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Glucan Common to the Microcyst Walls of Cyst-Forming Bacteria IAN W

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Glucan Common to the Microcyst Walls of Cyst-Forming Bacteria IAN W JOURNAL OF BACTRIOLOGY, Feb. 1977, p. 599-605 Vol. 129, No. 2 Copyright C 1977 American Society for Microbiology Printed in U.S.A. Glucan Common to the Microcyst Walls of Cyst-Forming Bacteria IAN W. SUTHERLAND* AND C. LINDSEY MACKENZIE Department ofMicrobiology, University ofEdinburgh, Edinburgh, EH9 3JG Scotland Received for publication 21 September 1976 Chemical analysis indicated that 1)-glucose is the major neutral monosacchar- ide present in the microcysts of a range of gram-negative bacteria. Varying amounts ofother neutral sugars were found. The glucose was mainly present as a glucan that could be extracted from microcysts of representative strains with alkali or mild acid treatment. The glucan could be identified as an a-1,3-linked polymer on the basis of (i) periodate resistance ofthe extracted polymer and the material present in microcysts; (ii) lectin agglutination of the microcysts; (iii) lectin precipitation ofthe extracted glucans; and (iv) susceptibility ofthe glucan either in the walls or after extraction to a specific a-1,3-glucanase from Asper- gillus nidulans, yielding glucose as the sole hydrolysis product. The galactosa- mine found in microcysts of Myxococcus xanthus by other workers is clearly a component of another polymer, distinct from the glucan. The presence of an a- 1,3-linked glucan, common to microcyst walls ofvarious bacterial genera, proba- bly contributes to the rigidity ofthe walls ofthese forms and, inter alia, to their resistance to ultrasonic treatment. Preliminary experiments indicate that the glucan is discarded on germination of the microcysts rather than being broken down by specific enzymes. Gram-negative bacteria are unable to form ance were associated with the synthesis of an endospores, but a limited number ofspecies can additional cell layer. The presence of glucose, form a resting phase resistant to ultrasonic ir- associated with peptidoglycan in both vegeta- radiation but not to heat. The resting form is tive bacteria and microcysts ofM. xanthus, was generally termed a microcyst (or, in myxobac- reported (8). The amount of glucose present in teria, a myxospore). Such microcysts are con- the lipopolysaccharide of bacilli is much fined in their occurrence to four groups ofbacte- smaller than that found in cysts, and the proba- ria: Azotobacteriaceae, Methylomonadaceae, ble loss oflipopolysaccharide during cyst forma- Sporocytophaga species, and Myxobacterales tion suggests that the sugar is present in differ- (2). In myxobacteria the microcysts are gener- ent polymers (17). The glucose is probably not ally found within fruiting bodies of varying part ofthe same polymer as the galactosamine, complexity, but in the other three bacterial since the neutral sugar content of myxospores groups they are found free. The morphological was variable but increased after galactosamine changes occurring during microcyst formation synthesis had apparently ceased, the total car- and germination in Azotobacter vinelandii and bohydrate continuing to increase over a 24-h in Myxococcus xanthus are well documented period (1, 10). and have been recently reviewed by Sadoff (15) The walls of A. vinelandii microcysts also and Sudo and Dworkin (16), respectively. Use differ in their carbohydrate composition from of these two species has the advantage that, the walls of the vegetative cells (15). 1-Glucose through the use of appropriate inducers, syn- is again present, although mannose, xylose, chronous conversion ofbacillary forms to micro- and rhamnose were also reported. Associated cysts can be achieved and relatively large with the neutral sugars are -mannuronic acid amounts of material can be prepared. and L-guluronic acid from bacterial alginate, Microcysts of M. xanthus contain relatively the proportions of the two uronic acids, not - large amounts of n-glucose and r.-galactosa- and L-isomers of the same acid (12), varying in mine, and the increase in amino sugar content the intine and exine layers of the microcysts after microcyst induction coincides with the in- (15). creased resistance to ultrasonic treatment (21). It seemed possible that similarities existed Dworkin (6) suggested that optical refractility, between the composition of the walls of all mi- increased galactosamine, and ultrasonic resist- crocyst-forming bacteria, and this paper reports 599 600 SUTHERLAND AND MACKENZIE J. BACTERIOL. a comparison of the chemical composition of Polysaccharide extracts. Microcysts (myxo- microcysts from different genera and species spores) were extracted with 1 N NaOH at 50°C for 30 representing three of the four microcyst-form- min (21) or with 5% (wt/vol) KOH for 16 h at 37°C ing bacterial groups. Attempts to obtain ade- (9). After neutralization, the extracts were dialyzed exhaustively against running tap water and dis- quate quantities of Sporocytophaga myxococ- tilled water and lyophilized. Acetic acid extracts coides microcysts, free from medium-derived were prepared as described earlier (17). cellulose, were unsuccessful. Chemical analysis. Polysaccharide preparations were hydrolyzed with 1 N H2SO4 for 12 h in sealed MATERIALS AND METHODS tubes, neutralized with saturated Ba(OH)2, and sep- Bacterial strains and methods of culture. Azoto- arated into neutral and basic fractions by prepara- bacter species were grown routinely in Burks nitro- tive paper electrophoresis. Acidic material was gen-free medium with glucose as carbon source. found in Azotobacter preparations, and its identity After 36 h at 30°C in 250-ml flasks on a reciprocal as a mixture of D-mannuronic acid and i-glucuronic shaker, yielding mid- to late-log-phase cells (E530 = acid was confirmed by paper chromatography. 