Characterization of a unique type IA topoisomerase in Bacillus cereus

Bacillus cereus topoisomerase IIIbeta (bcTopo IIIbeta) has been cloned, overexpressed and biochemically characterized. This enzyme exhibits 64% and 33% sequence identity to Bacillus subtilis topoisomerase III (bsTopo III) and Escherichia coli topoisomerase III (ecTopo III) respectively. The enzymatic properties of bcTopo IIIbeta differ substantially from other bacterial type IA topoisomerases, including E. coli type IA topoisomerases and B. cereus topoisomerase I (bcTopo I) and IIIalpha (bcTopo IIIalpha). bcTopo IIIbeta only partially relaxes negatively supercoiled DNA and appears incapable of generating fully relaxed topoisomers. In contrast to ecTopo III and bcTopo IIIalpha, bcTopo IIIbeta is not a decatenase. bcTopo IIIbeta is unable to compensate the loss of ecTopo III in vivo. Therefore, bcTopo IIIbeta is a unique prokaryotic type IA topoisomerase that is different from previously characterized topoisomerases.

Characterization of a mercury-reducing Bacillus cereus strain isolated from the Pulicat Lake sediments, south east coast of India

Pulicat Lake sediments are often severely polluted with the toxic heavy metal mercury. Several mercury-resistant strains of Bacillus species were isolated from the sediments and all the isolates exhibited broad spectrum resistance (resistance to both organic and inorganic mercuric compounds). Plasmid curing assay showed that all the isolated Bacillus strains carry chromosomally borne mercury resistance. Polymerase chain reaction and southern hybridization analyses using merA and merB3 gene primers/probes showed that five of the isolated Bacillus strains carry sequences similar to known merA and merB3 genes. Results of multiple sequence alignment revealed 99% similarity with merA and merB3 of TnMERI1 (class II transposons). Other mercury resistant Bacillus species lacking homology to these genes were not able to volatilize mercuric chloride, indicating the presence of other modes of resistance to mercuric compounds.

The autolytic phenotype of the Bacillus cereus group

AIM

To determine the autolytic phenotype of five species in the Bacillus cereus group.

METHODS AND RESULTS

The autolytic rate of 96 strains belonging to five species in the B. cereus group was examined under starvation conditions at pH 6, 6.5 and 8.5 in different buffers. The autolytic rate was strain-dependent with a wide variability at pH 6, but higher and more uniform at pH 6.5. At pH 8.5, and respect to the extent of autolysis at pH 6.5, it was relatively low for most of the strains with the lowest values between 13 and 52% in Bacillus mycoides and Bacillus pseudomycoides. Peptidoglycan hydrolase patterns evaluated by renaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis using cells of Bacillus thuringiensis ssp. tolworthi HD125 as an indicator, revealed complex profiles with lytic bands of about 90, 63, 46, 41, 38, 32, 28 and 25 kDa in B. cereus, B. thuringiensis and Bacillus weihenstephanensis. Bacillus mycoides and B. pseudomycoides had simpler profiles with lytic bands of 63, 46 and 38 kDa. Changes in the autolytic pattern were observed for cells harvested at the stationary phase of growth (72 h) showing an increase in the intensity of the 25 kDa band in the case of B. cereus, B. thuringiensis and B. weihenstephanensis, while no changes were observed for B. mycoides. Using Micrococcus lysodeicticus and Listeria monocytogenes as indicators lytic activity was retained by proteins of 63, 46, 38, 32 and 25 kDa and a new one of about 20 kDa in B. mycoides. Growth in the different media did not affect the autolytic pattern. NaCl abolished the activity of all the peptidoglycan hydrolases except for those of B. mycoides and B. weihenstephanensis. Lytic activity was retained in the presence of MgCl(2), MnCl(2) and EDTA and increased at basic pH.

