 10(-4) (mM·s)(-1) and 2.7 × 10(-4) s(-1), respectively. Meanwhile, the inhibition was more significant than that of L-glucose, D-mannose, D-galactose, D-aminoglucose, acetyl-D-glucose, and D-fructose. This work reveals how to increase sugar yields and reduce product inhibition during enzymatic saccharification of cellulose.

Cellulase hydrolysis of rice straw and inactivation of endoglucanase in urea solution

In order to optimize the cellulase (from Aspergillus glaucus) hydrolysis of pretreated rice straw, the effects of varying enzyme concentration, temperature, and pH were studied. The best experimental conditions found to degrade the pretreated rice straws were 24 h of incubation at 55 °C and pH 5.0, with an enzyme concentration of 48 mg/L. Urea is one of the important nitrogen sources used in fungi culture, but it is also a denaturant. The model of denaturation of endoglucanase (EG) in urea solutions was established. The denaturation was a slow, reversible reaction. Determination of microscopic rate constants showed k(+0) > k'(+0), indicating that EG was protected by the substrate to a certain extent during denaturation. Comparison with the results from fluorescence emission spectroscopy revealed that the inactivation of EG occurred before the marked conformational changes could be detected.

Purification and properties of endoglucanase from a sugar cane bagasse hydrolyzing strain, Aspergillus glaucus XC9

An endoglucanase (EG) from Aspergillus glaucus XC9 grown on 0.3% sugar cane bagasse as a carbon source was purified from the culture filtrate using ammonium sulfate, an anion exchange DEAE Sepharose fast flow column, and a Sephadex G-100 column, with a purification fold of 21.5 and a recovery of 22.3%. The ideal time for EG production is on the fourth day at 30 degrees C using bagasse as a substrate. Results obtained indicate that the enzyme was a monomer protein, and the molecular weight was determined to be 31 kDa. The optimum pH and temperature of EG for the hydrolysis of carboxymethylcellulose sodium (CMC-Na) were pH 4.0 and 50 degrees C, respectively. EG was stable over the pH range from 3.5 to 7.5 and at temperatures below 55 degrees C. Kinetic behavior of EG in the hydrolysis of CMC-Na followed Michaelis-Menten kinetics with constant K(m) of 5.0 mg/mL at pH 4.0 and 50 degrees C. The enzyme activity was stimulated by Fe(2+) and Mn(2+) but inhibited by Cd(2+), Pb(2+), and Cu(2+). The EDC chemical modification suggested that at least one carboxyl group probably acted as a proton donor in the enzyme active site.

[Isolation and characterization of H2-producing strains Enterobacter sp. and Clostridium sp]

Two hydrogen-producing bacterial strains were newly isolated and identified as Enterobacter sp. Z-16 and Clostridium sp. C-32 by 16S rDNA sequence analysis. Various parameters for hydrogen production, including substrates, initial pH and temperature, have been studied. The optimum condition for hydrogen production of strain Z-16 were achieved as: initial pH7.0, temperature 35 degrees C , sucrose as the favorite substrate. In comparison, The optimum condition for hydrogen production of strain C-32 were obtained as: initial pH8.0, temperature 35 degrees C , maltose as the favorite substrate . Under batch fermentative hydrogen production conditions, the maximal hydrogen conversion rate for strain Z-16 and strain C-32 were 2.68 mol H2/mol sucrose and 2.71mol H2/mol maltose, respectively. Using glucose as substrate, the hydrogen conversion rate of strain Z-16 and strain C-32 were 2.35 and 2.48 mol H2/mol glucose, respectively. This research suggest a good application potential of strain Z-16 and C-32 in the future biological hydrogen production.

[Recent advances on the structure and catalytic mechanism of hydrogenase]

Hydrogenases are enzymes that catalyse the oxidation of hydrogen and the reduction of protons. It plays an important role in the process of biohydrogen production. According to the metal atoms within hydrogenase, it can be classified as NiFe-hydrogenase, Fe-hydrogenase and metal-free hydrogenase. The overwhelming majority of hydrogenases are metalloenzymes. The metal atoms are involved in the forming of active site and [Fe-S] clusters. The active site directly catalyzes the reduction of protons and the oxidation of hydrogen. The [Fe-S] clusters are involved in the transport of electrons between the H2-activating site and the redox partners of hydrogenase. Presently, the crystal structures of NiFe-hydrogenase and Fe-hydrogenase from a few kinds of microorganism have been revealed. The metal-free hydrogenase, characterized by the absence of [Fe-S] cluster and the presence of an iron-containing cofactor, shows a great diversity comparing with those of NiFe-hydrogenases and Fe-hydrogenases. Recent progress have also indicated the mechanisms of activation.

