Lysis of Streptococcus mutans cells with mutanolysin, a lytic enzyme prepared from a culture liquor of Streptomyces globisporus 1829

Cell Wall Damage Reveals Spatial Flexibility in Peptidoglycan Synthesis and a Nonredundant Role for RodA in Mycobacteria

Cell wall peptidoglycan is a heteropolymeric mesh that protects the bacterium from internal turgor and external insults. In many rod-shaped bacteria, peptidoglycan synthesis for normal growth is achieved by two distinct pathways: the Rod complex, comprised of MreB, RodA, and a cognate class B penicillin-binding protein (PBP), and the class A PBPs (aPBPs). In contrast to laterally growing bacteria, pole-growing mycobacteria do not encode an MreB homolog and do not require SEDS protein RodA for in vitro growth. However, RodA contributes to the survival of Mycobacterium tuberculosis in some infection models, suggesting that the protein could have a stress-dependent role in maintaining cell wall integrity. Under basal conditions, we find here that the subcellular distribution of RodA largely overlaps that of the aPBP PonA1 and that both RodA and the aPBPs promote polar peptidoglycan assembly. Upon cell wall damage, RodA fortifies Mycobacterium smegmatis against lysis and, unlike aPBPs, contributes to a shift in peptidoglycan assembly from the poles to the sidewall. Neither RodA nor PonA1 relocalize; instead, the redistribution of nascent cell wall parallels that of peptidoglycan precursor synthase MurG. Our results support a model in which mycobacteria balance polar growth and cell-wide repair via spatial flexibility in precursor synthesis and extracellular insertion. IMPORTANCE Peptidoglycan synthesis is a highly successful target for antibiotics. The pathway has been extensively studied in model organisms under laboratory-optimized conditions. In natural environments, bacteria are frequently under attack. Moreover, the vast majority of bacterial species are unlikely to fit a single paradigm of cell wall assembly because of differences in growth mode and/or envelope structure. Studying cell wall synthesis under nonoptimal conditions and in nonstandard species may improve our understanding of pathway function and suggest new inhibition strategies. Mycobacterium smegmatis, a relative of several notorious human and animal pathogens, has an unusual polar growth mode and multilayered envelope. In this work, we challenged M. smegmatis with cell wall-damaging enzymes to characterize the roles of cell wall-building enzymes when the bacterium is under attack.

Inhibition of nicotine-induced Streptococcus mutans biofilm formation by salts solutions intended for mouthrinses

OBJECTIVES

Biofilm formation is critical to dental caries initiation and development. The aim of this study was to investigate the effects of nicotine exposure on Streptococcus mutans (S. mutans) biofilm formation concomitantly with the inhibitory effects of sodium chloride (NaCl), potassium chloride (KCl) and potassium iodide (KI) salts. This study examined bacterial growth with varying concentrations of NaCl, KCl, and KI salts and nicotine levels consistent with primary levels of nicotine exposure.

MATERIALS AND METHODS

A preliminary screening experiment was performed to investigate the appropriate concentrations of NaCl, KCl, and KI to use with nicotine. With the data, a S. mutans biofilm growth assay was conducted using nicotine (0-32 mg/mL) in Tryptic Soy broth supplemented with 1% sucrose with and without 0.45 M of NaCl, 0.23 M of KCl, and 0.113 M of KI. The biofilm was stained with crystal violet dye and the absorbance measured to determine biofilm formation.

RESULTS

The presence of 0.45 M of NaCl, 0.23 M of KCl, and 0.113 M of KI significantly inhibited (p < 0.05) nicotine-induced S. mutans biofilm formation by 52%, 79.7%, and 64.1%, respectively.

CONCLUSIONS

The results provide additional evidence regarding the biofilm-enhancing effects of nicotine and demonstrate the inhibitory influence of these salts in reducing the nicotine-induced biofilm formation. A short-term exposure to these salts may inhibit S. mutans biofilm formation.

Peptidoglycan remodeling by the coordinated action of multispecific enzymes

The peptidoglycan (PG) cell wall constitutes the main defense barrier of bacteria against environmental insults and acts as communication interface. The biochemistry of this macromolecule has been well characterized throughout the years but recent discoveries have unveiled its chemical plasticity under environmental stresses. Non-canonical D-amino acids (NCDAA) are produced and released to the extracellular media by diverse bacteria. Such molecules govern cell wall adaptation to challenging environments through their incorporation into the polymer, a widespread capability among bacteria that reveals the inherent catalytic plasticity of the enzymes involved in the cell wall metabolism. Here, we analyze the recent structural and biochemical characterization of Bsr, a new family of broad spectrum racemases able to generate a wide range of NCDAA. We also discuss the necessity of a coordinated action of PG multispecific enzymes to generate adequate levels of modification in the murein sacculus. Finally, we also highlight how this catalytic plasticity of NCDAA-incorporating enzymes has allowed the development of new revolutionary methodologies for the study of PG modes of growth and in vivo dynamics.