CONCLUSIONS

Bacillus cereus/B. thuringiensis/B. weihenstephanensis showed a high extent of autolysis around neutral pH, even though they presented relatively complex autolysin profiles at alkaline pH. Bacillus mycoides/B. pseudomycoides had a higher extent of autolysis at acidic pH and a simpler autolysin pattern.

SIGNIFICANCE AND IMPACT OF THE STUDY

Information on the autolytic phenotype expand the phenotypic characterization of the different species in the B. cereus group.

Isolation and identification of nitrogen-fixing bacilli from plant rhizospheres in Beijing region

AIMS

To isolate and identify nitrogen-fixing bacilli from the plant rhizospheres in Beijing region of China.

METHODS AND RESULTS

A total of 29 isolates were selectively obtained from the rhizospheres of wheat, maize, ryegrass and willow based on their growth on nitrogen-free medium and their resistance to 100 degrees C for 10 min. Of the 29 isolates, seven had nifH gene determined by PCR amplification. The seven isolates were found to belong to the genera Bacillus and Paenibacillus based on phenotypic characterization, 16S rDNA sequence, G+C content and DNA-DNA hybridization. Isolates T1 and W5 were identified as Bacillus cereus and Bacillus marisflavi respectively. Isolates G1, C4 and C5 were identified as Bacillus megaterium. Isolate G2 was identified as Paenibacillus polymyxa and isolate T7 as Paenibacillus massiliensis.

CONCLUSIONS

This study suggests that nifH gene could be detected in the both genera Bacillus and Paenibacillus. These degenerate primers for nifH gene fragment used in this study were shown to be useful for identifying nitrogen-fixing bacilli.

SIGNIFICANCE AND IMPACT OF THE STUDY

It is the first demonstration that nitrogen fixation exists in B. marisflavi and P. massiliensis and the first report of the sequences of the nifH gene from B. megaterium and B. cereus. The nitrogen-fixing bacilli obtained in this study will be used in our future research for investigating the mechanisms of nitrogen fixation in bacilli.

FlhA influences Bacillus thuringiensis PlcR-regulated gene transcription, protein production, and virulence

Bacillus thuringiensis and Bacillus cereus are closely related. B. thuringiensis is well known for its entomopathogenic properties, principally due to the synthesis of plasmid-encoded crystal toxins. B. cereus appears to be an emerging opportunistic human pathogen. B. thuringiensis and B. cereus produce many putative virulence factors which are positively controlled by the pleiotropic transcriptional regulator PlcR. The inactivation of plcR decreases but does not abolish virulence, indicating that additional factors like flagella may contribute to pathogenicity. Therefore, we further analyzed a mutant (B. thuringiensis 407 Cry(-) DeltaflhA) previously described as being defective in flagellar apparatus assembly and in motility as well as in the production of hemolysin BL and phospholipases. A large picture of secreted proteins was obtained by two-dimensional electrophoresis analysis, which revealed that flagellar proteins are not secreted and that production of several virulence-associated factors is reduced in the flhA mutant. Moreover, we quantified the effect of FlhA on plcA and hblC gene transcription. The results show that the flhA mutation results in a significant reduction of plcA and hblC transcription. These results indicate that the transcription of several PlcR-regulated virulence factors is coordinated with the flagellar apparatus. Consistently, the flhA mutant also shows a strong decrease in cytotoxicity towards HeLa cells and in virulence against Galleria mellonella larvae following oral and intrahemocoelic inoculation. The decrease in virulence may be due to both a lack of flagella and a lower production of secreted factors. Hence, FlhA appears to be an essential virulence factor with a pleiotropic role.

