Release of autolytic enzyme from Streptococcus, faecium cell walls by treatment with dilute alkali

The autolytic enzyme (endo-beta-1,4-N-acetylmuramoylhydrolase) of Streptococcus faecium (S. faecalis ATCC 9790) was released in a soluble form from insoluble cell wall-autolytic enzyme complexes by treatment with dilute NaOH at 0 degree C. Treatment of cell wall-enzyme complexes, obtained from either exponential- or stationary-phase cells, with 0.008 to 0.01 N NaOH gave maximum yields of autolytic enzyme activity. At a fixed concentration of NaOH, the yield of autolysin increased with increasing wall densities and was accompanied by the release of methylpentose and phosphorus in amounts proportional to the autolysin. Since extraction of wall-enzyme complexes with 4.5 M LiCl at 0 degree C also removed methylpentose and phosphorus, release of enzyme with NaOH did not appear to result from hydrolysis of covalent linkages. The autolytic enzyme activity released from intact cells, or cell walls, was predominantly in the later (proteinase activable) form which could be activated by trypsin or a proteinase present in commerical bovine plasma albumin.

Morphological and physiological study of autolytic-defective Streptococcus faecium strains

Three autolytic-defective mutants of Streptococcus faecium (S. faecalis ATCC 9790) were isolated. All three autolytic-defective mutants exhibited the following properties relative to the parental strain: (i) slower growth rates, especially in chemically defined medium; (ii) decreased rates of cellular autolysis and increased survival after exposure to antibiotics which block cell wall biosynthesis; (iii) decreased rates of cellular autolysis when treated with detergents, suspended in autolysis buffers, or grown in medium lacking essential cell wall precursors; (iv) a reduction in the total level of cellular autolytic enzyme (active plus latent forms of the enzyme); (v) an increased ratio of latent to active forms of autolysin; and (vi) increased levels of both cellular lipoteichoic acid and lipids.

Enzymatic deacylation of lipoteichoic acid by protoplasts of Streptococcus faecium (Streptococcus faecalis ATCC 9790)

High-molecular-weight, micellar lipoteichoic acid (LTA) was converted to a lower-molecular-weight, apparently deacylated polymer when the former was incubated in the presence of growing protoplasts of Streptococcus faecium (S. faecalis ATCC 9790), but not when incubated in fresh or spent protoplast medium. The mobility of the low-molecular-weight polymer upon agarose gel electrophoresis was indistinguishable from that of native extracellular lipoteichoic acid LTA(X) from this organism or from chemically deacylated LTA. Native LTA(X) was shown to contain less than one fatty acid equivalent per 18 LTA(X) molecules, in contrast to the 4:1 ratio of fatty acids to polyglycerolphosphate chains in micellar LTA.

Precursor-product relationship of intracellular and extracellular lipoteichoic acids of Streptococcus faecium

Exponential biosynthesis and excretion of lipoteichoic acid (LTA) during the exponential phase of growth, and continued synthesis and excretion during valine starvation of Streptococcus faecium (S. faecalis ATCC 9790), were shown. During exponential growth, extracellular LTA (LTAx) accounted for approximately 13% of the total LTA in cultures, whereas during valine starvation, this percentage increased to approximately 60% within 4 h. LTAx was present in a low-molecular-weight, apparently deacylated form, whereas intracellular (LTAi) was present primarily in an apparently high-molecular-weight, acylated and micellar form. Experiments utilizing chases of either fully equilibrated or short pulses of [14C]- or [3H]glycerol were used to demonstrate that LTAx was derived directly from LTAi.

Cellular lysis of Streptococcus faecalis induced with triton X-100

Lysis of exponential-phase cultures of Streptococcus faecalis ATCC 9790 was induced by exposure to both anionic (sodium dodecyl sulfate) and nonionic (Triton X-100) surfactants. Lysis in response to sodium dodecyl sulfate was effective only over a limited range of concentrations, whereas Triton X-100-induced lysis occurred over a broad range of surfactant concentrations. The data presented indicate that the bacteriolytic response of growing cells to Triton X-100: (i) was related to the ratio of surfactant to cells and not the surfactant concentration per se; (ii) required the expression of the cellular autolytic enzyme system; and (iii) was most likely due to an effect of the surfactant on components of the autolytic system that are associated with the cytoplasmic membrane. The possibility that Triton X-100 may induce cellular lysis by releasing a lipid inhibitor of the cellular autolytic enzyme is discussed.

