Transfer of persistent Listeria monocytogenes contamination between food-processing plants associated with a dicing machine

The possibility of the transfer of persistent Listeria monocytogenes contamination from one plant to another with a dicing machine was evaluated, and possible reasons for persistent contamination were analyzed. A dicing machine that diced cooked meat products was transferred from plant A to plant B and then to plant C. After the transfer of the dicing machine, L. monocytogenes PFGE type I, originally found in plant A, was soon also found in plants B and C. This L. monocytogenes PFGE type I caused persistent contamination of the dicing lines in plants B and C. The persistent L. monocytogenes strain and three nonpersistent L. monocytogenes strains found in the dicing line of plant C were tested for adherence to stainless steel surfaces and minimal inhibitory concentrations of a quaternary ammonium compound and sodium hypochlorite, disinfectants widely used in the dicing lines. The persistent strain showed significantly higher adherence to stainless steel surfaces than did the nonpersistent strains. The minimal inhibitory concentrations of sodium hypochlorite were similar for all strains, and the minimal inhibitory concentrations of the quaternary ammonium compound for three of the L. monocytogenes PFGE types, including the persistent PFGE type, were high. All persistent L. monocytogenes PFGE type I isolates were found in an area with high hygienic standards, with the dicing machine being the first point of contamination. These observations show that the dicing machine sustained the contamination and suggest that the dicing machine transferred the persistent L. monocytogenes PFGE type from one plant to another.     

Adaptive and cross-adaptive responses of persistent and non-persistent Listeria monocytogenes strains to disinfectants

Persistent and non-persistent Listeria monocytogenes strains were tested for initial resistance and adaptive and cross-adaptive responses towards two quaternary ammonium compounds, alkyl-benzyl-dimethyl ammonium chloride and n-alkyldimethyl ethylbenzyl ammonium chloride, one tertiary alkylamine, 1,3-propanediamine-N-(3-aminopropyl)N-dodecyl, sodium hypochlorite and potassium persulphate. The initial resistance of two persistent and two non-persistent L. monocytogenes strains was observed to differ. Both types of strains adapted after a 2-h sublethal exposure to the quaternary ammonium compounds and the tertiary alkylamine, the highest increase in the minimum inhibitory concentration (MIC) being 3-fold. Progressively increasing disinfecting concentrations at 10 and 37 degrees C resulted in adaptation of L. monocytogenes to all disinfectants except potassium sulphate. The highest observed increase in MIC was over 15-fold, from 0.63 to 10 microg/ml of n-alkyldimethyl ethylbenzyl ammonium chloride. All strains reached approximately similar MICs. Stability of the increased resistance was tested by measuring MICs every seventh day for 28 days. The increased resistance to sodium hypochlorite disappeared in 1 week, but the quaternary ammonium compounds and the tertiary alkylamine showed increased resistance for 28 days. These results suggest that cellular changes due to adaptive responses continue to have an effect on the resistance some time after the exposure. All disinfectants were shown to cause cross-adaptation of L. monocytogenes, the highest increase in MIC being almost 8-fold. The only agent that L. monocytogenes could not be shown to cross-adapt to was potassium persulphate which did, however, cause cross-adaptation to the other disinfectants. The mechanism behind these adaptive responses seemed to be non-specific as cross-adaptation was observed not only between related but also unrelated disinfectants. These findings suggest that sustaining high disinfectant effectiveness may be unsuccessful by rotation, even when using agents with different mechanisms of action.     

Directed evolution of a non-heme-iron-dependent extradiol catechol dioxygenase: identification of mutants with intradiol oxidative cleavage activity