Gene-based identification of bacterial colonies with an electric chip

A method for the identification of bacterial colonies based on their content of specific genes is presented. This method does not depend on DNA separation or DNA amplification. Bacillus cereus carrying one of the genes (hblC) coding for the enterotoxin hemolysin was identified with this method. It is based on target DNA hybridization to a capturing probe immobilized on magnetic beads, followed by enzymatic labeling and measurement of the enzyme product with a silicon-based chip. An hblC-positive colony containing 10(7) cells could be assayed in 30 min after ultrasonication and centrifugation. The importance of optimizing the ultrasonication is illustrated by analysis of cell disruption kinetics and DNA fragmentation. An early endpoint PCR analysis was used to characterize the DNA fragmentation as a function of ultrasonication time. The first minutes of sonication increased the signal due to both increased DNA release and increased DNA fragmentation. The latter is assumed to increase the signal due to improved diffusion and faster hybridization of the target DNA. Too long sonication decreased the signal, presumably due to loss of hybridization sites on the targets caused by extensive DNA fragmentation. The results form a basis for rational design of an ultrasound cell disruption system integrated with analysis on chip that will move nucleic acid-based detection through real-time analysis closer to reality.

Stationary-phase expression and aminoacylation of a transfer-RNA-like small RNA

Genome-scale analyses have shown numerous functional duplications in the canonical translational machinery. One of the most striking examples is the occurrence of unrelated class I and class II lysyl-transfer RNA synthetases (LysRS), which together may aminoacylate non-canonical tRNAs. We show that, in Bacillus cereus, the two LysRSs together aminoacylate a small RNA of unknown function named tRNA(Other), and that the aminoacylated product stably binds translation elongation factor Tu. In vitro reconstitution of a defined lysylation system showed that Lys-tRNA(Other) is synthesized in the presence of both LysRSs, but not by either alone. In vivo analyses showed that the class 2 LysRS was present both during and after exponential growth, whereas the class I enzyme and tRNA(Other) were predominantly produced during the stationary phase. Aminoacylation of tRNA(Other) was also found to be confined to the stationary phase, which suggests a role for this non-canonical tRNA in growth-phase-specific protein synthesis.

Attachment of different soil bacteria to arbuscular mycorrhizal fungal extraradical hyphae is determined by hyphal vitality and fungal species

Attachment of certain bacteria to living arbuscular mycorrhizal fungal extraradical hyphae may be an important prerequisite for interactions between these microorganisms, with implications for nutrient supply and plant health. The attachment of five different strains of gfp-tagged soil bacteria (Paenibacillus brasilensis PB177 (pnf8), Bacillus cereus VA1 (pnf8), Pseudomonas fluorescens SBW25 :: gfp/lux, Arthrobacter chlorophenolicus A6G, and Paenibacillus peoriae BD62 (pnf8)) to vital and nonvital extraradical hyphae of the arbuscular mycorrhizal fungi Glomus sp. MUCL 43205 and Glomus intraradices MUCL 43194 was examined. Arthrobacter chlorophenolicus did not attach to hyphae, whereas the other bacterial strains did to a varying degree. Only P. brasilensis showed greater attachment to vital hyphae than nonvital hyphae of both Glomus species tested. Pseudomonas fluorescens showed a higher attachment to vital compared with nonvital Glomus sp. MUCL 43205 hyphae, whereas this relationship was opposite for attachment to G. intraradices. Both B. cereus and P. peoriae showed higher attachment to nonvital hyphae. This study provides novel evidence that under laboratory conditions soil bacteria differ in their ability to colonize vital and nonvital hyphae and that this can also be influenced by the arbuscular mycorrhizal fungal species involved. The significance of bacterial attachment to mycorrhizal fungal extraradical hyphae is discussed.

Detection of enterotoxic Bacillus cereus producing hemolytic and non hemolytic enterotoxins by PCR test

Nine strains belonging to Bacillus cereus group has been isolated from food and environmental samples. Their taxonomic position was confirmed by RFLP analysis of 16S rRNA gene digested with TaqI. The detection of DNA sequences encoding the hemolysin BL complex and enterotoxin NHE, was studied in Bacillus sp. isolates. Set of primers was used to amplify fragment of hblD gene by PCR. For the detection of nheB gene a new primer set was developed which allowed to amplify 273 bp fragment from wide number of strains belonging to B. cereus group. The hblD gene was present in 7 out of 9 isolates whereas nheB gene occurred in all of them. Reference strains of B. cereus LOCK 0807, and B. thuringiensis NCAIM 01262 contained both genes. Strains of B. subtilis ATCC 6633 and B. pumilus LOCK 0814 do not contain both genes. Obtained results showed that B. thuringiensis NCAIM 01262 contains both genes and therefore may be harmful for human beings.