0.7 to 0.8), vegetative cells were centrifuged asepti- Paper chromatography and microanalysis. These cally and microcysts were induced by resuspension assays were done by methods previously described in Burks medium containing 0.2% (wt/vol) 6-hy- (16, 17). Glucose was determined by the oxidase droxybutyric acid and incubated for a further 48 h at reagent; galactosamine was assayed with the galac- 30°C. Cysts were then recovered by centrifugation tose oxidase procedure. Xylose was used as an inter- and washed in saline and water before lyophiliza- nal standard. to correct for losses of carbohydrates tion. Bacilli for lectin studies were taken from the during separation procedures. All chromatograms initial cultures. M. xanthus strain FB was grown in were run in descending solvent systems. These Casitone broth (medium C10 of Reichenbach and were: butan-1-ol-pyridine-water (6:4:3), solvent A; Dworkin) (14) for 4 h at 30°C in 1-liter amounts in 2- ethyl acetate-pyridine-acetic acid-water (5:5:1:3), liter Erlenmeyer flasks on a shaker, to give early- solvent B; and butan-1-ol-acetic acid-water log-phase cells (E530 = ca. 0.4). Microcysts were (50:12:25), solvent C. Irrigation times were 24, 24, induced by the addition of 0.5 M glycerol directly to and 60 h, respectively. the cultures and harvested after a further 4 or 16 h Enzymes. Cellulase from Trichomonas viride and (7). To obtain fruiting bodies of other myxobacterial a Cytophaga 8-glucanase preparation were pur- strains, cultures were grown on cell agar (0.1% [wt/ chased from British Drug Houses, Poole, England; vol] Enterobacter aerogenes cells in tap water agar) a- and 8-amylases were from the Boehringer Corp., for 6 to 14 days at 30°C. Fruiting bodies were re- Lewes, England. The specific exo a-1,3-glucanase covered as described by Sutherland and Thomson fromAspergillus nidulans (25) was the generous gift (18). The single strain of Cystobacter velatus was of B. J. M. Zonneveld. grown similarly on yeast cell agar. Vegetative cells Lectin tests. Slide agglutination tests were per- were isolated from cultures on 2% (wt/vol) Casitone formed according to Cruickshank (5), using lectin agar. All myxobacterial cultures were incubated in solutions (5 mg/ml) and vegetative cell or microcyst sealed polythene bags to maintain the humidity of suspensions containing 109 and 108/ml, respectively. the cultures. Results were viewed after 2 min at 20°C and reexam- The culture of "Methylomonas albus" was grown ined at 5 and 10 min. in a salts medium gassed with methane for 4 to 5 Capillary precipitation tests were done by using days at 30°C as described by Whittenbury et al. (23). soluble extracts (10 mg/ml) added to equal volumes Mycrocyst isolation. Microcysts from Azotobacter of aqueous lectin solution (60 or 120 ,ug/ml) in fine species were the only form ofcells present in induced capillary tubes. The tubes were examined under a cultures and were recovered directly by centrifuga- hand lens after 16 h at 20°C. tion. The other bacteria investigated contained a mixture of vegetative forms and myxospores in both RESULTS induced cultures and fruiting bodies. In addition, Composition of microcysts. The properties most produced relatively large amounts of exopoly- common to microcysts or myxospores from dif- saccharides. Myxospores were separated from con- taminating material by centrifugation after de- ferent bacterial species and genera indicated struction of vegetative bacteria by ultrasonic treat- the possibility that similar or identical chemi- ment: 15-s. exposure to a nominal 20 kc/s in an MSE cal components were present to confer these (London) sonic oscillator. The myxospores were properties. Only Azotobacter vinelandii micro- thoroughly washed in distilled water. For lectin ag- cysts are readily lysed to yield good wall prepa- glutination, some of the myxospores from fruiting rations. Alternatively, acetic acid extracts can bodies or induced with glycerol were also suspended be obtained from most microcysts, yielding directly in distilled water without ultrasonic treat- much ofthe cell wall carbohydrate. Analyses of ment to obviate the possible removal of surface lay- these preparations revealed that all contained ers. Methylobacterial microcysts were prepared in the same way as those from myxobacteria. 1-glucose as the major identifiable carbohy- Lectins. Lectins were obtained from Pharmacia drate component (Table 1). Most myxospores Ltd.
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