Relationship between cellular autolytic activity, peptidoglycan synthesis, septation, and the cell cycle in synchronized populations of Streptococcus faecium

Synchronized, slowly growing (TD = 70 to 80 min) cultures were used to study several wall-associated parameters during the cell cycle: rate of peptidoglycan synthesis, septation, and cellular autolytic activity. The rate of peptidoglycan synthesis per cell declined during most of the period of chromosome replication (C), but increased during the latter part of C and into the period between chromosome termination and cell division (D). An increase in cellular septation was correlated with the increased rate of peptidoglycan synthesis. Cellular autolytic capacity increased during the early portion of C, reached a maximum late in C or early in D, and declined during D. Inhibition of DNA synthesis during C prevented the decline in autolytic capacity at the end of the cell cycle, caused a slight reduction in the rate of peptidoglycan synthesis, delayed but did not prevent septation, and prevented the impending cell division by inhibiting cell separation. Inhibition of DNA synthesis during D did not prevent the increase in autolytic capacity during the next C phase, but, once again, prevented the decline at the end of the subsequent cycle. Thus, increased autolytic capacity at the beginning of the cell cycle did not seem to be related to chromosome initiation, whereas decreased autolytic capacity at the end of the cell cycle seemed to be related to chromosome termination. The data presented are consistent with the role of autolytic enzyme activity in the previously proposed model for cell division of S. faecium (G.D. Shockman et al., Ann. N.Y Acad. Sci. 235:161-197, 1974).

Approximation of the cell cycle in synchronized populations of Streptococcus faecium

Slowly growing populations (TD = 70 to 80 min) of Streptococcus faecium (S. faecalis ATCC 9790) were synchronized by selection after sucrose gradient fractionation. The cell cycle was approximated by correlating the patterns of DNA accumulation and cell division. More specifically, the beginning of cell cycle was equated with the beginning of a rapid linear increase in DNA accumulation. The DNA content of the culture approximately doubled during the period of accumulation, which lasted about 51 min. The period of rapid DNA accumulation, was followed by a period of reduced accumulation that lasted about 24 min. During synchronized growth, cell numbers increased rapidly in coordination with the period of rapid DNA accumulation and exhibited a plateau during the period of reduced DNA accumulation. In contrast, RNA and protein appeared to accumulate exponentially throughout the cell cycle at the same rate as culture mass.

Autolytic defective mutant of Streptococcus faecalis

Properties of a variant of Streptococcus faecalis ATCC 9790 with defective cellular autolysis are described. The mutant strain was selected as a survivor from a mutagenized cell population simultaneously challenged with two antibiotics which inhibit cell wall biosynthesis, penicillin G and cycloserine. Compared to the parental strain, the mutant strain exhibited: (i) a thermosensitive pattern of cellular autolysis; (ii) an autolytic enzyme activity that had only a slightly increased thermolability when tested in solution in the absence of wall substrate; and (iii) an isolated autolysin that had hydrolytic activity on isolated S. faecalis wall substrate indistinguishable from that of the parental strain, but that was inactive when tested on walls of Micrococcus lysodeikticus as a substrate. These data indicate an alteration in the substrate specificity of the autolytic enzyme of the mutant which appears to result from the synthesis of an altered form of autolytic enzyme.

Cellular autolytic activity in synchronized populations of Streptococcus faecium

The autolytic capacity of Streptococcus faecium (S. faecalis ATCC 9790) varied during synchronous cell division. This phenomenon was initially observed in rapidly dividing populations (TD=30 to 33 min) synchronized by a combination of induction and size selection techniques. To minimize the problems inherent in studies of cells containing overlapping chromosome cycles and possible artifacts generated by induction techniques, the autolytic capacities of slowly dividing populations (TD=60 to 110 min) synchronized by selection only were examined. Although the overall level of cellular autolytic capacity was observed to decline with decreasing growth rate, sharp, periodic fluctuations in cellular autolytic capacity were seen during synchronous growth at all growth rates examined. On the basis of similar patterns of cyclic fluctuations in autolytic capacity of cultures synchronized by (i) selection, (ii) amino acid starvation followed by size selection, and (iii) amino acid starvation followed by inhibition of DNA synthesis, a link of such fluctuations with the cell division cycle has been postulated.

