H. H. Yusef

Production of Aspergillus terreus xylanase in solid-state cultures : application of the Plackett-Burman experimental design to evaluate nutritional requirements

Abstract Xylanase was produced by Aspergillus terreus cultivated on finely ground wheat straw in solid-state fermentation. The optimal medium composition was developed by applying the Plackett–Burman experimental design. Best enzymic activity was obtained in a medium containing 10 g wheat straw/flask moistened with a concentrated nutrient salt solution to 75% initial water content and incubated for 4 days at 30°C. A. terreus xylanase was fractionated by ammonium sulfate precipitation and purified by chromatography on DEAE Bio-Gel A followed by gel-filtration on Sephadex G-75. The enzyme was characterized by apparent Vmax and Km values of 333.3 U/mg protein and 16.7 mg xylan/ml, respectively, obtained for xylanase with oat spelt xylan as substrate. The optimal pH and temperature for maximum activity were 7 and 50°C, respectively. The enzyme showed high specificity towards oat spelt xylan and minute activities were observed with carboxymethyl cellulose and cellobiose. About 48.02% of the activity remained after the enzyme had been incubated at 60°C for 30 min. Metal ions such as Hg2+, Cu2+, Co2+, Fe3+, Pb2+ strongly inhibited xylanase, whereas, Ca2+ activated the enzyme.

Diversity and cold-active hydrolytic enzymes of culturable bacteria associated with Arctic sea ice, Spitzbergen

The diversity of culturable bacteria associated with sea ice from four permanently cold fjords of Spitzbergen, Arctic Ocean, was investigated. A total of 116 psychrophilic and psychrotolerant strains were isolated under aerobic conditions at 4°C. The isolates were grouped using amplified rDNA restriction analysis fingerprinting and identified by partial sequencing of 16S rRNA gene. The bacterial isolates fell in five phylogenetic groups: subclasses α and γ of Proteobacteria, the Bacillus–Clostridium group, the order Actinomycetales, and the Cytophaga–Flexibacter–Bacteroides (CFB) phylum. Over 70% of the isolates were affiliated with the Proteobacteria γ subclass. Based on phylogenetic analysis (<98% sequence similarity), over 40% of Arctic isolates represent potentially novel species or genera. Most of the isolates were psychrotolerant and grew optimally between 20 and 25°C. Only a few strains were psychrophilic, with an optimal growth at 10–15°C. The majority of the bacterial strains were able to secrete a broad range of cold-active hydrolytic enzymes into the medium at a cultivation temperature of 4°C. The isolates that are able to degrade proteins (skim milk, casein), lipids (olive oil), and polysaccharides (starch, pectin) account for, respectively, 56, 31, and 21% of sea-ice and seawater strains. The temperature dependences for enzyme production during growth and enzymatic activity were determined for two selected enzymes, α-amylase and β-galactosidase. Interestingly, high levels of enzyme productions were measured at growth temperatures between 4 and 10°C, and almost no production was detected at higher temperatures (20–30°C). Catalytic activity was detected even below the freezing point of water (at −5°C), demonstrating the unique properties of these enzymes.

Antimicrobial and biocide resistance of bacteria in a Lebanese tertiary care hospital

The environment in a Lebanese tertiary-level care hospital was examined for bacterial contamination. In total, 8 open-air and 62 surface samples were taken and CFU were enumerated. Morphologically different isolates (n=104) were obtained and tested for resistance to a wide range of antibiotics by the Kirby-Bauer disc diffusion method. Minimum bactericidal concentrations of isolates to seven different biocides used within this facility were also determined using both qualitative and quantitative methods. Gram-positive bacteria (82.7% of isolates) were encountered more often than Gram-negative bacteria (17.3%), and the genus Staphylococcus was most prevalent among isolates (59.6% of the isolates). Gram-positive isolates showed the highest resistance to penicillin (60.6%), clindamycin (54.5%) and ceftazidime (39.4%); the lowest frequencies of resistance were seen with vancomycin (0% of isolates), nitrofurantoin (3.5%), tetracycline (12.1%) and ciprofloxacin (12.1%). The prevalence of meticillin-resistant Staphylococcus aureus (MRSA) and meticillin-resistant coagulase negative staphylococci among all environmental samples was 3.8% and 11.5% of isolates, respectively. All Gram-negative isolates were multidrug-resistant (MDR). With respect to biocides, the highest frequency of resistance was to quaternary ammonium compounds (QACs) (13.5% of isolates) and the lowest frequency of resistance was to QAC/biguanide complex (<1% of isolates) and Anios DVA (3.8%). MDR bacteria can pose a serious threat in the hospital environment, showing the need for prudent use of antibiotics and correct infection control procedures.