Bacillus cereus autolytic endoglucosaminidase active on cell wall peptidoglycan with N-unsubstituted glucosamine residues

Statistical analysis of the Bacterial Carbohydrate Structure Data Base (BCSDB): characteristics and diversity of bacterial carbohydrates in comparison with mammalian glycans

Background

There are considerable differences between bacterial and mammalian glycans. In contrast to most eukaryotic carbohydrates, bacterial glycans are often composed of repeating units with diverse functions ranging from structural reinforcement to adhesion, colonization and camouflage. Since bacterial glycans are typically displayed at the cell surface, they can interact with the environment and, therefore, have significant biomedical importance.

Results

The sequence characteristics of glycans (monosaccharide composition, modifications, and linkage patterns) for the higher bacterial taxonomic classes have been examined and compared with the data for mammals, with both similarities and unique features becoming evident. Compared to mammalian glycans, the bacterial glycans deposited in the current databases have a more than ten-fold greater diversity at the monosaccharide level, and the disaccharide pattern space is approximately nine times larger. Specific bacterial subclasses exhibit characteristic glycans which can be distinguished on the basis of distinctive structural features or sequence properties.

Conclusion

For the first time a systematic database analysis of the bacterial glycome has been performed. This study summarizes the current knowledge of bacterial glycan architecture and diversity and reveals putative targets for the rational design and development of therapeutic intervention strategies by comparing bacterial and mammalian glycans.

Peptidoglycan N-acetylglucosamine deacetylation decreases autolysis in Lactococcus lactis

The gene xynD (renamed pgdA) of Lactococcus lactis IL1403 was shown to encode a peptidoglycan N-acetylglucosamine deacetylase. Inactivation of pgdA in L. lactis led to fully acetylated peptidoglycan, whereas cloning of pgdA on a multicopy plasmid vector resulted in an increased degree of peptidoglycan deacetylation, as shown by analysis of peptidoglycan constituent muropeptides. An increased amount of N-unsubstituted glucosamine residues in peptidoglycan resulted in a reduction of the rate of autolysis of L. lactis cells. The activity of the L. lactis major autolysin AcmA was tested on L. lactis cells or peptidoglycan with different degrees of de-N-acetylation. Deacetylated peptidoglycan exhibited decreased susceptibility to AcmA hydrolysis. This reduced susceptibility to AcmA did not result from reduced AcmA binding to peptidoglycan with an increasing degree of de-N-acetylation. In conclusion, enzymic N-acetylglucosamine deacetylation protects peptidoglycan from hydrolysis by the major autolysin AcmA in L. lactis cells, and this leads to decreased cellular autolysis.

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.

Mode of action of Clostridium perfringens initiation protein (spore-lytic enzyme)

The extracellular initiation protein (IP; spore germination enzyme) produced by Clostridium perfringens was further purified and characterized. IP hydrolysed spore cortical fragments with the release of free amino groups. End group analysis of hydrolysed fragments indicated the presence of N-terminal alanine but no reducing sugars. Molecular weight analysis of IP- and lysozyme-treated fluorescamine-labelled cortical fragments indicated that IP acts only on peptidoglycan chains containing cross-linked peptide subunits. IP failed to hydrolyse a number of nitrophenyl-conjugated glucopyranosides and galactopyranosides. The results indicate that IP is an N-acetylmuramyl-L-alanine amidase.

Separation and characterization of an autolytic endo-beta-glucosaminidase from Bacillus cereus

1. An autolytic endo-beta-glucosaminidase, capable of cleaving the glycoside linkages of N-unsubstituted glucosamine in the glycan moiety of cell wall peptidoglycan, was purified 470-fold from a salt extract of the 2,000 x g precipitate fraction obtained after sonication of a lysozyme-resistant strain of Bacillus cereus. The properties of this enzyme were studied. 2. The purified enzyme preparation was also active towards the glycan chain of fully N-acetylated cell wall peptidoglycan. 3. The endo-beta-glucosaminidase was inactive towards the cell wall peptidoglycan unless the peptide portion of this polymer was removed either by the action of N-acetylmuramyl-L-alanine amidase or by the treatment with alkali in aqueous dimethyl sulfoxide. 4. Studies on the action of this enzyme towards chemically modified glycans revealed that the carboxyl groups of muramic acid residues are indispensable to a substrate for this enzyme.