Structural arrangement of polymers within the wall of Streptococcus faecalis

The structure of the cell wall of Streptococcus faecalis was studied in thin sections and freeze fractures of whole cells and partially purified wall fractions. Also, the structures of wall preparations treated with hot trichloroacetic acid to remove non-peptidoglycan wall polymers were compared with wall preparations that possess a full complement of accessory polymers. The appearance of the wall varied with the degree of hydration of preparations and physical removal of the cell membrane from the wall before study. Seen in freeze fractures of whole cells, the fully hydrated wall seemed to be a thick, largely amorphic layer. Breaking cells with beads caused the cell membrane to separate from the wall and transformed the wall from a predominantly amorphic layer to a structure seemingly made up of two rows of "cobblestones" enclosing a central channel of lower density. Dehydration of walls seemingly caused the cobblestones to be transformed into two bands which continued to be separated by a channel. This channel was also observed in isolated wall preparations treated with hot trichloroacetic acid to remove non-peptidoglycan polymers. These observations are consistent with the interpretation that both peptidogylcan and non-peptidoglycan polymers are concentrated at the outer and inner surfaces of cell walls. These observations are discussed in relation to possible models of wall structure and assembly.

Factors regulating cell wall thickening and intracellular iodophilic polysaccharide storage in Streptococcus mutans

The effects of a series of different antibiotics on the synthesis and accumulation of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), protein, cell wall peptidoglycan (PG), and intracellular iodophilic polysaccharide (IPS) in Streptococcus mutans FA-1 were examined. d-Cycloserine, penicillin G, or vancomycin treatment resulted in rapid inhibitions of PG synthesis and a consequent decrease in the relative amount of lysine found in PG fractions. Decreases in culture turbidity, an indicator of gross cellular lysis, were not observed. Secondary inhibitions of the rates and extent of syntheses of DNA, RNA, and protein were observed. With all three inhibitors of PG synthesis, IPS synthesis continued for varying time intervals but, at most, resulted in only relatively small and transient increases in cellular IPS content. Chloramphenicol inhibited protein synthesis but permitted continued synthesis of RNA and PG. After 6 h, the cells contained 42% of their [(3)H] lysine in the PG fraction compared with 25% in exponential-phase cells, a good indication of thickened cell walls. In the presence of chloramphenicol, cellular IPS content increased about 2.5-fold during the first 45 min and then decreased to a level (13%) at 6 h very similar to that of exponential-phase cells (about 10%). Rifampin inhibition of RNA (and, consequently, also protein) synthesis resulted in accumulation of cellular PG and IPS. After 6 h, IPS accounted for 38% of the cellular dry weight, and the cells contained 43% of their lysine in PG. Thus, rifampin-inhibited cells appear to have both thickened walls and a high IPS content. The correlation between inhibition of RNA synthesis and IPS accumulation was confirmed by exposing cultures to rifampin for 60 min and then removing the drug, thus permitting the cells to regrow. Upon removal of rifampin and resumption of RNA synthesis, cellular IPS content rapidly decreased to the level expected for exponentialphase cells.

Monomer addition as a mechanism of forming peptide cross-links in the cell-wall peptidoglycan of Streptococcus faecalis ATCC 9790

The relative amounts of radioactively labelled disaccharide-peptide monomers and peptide-cross-linked dimers and trimers found in the peptidoglycan of Streptococcus faecalis ATCC 9790 were compared to the relative amounts to be expected from two different polymerization mechanisms (random condensation and monomer addition). Data from continuously-labeled, exponentially-growing cells are consistent with a monomer addition cross-linking process, not with a random condensation cross-linking mechanism. This conclusion was supported by data obtained from analyses of cells labeled during valine starvation (and wall thickening), recovery from valine starvation, and pulse and pulse-chase labeling of walls from exponentially-growing cultures.

Autolytic formation of protoplasts (autoplasts) of Streptococcus faecalis; location of active and latent autolysin

Over 80% of the active and porteinase-activatable, latent forms of the autolytic N-acetylmuramide glycanhydrolase of Streptococcus faecalis ATCC 9790 were released to the supernatant buffer during the autolytic formation of protoplasts (autoplasts) in the presence of absence of trypsin. Autolysin activity was not found in association with released mesosomal vesicles and had little affinity for binding to membranes or to the outer surface of the wall. Isolated walls were able to bind over four times as much autolysin activity as that present on wall exponential-phase cells. Using a rapid technique for wall isolation, evidence was obtained that the latent form (as well as the active form) was wall bound in intact cells. In addition, isotope labeling and ultrastructural studies were able to show that latent autolysin was concentrated in the newer, septally associated portion of the wall.