The role of cell surface architecture of lactobacilli in host-microbe interactions in the gastrointestinal tract

Lactobacillus species can exert health promoting effects in the gastrointestinal tract (GIT) through many mechanisms, which include pathogen inhibition, maintenance of microbial balance, immunomodulation, and enhancement of the epithelial barrier function. Different species of the genus Lactobacillus can evoke different responses in the host, and not all strains of the same species can be considered beneficial. Strain variations may be related to diversity of the cell surface architecture of lactobacilli and the bacteria's ability to express certain surface components or secrete specific compounds in response to the host environment. Lactobacilli are known to modify their surface structures in response to stress factors such as bile and low pH, and these adaptations may help their survival in the face of harsh environmental conditions encountered in the GIT. In recent years, multiple cell surface-associated molecules have been implicated in the adherence of lactobacilli to the GIT lining, immunomodulation, and protective effects on intestinal epithelial barrier function. Identification of the relevant bacterial ligands and their host receptors is imperative for a better understanding of the mechanisms through which lactobacilli exert their beneficial effects on human health.

Characterization of structural variations in the peptidoglycan of vancomycin-susceptible Enterococcus faecium: understanding glycopeptide-antibiotic binding sites using mass spectrometry

Enterococcus faecium, an opportunistic pathogen that causes a significant number of hospital-acquired infections each year, presents a serious clinical challenge because an increasing number of infections are resistant to the so-called antibiotic of last resort, vancomycin. Vancomycin and other new glycopeptide derivatives target the bacterial cell wall, thereby perturbing its biosynthesis. To help determine the modes of action of glycopeptide antibiotics, we have developed a bottom-up mass spectrometry approach complemented by solid-state nuclear magnetic resonance (NMR) to elucidate important structural characteristics of vancomycin-susceptible E. faecium peptidoglycan. Using accurate-mass measurements and integrating ion-current chromatographic peaks of digested peptidoglycan, we identified individual muropeptide species and approximated the relative amount of each. Even though the organism investigated is susceptible to vancomycin, only 3% of the digested peptidoglycan has the well-known D-Ala-D-Ala vancomycin-binding site. The data are consistent with a previously proposed template model of cell-wall biosynthesis where D-Ala-D-Ala stems that are not cross-linked are cleaved in mature peptidoglycan. Additionally, our mass-spectrometry approach allowed differentiation and quantification of muropeptide species seen as unresolved chromatographic peaks. Our method provides an estimate of the extent of muropeptides containing O-acetylation, amidation, hydroxylation, and the number of species forming cyclic imides. The varieties of muropeptides on which the modifications are detected suggest that significant processing occurs in mature peptidoglycan where several enzymes are active in editing cell-wall structure.

Structural variation in the glycan strands of bacterial peptidoglycan

The normal, unmodified glycan strands of bacterial peptidoglycan consist of alternating residues of beta-1,4-linked N-acetylmuramic acid and N-acetylglucosamine. In many species the glycan strands become modified after their insertion into the cell wall. This review describes the structure of secondary modifications and of attachment sites of surface polymers in the glycan strands of peptidoglycan. It also provides an overview of the occurrence of these modifications in various bacterial species. Recently, enzymes responsible for the N-deacetylation, N-glycolylation and O-acetylation of the glycan strands were identified. The presence of these modifications affects the hydrolysis of peptidoglycan and its enlargement during cell growth. Glycan strands are frequently deacetylated and/or O-acetylated in pathogenic species. These alterations affect the recognition of bacteria by host factors, and contribute to the resistance of bacteria to host defence factors such as lysozyme.