The non-heme-iron(II)-dependent extradiol catechol dioxygenases catalyse the oxidative cleavage of substituted catechols found on bacterial aromatic degradation pathways. The reaction mechanism of the extradiol dioxygenases is believed to proceed through the same proximal hydroperoxide intermediate as the iron(III)-dependent intradiol catechol dioxygenases. Directed evolution was carried out on members of the class III extradiol catechol dioxygenases, by using 1) error-prone polymerase chain reaction, 2) a primer-based cross-over method; the mutant dioxygenases were then screened for their ability to process a range of substituted catechols. Several mutant enzymes were found to show higher activity towards certain substituted catechols, including 4-chlorocatechol, and higher affinity for the iron(II) cofactor. Two mutants isolated from error-prone PCR of Escherichia coli MhpB (mutants R215W and K273R) were found to produce a mixture of extradiol and intradiol cleavage products, as detected by GC-MS and 1H NMR spectroscopy. The residue corresponding to K273 in protocatechuate 4,5-dioxygenase (LigAB), Val244, is located approximately 12 A from the iron(II) centre, but close to the putative dioxygen channel; R215 is found on a sequence loop not present in LigB.     

Book reviews

URBAN TRANSPORT. Kevin Hay and John Sheldrake (editors). Ashgate, Aldershot, UK, 1997. ISBN 1 85972 466 3 (hardback). 133 pp, bibliography and index. rrp $84.50.

ASIANS IN AUSTRALIA: Patterns of Migration and Settlement. James E. Coughlan and Deborah J. McNamara (editors). Macmillan Education Australia, Melbourne, 1997. ISBN 0 7329 4561 5 (hardback). 330pp, bibliography and index. rrp $64.95.

SOCIAL EXCLUSION IN EUROPEAN CITIES: Processes, Experiences and Responses. Ali Madanipour, Goran Cars and Judith Allen (editors). Jessica Kingsley and the Regional Studies Association, London, 1998. ISBN 1 85302 609 3 (paperback). 301pp, bibliography and index. rrp UK17.95stlg.

THE PROPERTY DEVELOPMENT PROCESS: Western Australia. Hugh Reynolds and Peter Solomon. Victor Publishing, Perth, 1998. ISBN 0 646 35442 6 (paperback). 174pp. rrp $65.00.

THE ANTHROPOLOGY OF POLICY: Critical Perspectives on Governance and Power. Shore, C. and Wright, S. (editors). Routledge, London, 1998. ISBN 0 415 13221 5. 294pp, bibliography and index. rrp $39.95.

THE CHANGING SHAPE OF WORK. Richard K. Brown (editor). Macmillan Press Limited, London, 1997. ISBN 0 312 17251 6 (paperback). 223pp including bibliography and index. rrp $42.95.     

Characterization of ten H2 producing cyanobacteria isolated from the Baltic Sea and Finnish lakes

The genetic background and activities of the enzymes involved in H₂ production were investigated from ten distinct H₂ producing cyanobacteria, revealed by a recent screening. All strains are N₂-fixing, filamentous and heterocystous. Southern hybridization revealed that the tested strains possess the genes encoding the conventional nitrogenase (nifHDK1), and lack the alternative nitrogenases. The high H₂ production rate of these strains was shown not to be dependent on the presence of highly active nitrogenase or bidirectional hydrogenase enzymes. Moreover, most of the strains possessed a highly active uptake hydrogenase enzyme. We also examined the structure of the nif and hup operons encoding nitrogenase and uptake hydrogenase enzymes in the Calothrix 336/3 strain, the best H₂ producer in the screening. We concluded that the ability of the cyanobacteria to produce high levels of H₂ is not directly linked to the maximum capacities of the enzymes involved in H₂ production.     

Electronic structure of the SrAl2O4：Eu^2＋ persistent luminescence material

The electronic structure of the strontium aluminate (SrAl₂O₄:Eu²⁺) materials was studied with a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structure of the non-optimized and optimized crystal structure were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands as well as the band gap energy of the material together with the position of the Eu²⁺ 4f^{7 8}S_7/2 ground state were calculated. The calculated band gap energy (6.4 eV) agreed well with the experimental value of 6.6 eV. The valence band consisted mainly of oxygen states whereas the bottom of the conduction band of strontium states. In agreement with the experimental results, the calculated 4f^{7 8}S_7/2 ground state of Eu²⁺ lies in the energy gap of the host. The position of the 4f⁷ ground state depended on the Coulomb repulsion strength. The position of the 4f⁷ ground state with respect to the valence and conduction bands was discussed using theoretical and experimental evidence available.