Study of cycle of cell wall assembly in Streptococcus faecalis by three-dimensional reconstructions of thin sections of cells

A new ultrastructural method was used to study rounds of envelope synthesis that occur in Streptococcus faecalis in "growth zones" found between pairs of naturally occurring surface markers. The technique consists of producing three-dimensional reconstructions of these growth zones from the mathematical rotation, about a central axis, of measurements taken from central, longitudinal thin sections of cells. A cycle of exponential-phase envelope growth was then simulated by arranging a series of these reconstructions in increasing order of the amount of peripheral wall surface area or the amount of cell volume that each was calculated to contain. Using this simulated cycle of growth, the geometry of a single growth zone during a round of synthesis was studied. Based on this analysis, a model was developed for the assembly of the cell wall of S. faecalis. The model states that new cell wall surface is synthesized by the regulated flow of essentially two channels of cell wall precursors into a single growth zone. One channel of precursors would be involved in the assembly of a bilayered cross wall that would proceed at a fairly constant rate until the cross wall closes. The second channel of precursors would be involved in the separation of the bilayered cross wall into two segments of peripheral wall. These precursors would intercalate into and thicken the separating layers of the cross wall. The flow of precursors through this channel would be progressively reduced through a cycle. These decreases, when coupled with internal hydrostatic pressure, apparently would result in the enlarging peripheral wall becoming increasingly more curved and would also promote cell division by reducing the total amount of cell wall that must be assembled in order for septation to occur.

Effect of lipoteichoic acid and lipids on lysis of intact cells of Streptococcus faecalis

Autolysis of intact cells of Streptococcus faecalis was inhibited to a greater extent by phospholipids than by lipoteichoic acid, suggesting a possible difference in the accessibility of native autolysin to these substances.

Inhibition of wall autolysis in Streptococcus faecalis by lipoteichoic acid and lipids

Fully acylated lipoteichoic acid (LTA) isolated from Streptococcus faecalis ATCC9790 (S. faecium) inhibited autolysis of walls from the same organism at concentrations (1.0 to 1.5 nmol of LTA per mg of wall) comparable to those found in intact cells. Partially deacylated LTA isolated from S. faecalis or chemically deacylated LTA failed to inhibit significantly in the same concentration range. Beef heart cardiolipin and commercially obtained dipalmitoyl phosphatidyl glycerol were also found to inhibit wall autolysis in S. faecalis. Chemical deacylation of beef heart cardiolipin also removed the inhibitory activity of this molecule. Lipid fractions isolated from S. faecalis that inhibited wall autolysis were: diphosphatidyl glycerol (cardiolipin), phosphatidyl glycerol, aminoacyl phosphatidyl glycerol, and a neutral lipid fraction. Glycolipids were not found to be effective inhibitors. The possible role of LTA and/or certain lipids as regulators of cellular autolytic activity is discussed.

Unbalanced growth and macromolecular synthesis in Streptococcus mutans FA-1

The continued synthesis of deoxyribonucleic acid, protein, cell wall peptidoglycan and intracellular iodophilic polysaccharide (IPS) by Streptococcus mutans strain FA-1 after several treatments intended to inhibit protein synthesis was studied. Exponential-phase cultures were: (i) simultaneously deprived of two required amino acids (cystine and leucine) that are not present in the cell wall peptidoglycan of this species; (ii) depreived of required amino acids (lysine or glutamate plus glutamine ) that are present in both peptidoglycan and protein; or (iii) treated with tetracycline. Each of these three types of treatment was accompanied by a different pattern of unbalanced growth. The patterns of unbalanced growth that accompanied treatments (i) or (ii) differed substantially from the patterns observed previously for Streptococcus faecalis ATCC 9790, a noncariogenic organism that does not contain IPS. In contrast to S. faecalis 9790, S. mutans FA-1 failed to accumulate peptidoglycan and thicken its wall when deprived of non-wall amino acids. Instead, S. mutans FA-1 continued to accumulate IPS to levels substantially higher than those found in exponential-phase cells. Again, in contrast to S. faecalis, S. mutans FA-1 failed to autolyze upon deprivation of essential precursors of wall peptidoglycan. Under conditions of lysine of glutamate/glutamine deprivation, S. mutans FA-1 continued to accumulate IPS to very high levels. Treatment with tetracycline did result in peptidoglycan accumulation and wall thickening in a manner very similar to that observed previously for inhibition of protein synthesis in S. faecalis. Realtively little IPS synthesis continued after tetracycline treatment. Accumulation of IPS appeared to occur when both ribonucleic acid and peptidoglycan synthesis were severely inhibited. The observations are discussed in terms of the survival of cariogenic organisms in the oral environment.