The putative autolysin regulator LytR in Streptococcus mutans plays a role in cell division and is growth-phase regulated

Streptococcus mutans is the primary odontopathogen present in supragingival plaque and causes the oral disease known as dental caries. Colonization of the oral cavity by S. mutans requires the bacteria to adhere to the tooth surface and occurs by both sucrose-dependent and -independent mechanisms. Sucrose-independent adhesion of S. mutans in vitro has been shown to involve an ORF (ORF0317) encoding a homologue (39 %) to LytR, a regulator of autolysin activity in Bacillus subtilis. The protein encoded by ORF0317, LytR, belongs to the LytR/CpsA/Psr protein family. This family has a putative role in cell-wall structural maintenance, possibly through autolysin regulation. Autolysins have also been shown to be important in surface adhesion in Lactococcus lactis and in the pathogenic properties of Streptococcus pneumoniae. To investigate the role of autolysins in the adhesion and pathogenesis of S. mutans, a LytR mutant was constructed. The mutant grows in long chains, which may indicate a defect in cell division. Further experiments with the mutant strain show increased autolytic activity, indicating that LytR attenuates S. mutans autolytic activity, possibly through regulation of the expression of autolytic enzymes. No defect in cell-to-surface adherence or biofilm growth was seen in the LytR mutant. However, a connection between cell growth phase and transcription of lytR was found.

Real-time PCR for quantification of Streptococcus mutans

A real-time polymerase chain reaction (PCR) assay was developed for the quantification of Streptococcus mutans. Primers targeting gtf genes of S. mutans were designed and tested for their specificity using 28 oral streptococcal strains, three other bacterial strains, and human DNA. The primers could amplify specifically the target DNA fragment from a mixture of oral streptococcus genomic DNA containing about 10 fg to 10 ng of S. mutans genome DNA. The real-time PCR produced a linear quantitative detection range over concentrations spanning seven exponential values, with a detection limit of a few copies of S. mutans' genomic DNA per reaction tube. The results of the real-time PCR assay corresponded well to those of conventional culture assays for S. mutans in saliva samples. A real-time PCR assay for Streptococcus sobrinus and Streptococcus downei was also established and produced results that corresponded well to those from conventional culture assays for S. sobrinus in saliva samples. These assays will be useful as a new means to assess one of the important risk factors for caries.

The role of O-acetylation in the metabolism of peptidoglycan in Providencia stuartii

The gentamicin 2'-N-acetyltransferase [EC 2.3.1.59; AAC(2')-Ia] of Providencia stuartii was shown to contribute to the O-acetylation of peptidoglycan and mutants that either under- or overexpress the aac(2')-Ia gene was characterized phenotypically to possess either lower or higher levels of peptidoglycan O-acetylation, respectively, compared to the wild-type. These mutants were subjected to scanning electron microscopy. P. stuartii PR100, with 42-44% peptidoglycan O-acetylation compared to 54% for the wild-type, appeared as irregular rods. In direct contrast, strains PR50.LM3 and PR51, with increased levels of peptidoglycan O-acetylation (63 and 65%, respectively), appeared as coccobacilli or chain formers, respectively. Zymogram analysis of the autolysins produced by another member of the closely related Proteeae group of bacteria, Proteus mirabilis, indicated the presence of three classes of enzymes: one that acts preferentially on native, O-acetylated peptidoglycan, a second that hydrolyses non-O-acetylated peptidoglycan, and a third that is not distinguished by the two forms of substrate. On the basis of the apparent morphological changes directly related to levels of O-acetylation combined with the presence of different classes of autolysins, a model is proposed that invokes the role of this modification in the control of autolysins for the maintenance of the structure of the peptidoglycan sacculus.

Multiple changes in cell wall antigens of isogenic mutants of Streptococcus mutans

Isogenic mutants of Streptococcus mutans LT11, deficient in the production of the wall-associated protein antigens A and B, were generated by recombinant DNA technology. The hydrophobicity, adherence, and aggregation of the mutants were compared with those of the parent strain. These studies indicated that hydrophobicity, adherence, and saliva- or sucrose-induced aggregation were unaltered in the A- mutant but that hydrophobicity and adherence to saliva-coated hydroxylapatite were greatly reduced in the B- mutant whilst sucrose-dependent adherence and aggregation were increased. To determine whether these changes correlated with changes in the mutated gene product alone, the levels of a number of cell wall antigens were determined in each of the mutants. The loss of antigen A resulted in significantly reduced levels of wall-associated lipoteichoic acid, and loss of antigen B resulted in reductions in both antigen A and lipoteichoic acid. Data presented here thus suggest that changes in the expression of one wall antigen can have a dramatic effect on the levels of others.