Comparison of minisatellite polymorphisms in the Bacillus cereus complex: a simple assay for large-scale screening and identification of strains most closely related to Bacillus anthracis

Polymorphism of five tandem repeats that are monomorphic in Bacillus anthracis was investigated in 230 isolates of the B. cereus group and in 5 sequenced B. cereus genomes in search for markers allowing identification of B. cereus and B. thuringiensis strains most closely related to B. anthracis. Using this multiple-locus variable number of tandem repeat analysis (MLVA), a cluster of 30 strains was selected for further characterization. Eventually, six of these were characterized by multilocus sequence type analysis. One of the strains is only six point mutations (of almost 3,000 bp) away from B. anthracis and was also proposed to be closest to B. anthracis by MLVA analysis. However, this strain remains separated from B. anthracis by a number of significant genetic events observed in B. anthracis, including the loss of the hemolysin activity, the presence of four prophages, and the presence of the two virulence plasmids, pXO1 and pXO2. One particular minisatellite marker provides an efficient assay to identify the subset of B. cereus and B. thuringiensis strains closely related to B. anthracis. Based on these results, a very simple assay is proposed that allows the screening of hundreds of strains from the B. cereus complex, with modest equipment and at a low cost, to eventually fill the gap with B. anthracis and better understand the origin and making of this dangerous pathogen.

Deletion of the sigB gene in Bacillus cereus ATCC 14579 leads to hydrogen peroxide hyperresistance

The sigB gene of Bacillus cereus ATCC 14579 encodes the alternative sigma factor sigma(B). Deletion of sigB in B. cereus leads to hyperresistance to hydrogen peroxide. The expression of katA, which encodes one of the catalases of B. cereus, is upregulated in the sigB deletion mutant, and this may contribute to the hydrogen peroxide-resistant phenotype.

Distribution of genes encoding putative virulence factors and fragment length polymorphisms in the vrrA gene among Brazilian isolates of Bacillus cereus and Bacillus thuringiensis

One hundred twenty-one strains of the Bacillus cereus complex, of which 80 were isolated from a variety of sources in Brazil, were screened by PCR for the presence of sequences (bceT, hblA, nheBC, plc, sph, and vip3A) encoding putative virulence factors and for polymorphisms in variable-number tandem repeats (VNTR), using a variable region of the vrrA open reading frame as the target. Amplicons were generated from isolates of B. cereus and Bacillus thuringiensis for each of the sequences encoding factors suggested to play a role in infections of mammals. Intriguingly, the majority of these sequences were detected more frequently in Bacillus thuringiensis than in B. cereus. The vip3A sequence, which encodes an insecticidal toxin, was detected exclusively in B. thuringiensis. VNTR analysis demonstrated the presence of five different fragment length categories in both species, with two of these being widely distributed throughout both taxa. In common with data generated from previous studies examining European, Asian, or North American populations, our investigation of Brazilian isolates supports the notion that B. cereus and B. thuringiensis should be considered to represent a single species.