Analysis of growth rate in sucrose-supplemented cultures of Streptococcus mutans

In the presence of sucrose, Streptococcus mutans grows in large glucan-containing aggregates. Because of reports of linear rather than exponential growth of sucrose-grown cultures, the kinetics of growth of sucrose-grown cultures of S. mutans strain OMZ-176 were compared with those of glucose-grown cultures. Culture turbidity measurements indicated that growth of sucrose cultures was slower, did not follow exponential kinetics, and slowed and stopped at lower absorbance values than did glucose-grown cultures. However, measurements of the rates of accumulation of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein using fully equilibrated radioactively labeled precursors of each of these macromolecular species in sucrose and glucose-grown cultures showed that: (i) for glucose cultures the synthesis of each of the three informational molecules occurred at the same exponential rate, which was identical to the rate of turbidity increase; (ii) for sucrose cultures each macromolecular species was synthesized at the same exponential rate and these rates were identical to the rate of increase of turbidity of the glucose-grown culture for periods of up to 7 h. Furthermore, the ratios of DNA to RNA, RNA to protein, and protein to DNA for the sucrose cultures were identical to those for the glucose cultures for up to 10 doublings. From these data it was concluded that in the presence of sucrose S. mutans grows in a balanced fashion at the same exponential rate as it does in glucose. The deviation from an exponential growth model of the absorbance in sucrose cultures was attributed to an optical artifact due to the formation of large glucan-containing aggregates of cells. The addition of dextranase to sucrose cultures resulted in cultures which increased in turbidity at the same exponential rate as glucose-grown cultures, without affecting the rate or extent of macromolecular synthesis.

Evidence for the synthesis of soluble peptidoglycan fragments by protoplasts of Streptococcus faecalis

Growing protoplasts of Streptococcus faecalis 9790 were found to synthesize and excrete soluble peptidoglycan fragments. The presence of soluble peptidoglycan derivatives in culture supernatants was determined by (i) incorporation of three different radioactively labeled precursors (L-lysine, D-alanine, and acetate) into products which, after hen egg-white lysozyme hydrolysis, had the same KD values on gel filtration as muramidase hydrolysis products of isolated walls; (ii) inhibition of net synthesis of these products by cycloserine and vancomycin; and (iii) identification of disaccharide-peptide monomer using the beta-elimination reaction, gel filtration, and high-voltage paper electrophoresis. Under the conditions of these experiments the presence of newly synthesized, acid-precipitable (macromolecular) peptidoglycan was not detected. The predominance of monomer (70 to 80%) in lysozyme digests of peptidoglycan synthesized by protoplasts was in sharp contrast to digest of walls from intact streptococci which contain mostly peptide cross-linked products. Biosynthesis and release of relatively uncross-linked, soluble peptidoglycan fragments by protoplasts was related to the absence of suitable, preexisting acceptor wall.

Characterization of the presumed peptide cross-links in the soluble peptidoglycan fragments synthesized by protoplasts of Streptococcus faecalis

Protoplasts of Streptococcus faecalis ATCC 9790 were produced with the aid of lysozyme, and the ability of these bodies to synthesize soluble, peptide cross-linked peptidoglycan (PG) fragments was examined. Lysozyme digests of PG isolated using gel filtration from the supernatant medium of protoplasts grown in the presence of [14C]acetate and L-[3H]lysine contained small amounts of PG having KD expected for peptide cross-linked dimers and trimers. Addition of benzyl penicillin (300 mug/ml) to growing protoplast cultures did not affect the net amount of PG fragments synthesized but resulted in inhibition of synthesis of dimer and trimer fractions by 27 and 59%, respectively. Failure of penicillin to completely inhibit the accumulation of the dimer fraction was attributed to the presence of atypical forms of dimer. In fact, the supernatant medium of penicillin-treated cultures did not contain detectable amounts of typical peptide cross-linked dimer. The degree of peptide cross-linkage of protoplast PG was at most only 13% of that found in walls isolated from intact streptococci. The relative amounts of monomers, dimers, and trimers synthesized during early and late stages of protoplast growth was approximately the same. Protoplasts synthesized soluble PG fragments in amounts which were of the same order of magnitude as that expected for insoluble PG produced by an equivalent amount of intact streptococci.

Synthesis of peptidoglycan in the form of soluble glycan chains by growing protoplasts (autoplasts) of Streptococcus faecalis

Protoplasts (autoplasts) of Streptococcus faecalis were produced by the action of native autolytic N-acetylmuramidase in the absence of added peptidoglycan hydrolases and were grown in osmotically stabilized medium containing L-[3H]lysine and D-[14C]alanine. To reduce the level of muralytic hydrolysis of glycan chains during growth, heat-inactivated cell walls were added to the medium to bind autolytic enzyme, and tetracycline (1 mug/ml) was added to inhibit further enzyme synthesis. Under these conditions, protoplasts synthesized newly labeled peptidoglycan in the form of soluble, infrequently peptide cross-linked glycan chains which were released into the supernatant medium. These relatively large glycan chains were not transferred to exogenously added cell walls.