Extent of peptidoglycan O acetylation in the tribe Proteeae

The degree of peptidoglycan O acetylation in 18 strains of the different genera of the tribe Proteeae (Proteus, Providencia, and Morganella) has been determined by high-performance liquid chromatography-based organic acid analysis of mild-base-released acetic acid and quantitation of peptidoglycan concentrations by simultaneous amino sugar-amino acid analysis using high-performance anion-exchange chromatography with pulsed amperometric detection. The N,O-diacetylmuramyl content of all isolated and purified peptidoglycans was greater than 29% and ranged up to 57% relative to total muramic acid concentration. Each of the O-acetylated peptidoglycans was found to be resistant to solubilization by hen egg white lysozyme.

Association between the 65-kilodalton heat shock protein, Streptococcus sanguis, and the corresponding antibodies in Behçet's syndrome

The etiology of Behcet's syndrome (BS) is unknown, but a number of streptococcal species have been implicated. A hypothesis was postulated that a shared antigen, such as a stress protein, might account for some of these findings. Indeed, a rabbit antiserum against a 65-kDa heat shock protein of Mycobacterium tuberculosis revealed a corresponding 65-kDa band with all six Streptococcus sanguis strains examined and S. pyogenes but not with S. salivarius. By applying a panel of nine monoclonal antibodies to the mycobacterial 65-kDa heat shock protein, an approximately 65-kDa antigen was identified in the uncommon serotypes of S. sanguis ST3 and H.83 and one with a different Mr was identified in KTH-1 and S. pyogenes. Monoclonal antibodies Y1.2, C1.1, II H9, and ML30, which reacted with these streptococci, recognize residues 11 to 27, 88 to 123, 107 to 122, and 276 to 297 of the 65-kDa heat shock protein, respectively, suggesting that these residues are conserved among some uncommon serotypes of S. sanguis and S. pyogenes. Immunoblot analyses of sera from patients with BS for immunoglobulin A (IgA) and IgG antibodies revealed bands of 65 to 70 kDa with the mycobacterial heat shock protein, S. sanguis strains, and S. pyogenes, although these reactivities were also found to a lesser extent in controls. A 65- to 70-kDa band was found more frequently with S. sanguis KTH-2 or KTH-3 and IgA in serum from patients with BS than with serum from controls (P less than 0.02). Antibodies in serum were then studied by a radioimmunoassay, and in patients with BS this revealed significantly raised IgA antibodies to the recombinant 65-kDa mycobacterial heat shock protein and to soluble protein extracts of S. sanguis ST3, KTH-1, KTH-2, and KTH-3. Whereas significant anti-65-kDa heat shock protein and anti-S. sanguis ST3 antibodies were also found in sera from patients with rheumatoid arthritis and recurrent oral ulcers, the anti-S. sanguis KTH-1, KTH-2, and KTH-3 antibodies were confined to BS. The results are consistent with the hypothesis that some of the streptococcal antigens are associated with heat shock or stress proteins, which will need to be formally established by isolating heat shock proteins from streptococci.

Mutanolysin-induced lysis of actinomyces pyogenes determined by aggregometry

The lytic activity of mutanolysin from Streptomyces globisporus on 42 cultures of Actinomyces pyogenes could be effectively analyzed in an aggregometer. It was expressed as increase of transmittance at 546 nm after 20 min and 2 h at 37 degrees C. The A. pyogenes cultures revealed no uniform lysis pattern. Most of the cultures were lyzed within 20 to 40 min at 37 degrees C, others were lyzed only moderately or weakly within 2 h of incubation. The lytic activity was optimal at low (0.01 mol/l) molarity of the lysis buffer between pH 5.7 and 7 and could be inhibited by HgCl2.A. pyogenes was not lyzed by lysostaphin or lysozyme.

Biological and immunochemical identity of M protein on group G streptococci with M protein on group A streptococci

Previous evidence for the presence of an M or M-like protein on group G streptococci has been based on the ability of these strains to survive in human blood. In addition, cross-reactions between group A and group G streptococci have been demonstrated, but they have relied either on whole bacterial cell vaccine-induced polyclonal sera or crude protein extracts of these cells. In this study two monoclonal antibodies prepared against the purified, native group A streptococcal M6 protein demonstrated a high degree of cross-reactivity with group G streptococcal clinical isolates (9 and 19 of 22 strains examined, respectively). Ten of these strains exhibited resistance to phagocytosis when rotated in human blood. In addition, immunoblot analysis of crude mutanolysin extracts of group G streptococci with one of the M6 monoclonal antibodies illustrated a remarkable similarity in the protein pattern of these extracts as compared with those of group A streptococcal M protein. The immunoblots further demonstrated a variation in the relative molecular weights of the extracted proteins from strain to strain over a range of 57,000 to 77,000. In addition, a purified, pepsin-derived fragment (Mr, 43,000) from a group G strain was capable of eliciting rabbit antibodies that were opsonic for group G cells in a bactericidal assay. These functional and immunochemical data, in concert with DNA hybridization between group G streptococcal DNA and a group A M6 gene probe (J. R. Scott, W. M. Pulliam, S. K. Hollingshead, and V. A. Fischetti, Proc. Natl. Acad. Sci. USA 82:1822-1826, 1985), provide strong evidence for the presence of an M protein on these organisms and indicate its probable role as a virulence molecule on the surface of group G streptococci.