Phenotypic and genotypic comparisons reveal a broad distribution and heterogeneity of hemolysin BL genes among Bacillus cereus isolates

The presence of hemolysin BL (HBL; components L(2), L(1), and B)-encoding genes (hblC, hblD, and hblA) from 339 Bacillus cereus strains isolated in Thailand was determined. PCR analysis showed that all three hbl genes were detected in 222 strains (65.5%). Two, one or no hbl genes were detected in 3 (0.9%), 6 (1.8%), and 108 (31.8%) strains, respectively. Among the 222 strains in which all three hbl genes were detected, 210 (61.9%) displayed discontinuous hemolysis (DH) characteristic of HBL producers, while 12 (3.5%) showed continuous hemolysis (CH) on sheep blood agar. Among strains in which none of the hbl genes was detected, 97 (28.6%) displayed CH while 11 (3.2%) did not show hemolytic activity. Three strains in which two hbl genes were detected showed CH. hblC was present in five of six strains where only one hbl gene was detected, and all of them (designated SS-00-15, TG-00-06, TG-00-14, F-00-25, and NR-01-49) showed DH. The HpaII restriction profiles of PCR fragments amplified from the hblC-A region in these five strains using hblC forward (FHC) and hblA reverse (RHA(2)) primers displayed heterogeneous patterns, which indicated sequence variation. Western blot analysis using polyclonal antibodies (Pab) raised against HBL components purified from strain F837/76 showed that three of the five strains produced all three components, whereas strain TG-00-06 did not give a signal for any component, and strain TG-00-14 produced B and L(1) but not L(2). The production of L(2) by these five strains was further analyzed using the Oxoid RPLA test. Three strains gave high titers (>64) whereas strains TG-00-06 and TG-00-14 showed lower titers of 16 and 32, respectively. The data show that HBL-encoding genes are widely distributed among B. cereus isolated in Thailand and there is a high degree of heterogeneity in both the genes and proteins. This is the first report of a B. cereus strain showing DH in which all three hbl genes and their proteins were not detected by both PCR primers and antibodies derived from prototype and type strains. The data also suggest that the L(2) component from strains TG-00-06 and TG-00-14 may be antigenically very different from that of most B. cereus isolates.

Binding of harvested bacterial exopolymers to the surface of calcite

Biologically produced exopolysaccharides (EPS) affect calcite dissolution and precipitation. In this study, natural alkaliphilic microbial isolates were collected from biofilms on historic limestone. The isolates were screened for their ability to produce significant quantities of EPS in cultures. The most productive isolates were identified by 16S rRNA sequence analysis as a close relative of Bacillus cereus. EPS with different chemical structures were harvested from the isolates. Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of binding by the harvested EPS to calcite. The binding was described by a Langmuir adsorption isotherm. Characterization of the EPS showed that binding strength to calcite depended on the chemical nature of the polymer.

Use of oligonucleotide array for identification of six foodborne pathogens and Pseudomonas aeruginosa grown on selective media

Identification of presumptive foodborne pathogens grown on selective media may take one to several days and requires a different battery of biochemical tests for each microorganism. A molecular identification method was developed in which universal primers were used to amplify the 16S to 23S rDNA intergenic spacer of target microorganisms, and PCR products were hybridized to a panel of species-specific oligonucleotides that were immobilized on a nylon membrane. The seven target microorganisms were Bacillus cereus, Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella, Staphylococcus aureus, and Vibrio parahaemolyticus. After testing a large collection of target bacteria (29 to 51 strains) and nontarget bacteria (> 500 strains), the performances (sensitivity and specificity) of the oligonucleotide array were as follows: B. cereus (100 and 77%), E. coli (100 and 100%), L. monocytogenes (100 and 90%), P. aeruginosa (100 and 100%), Salmonella (100 and 100%), S. aureus (100 and 100%), and V. parahaemolyticus (100 and 94.2%). Other species in the B. cereus group cross-hybridized to the probes used for identification of B. cereus, and positive results should be confirmed by additional morphological observation of colonies. Listeria innocua cross-reacted with probes used to identify L. monocytogenes, but a simple hemolysis test was used to differentiate the two species. Some strains of Vibrio harveyi and Vibrio mimicus cross-hybridized with probes used for identification of V. parahaemolyticus and caused false-positive reactions. The advantage of the array is that a common protocol was used to identify the seven target microorganisms and multiple different microorganisms could be simultaneously identified on a single array.