Prospective, randomized study of ventilator-associated pneumonia in patients with one versus three ventilator circuit changes per week

OBJECTIVE

To assess the effect on the rate of ventilator-associated pneumonia (VAP) of decreasing the frequency of ventilator circuit changes from three times to once per week.

DESIGN

Prospective, randomized trial.

SETTING

Medical intensive care unit (MICU), a 12-bed, critical-care internal medicine unit, and neurosciences intensive care unit (NICU), a 21-bed, predominantly adult neurosurgical unit, of an urban university hospital.

PATIENTS

All 447 patients requiring mechanical ventilation during October 1992 through June 1993.

INTERVENTION

Patients were allocated randomly on the basis of permanent medical record numbers: those with odd numbers had circuits changed three times per week, those with even numbers once per week. Intensive-care-unit surveillance was conducted in accordance with definitions and methods of the National Nosocomial Infections Surveillance System.

RESULTS

In the MICU, the one-change-per-week group had a VAP rate of 7.3 per 1,000 ventilator days, versus 5.9 for the three-per-week group (P = .6). In the NICU, the one-change-per-week group had a rate of 12.2 per 1,000 ventilator days, versus 12.6 for the three-per-week group (P = .9). Considering patients in both units ventilated for no more than 7 days, the one-change-per-week group had a VAP rate of 5.9 per 1,000 ventilator days, versus 9.0 per 1,000 for the three-changes-per-week group (odds ratio [OR], 0.65; 95% confidence interval [CI95], 0.25 to 1.69). Including patients in the two units maintained on mechanical ventilation for more than 7 days, the one-change-per-week group had a VAP rate of 13.2 per 1,000 ventilator days, versus 9.6 per 1,000 for the three-changes-per-week group (OR, 1.37; CI95, 0.71 to 2.65).

CONCLUSIONS

Decreasing the frequency of ventilator circuit changes from three times to once per week had no adverse effect on the overall rate of VAP. Less frequent ventilator circuit changes may decrease the incidence of VAP among patients ventilated for no more than 1 week. However, the incidence of VAP may be higher among patients with once weekly circuit changes ventilated for more than 1 week.

Higher overall nosocomial infection rate because of increased attack rate of methicillin-resistant Staphylococcus aureus

BACKGROUND

Methicillin-resistant Staphylococcus aureus has become a frequent cause of hospital-acquired infectious disease. However, the impact of methicillin-resistant S. aureus on the overall nosocomial infection rate has not been clearly defined.

METHODS

The University of Alabama at Birmingham Hospital is a tertiary care facility and participant in the National Nosocomial Infections Surveillance system. Prospective, hospital-wide surveillance for nosocomial infections was conducted for 8 months per year during 1986 through 1991.

RESULTS

The total nosocomial infection rate increased from 4.5% in 1987 to 5.9% in 1990. This was temporally associated with a rise in the attack rate of methicillin-resistant S. aureus from 0% to 0.4%. The remainder of the increase was caused by other gram-positive microorganisms, including methicillin-sensitive S. aureus. We identified methicillin-resistant S. aureus most commonly in the surgical intensive care unit and as a cause of lower respiratory tract infections. In contrast, methicillin-sensitive S. aureus was most frequently found in the neurologic intensive care unit and as the etiologic agent of surgical wound infections.

CONCLUSIONS

Methicillin-resistant S. aureus has become a common nosocomial pathogen. There has been no decrease in the attack rate of methicillin-sensitive S. aureus or other gram-positive cocci. The total nosocomial infection rate has increased as a result equally of the emergence of methicillin-resistant S. aureus and of the renewed activity of other gram-positive pathogens. Methicillin-resistant S. aureus has added to the overall burden of nosocomial infectious disease.