Deletion ofsigBinBacillus cereusaffects spore properties

In Bacillus cereus and other gram-positive bacteria the alternative sigma factor sigma(B) is an important regulator of the stress response. Deletion of the sigB gene generally leads to a stress-sensitive phenotype of vegetative cells. In this study, we describe the effect of the deletion of the sigB gene in B. cereus on spore properties. In particular, spores of the sigB deletion mutant showed a defect in germination upon exposure to the germinants alanine and inosine.

Microarray analysis of Bacillus cereus group virulence factors

Bacillus cereus, B. thuringiensis and B. anthracis are closely related medically and economically important bacterial species that belong to the B. cereus group. Members of the B. cereus group carry genes encoding several important virulence factors, including enterotoxins, phospholipases and exotoxins. Since it is difficult to differentiate among B. cereus group members, and because Bacillus virulence factors are very important for pathogenesis, we explored the use of microarray-based detection of virulence factor genes as a tool for strain identification and for determining virulence. Our method requires an initial multiplex PCR amplification step, followed by identification of the PCR amplicons by hybridization to an oligonucleotide microarray containing genes for all three types of Bacillus virulence factors including B. anthracis virulence factors. The DNA chip described here contains 21 identical arrays used for analysis of seven samples in triplicates. Using the arrays, we found that virulence factors are present in several combinations in the strains analyzed. This work also demonstrates the potential of oligonucleotide microarrays for medical, food safety and biodefense analysis of microbial pathogens.

Cereulide-producing strains of Bacillus cereus show diversity

Producers of cereulide, the emetic toxin of Bacillus cereus, are known to constitute a specific subset within this species. We investigated physiological and genetic properties of 24 strains of B. cereus including two high cereulide producers (600-1,800 ng cereulide mg(-1) wet weight biomass), seven average producers (180-600 ng cereulide mg(-1) wet weight biomass), four low cereulide producers (20-160 ng cereulide mg(-1) wet weight biomass) and 11 non-producers representing isolates from food, food poisoning, human gut and environment. The 13 cereulide producers possessed 16S rRNA gene sequences identical to each other and identical to that of B. anthracis strains Ames, Sterne from GenBank and strain NC 08234-02, but showed diversity in the adk gene (two sequence types), in ribopatterns obtained with EcoRI and PvuII (three types of patterns), in tyrosin decomposition, haemolysis and lecithin hydrolysis (two phenotypes). The cereulide-producing isolates from the human gut represented two ribopatterns of which one was novel to cereulide-producing B. cereus and two phenotypes. We conclude that the cereulide-producing B. cereus are genetically and biochemically more diverse than hitherto thought.

Bacillus cereus DNA topoisomerase I and IIIalpha: purification, characterization and complementation of Escherichia coli TopoIII activity

The Bacillus cereus genome possesses three type IA topoisomerase genes. These genes, encoding DNA topoisomerase I and IIIα (bcTopo I, bcTopo IIIα), have been cloned into T7 RNA polymerase-regulated plasmid expression vectors and the enzymes have been overexpressed, purified and characterized. The proteins exhibit similar biochemical activity to their Escherichia coli counterparts, DNA topoisomerase I and III (ecTopo I, ecTopo III). bcTopo I is capable of efficiently relaxing negatively supercoiled DNA in the presence of Mg²⁺ but does not possess an efficient DNA decatenation activity. bcTopo IIIα is an active topoisomerase that is capable of relaxing supercoiled DNA at a broad range of Mg²⁺ concentrations; however, its DNA relaxation activity is not as efficient as that of bcTopo I. In addition, bcTopo III is a potent DNA decatenase that resolves oriC-based plasmid replication intermediates in vitro. Interestingly, bcTopo I and bcTopo IIIα are both able to compensate for the loss of ecTopo III in E.coli cells that lack ecTopo I. In contrast, ecTopo I cannot substitute for ecTopo III under these conditions.

Transfer and expression of the mosquitocidal plasmid pBtoxis in Bacillus cereus group strains

The toxicity of Bacillus thuringiensis subsp. israelensis to dipteran larvae (mosquitoes and black flies) depends on the presence of the pBtoxis plasmid. In this paper, two antibiotic resistance tagged pBtoxis were transferred by conjugation to other Bacillus cereus group strains. Among 15 potential recipients, only a lepidopteran active B. thuringiensis subspecies kurstaki and a B. cereus strain received the plasmid pBtoxis with a low transfer rate of about 10(-8) transconjugants/recipient. The resulting B. thuringiensis subspecies kurstaki transconjugant was active to both lepidopteran and dipteran targets and the B. cereus transconjugant was active against dipteran insects. Phase contrast microscopy showed that the B. cereus transconjugants could produce only round crystalline inclusion bodies while B. thuringiensis subspecies kurstaki transconjugant could produce both round and bipyramidal crystals during sporulation. SDS-PAGE revealed that all the major mosquitocidal proteins from pBtoxis could express in the two transconjugants, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa and Cyt1Aa. However, none of the experiment showed any indications of mobilising abilities of pBtoxis. The limited number of strains, which could receive and maintain pBtoxis using a conjugational helper plasmid, indicates a very narrow host range of the B. thuringiensis subsp. israelensis pBtoxis plasmid.

Mimicking natural evolution in metallo-beta-lactamases through second-shell ligand mutations

Metallo-beta-lactamases (MBLs) represent the latest generation of beta-lactamases. The structural diversity and broad substrate profile of MBLs allow them to confer resistance to most beta-lactam antibiotics. To explore the evolutionary potential of these enzymes, we have subjected the Bacillus cereus MBL (BcII) to a directed evolution scheme, which resulted in an increased hydrolytic efficiency toward cephalexin. A systematic study of the hydrolytic profile, substrate binding, and active-site features of the evolved lactamase reveal that directed evolution has shaped the active site by means of remote mutations to better hydrolyze cephalosporins with small, uncharged C-3 substituents. One of these mutations is found in related enzymes from pathogenic bacteria and is responsible for the increase in that enzyme's hydrolytic profile. The mutations lowered the activation energy of the rate-limiting step rather than improved the affinity of the enzyme toward these substrates. The following conclusions can be made: (i) MBLs are able to expand their substrate spectrum without sacrificing their inherent hydrolytic capabilities; (ii) directed evolution is able to mimic mutations that occur in nature; (iii) the metal-ligand strength is tuned by second-shell mutations, thereby influencing the catalytic efficiency; and (iv) changes in the position of the second Zn(II) ion in MBLs affect the substrate positioning in the active site. Overall, these results show that the evolution of enzymatic catalysis can take place by remote mutations controlling reactivity.

Fatal family outbreak of Bacillus cereus-associated food poisoning

Bacillus cereus is a well-known cause of food-borne illness, but infection with this organism is not commonly reported because of its usually mild symptoms. A fatal case due to liver failure after the consumption of pasta salad is described and demonstrates the possible severity of the emetic syndrome.

Occurrence and significance ofBacillus cereusandBacillus thuringiensisin ready-to-eat food

Among 48,901 samples of ready-to-eat food products at the Danish retail market, 0.5% had counts of Bacillus cereus-like bacteria above 10(4) cfu g(-1). The high counts were most frequently found in starchy, cooked products, but also in fresh cucumbers and tomatoes. Forty randomly selected strains had at least one gene or component involved in human diarrhoeal disease, while emetic toxin was related to only one B. cereus strain. A new observation was that 31 out of the 40 randomly selected B. cereus-like strains could be classified as Bacillus thuringiensis due to crystal production and/or content of cry genes. Thus, a large proportion of the B. cereus-like organisms present in food may belong to B. thuringiensis.