Effective murolytic solubilization of streptococcal-group-specific antigen

Streptococcal-group-specific antigens were solubilized with a murolytic enzyme contained in the culture supernatant of Streptomyces globisporus. This facilitated the effective serogrouping of streptococci from humans and animals.

Size variation of the M protein in group A streptococci

In addition to the type-specific antigenic variation that is a well-known characteristic for the group A streptococcal M protein, we have now found that the M molecules vary with respect to their molecular size, both between M types and within an M type. By the use of an M6 monoclonal antibody, which crossreacts with 20 different M protein types, and antibodies to the N-acetyl glucosamine determinant of the cell wall, we have been able to identify the M protein molecules released from the streptococcal cell wall with muralytic enzymes, particularly group C phage-associated lysin. Immunoblot analysis of the cell extract identified M protein molecules bound to various cell wall fragments, suggesting a peptidoglycan linkage for the M molecule. M protein extracted from 20 different streptococcal serotypes revealed size variations from 41,000 to 80,000 in molecular weight. This extreme variation is unusual for related proteins. Similar size variations in the M molecule were also found in random clinical isolates of type 6 streptococci. No size change was seen in M6 protein isolated from: (a) strains within a limited epidemic, (b) a strain passaged in mice 192 times, and (c) a strain passaged in the laboratory for 156 generations, suggesting that the observed variation is not a rapid process. The results indicate that, within the broad limits observed in this study, the size of the M protein may not be critical to the antiphagocytic activity of the molecule.

Treatment of experimental erosive arthritis in rats by injection of the muralytic enzyme mutanolysin

A single intravenous injection into rats of 0.4 mg of the muralytic enzyme mutanolysin, given as long as 3 d after an arthropathic dose of peptidoglycan-polysaccharide polymers derived from group A streptococci (PG-APS), resulted in a complete resolution of acute arthritis and the prevention of chronic joint disease. When administration of mutanolysin was delayed until 14 d after the injection of PG-APS, a great reduction in the severity of chronic inflammation was still observed. Quantitation of the amount of PG-APS present in the limbs, spleen, and liver by a solid phase enzyme-linked immunoassay indicated that the tissues of mutanolysin-treated rats contained as much PG-APS as tissues of PBS-treated control rats. In addition, rats treated with mutanolysin immediately after receiving an intraperitoneal injection of PG-APS developed a transient limb edema similar to that seen in rats after the injection of PG-APS digested to a small fragment size in vitro with mutanolysin. We hypothesize that mutanolysin acts in vivo by degrading PG-APS to small fragments that persist but are no longer arthropathic.

Effect of fructose and other carbohydrates on the surface properties, lipoteichoic acid production, and extracellular proteins of Streptococcus mutans Ingbritt grown in continuous culture

Streptococcus mutans Ingbritt was grown in a chemostat at destined dilution rates in either 0.5% fructose or 0.5% sorbitol and at destined pH values in 0.5% fructose. The yield of cells was affected by the carbohydrate source, as well as by the pH, with the lowest yield being at pH 5.5 in 0.5% fructose. Fructose-grown cells showed greater susceptibility to lysis by a muramidase than the corresponding glucose-grown cells, but there were no marked differences in the lytic susceptibilities of the corresponding cell wall preparations or in the serological reactivities of wall lysates with antiserum to S. mutans Ingbritt. The greatest amounts of cellular lipoteichoic acid were obtained at high dilution rates in both fructose and sorbitol, as well as at high pH values in fructose. The greatest amounts of extracellular lipoteichoic acid were found at low dilution rates, as estimated by rocket immunoelectrophoresis and also by hemagglutination. Three major extracellular protein components were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the effects of growth conditions on these components were determined. Results for batch-grown cultures showed that there was genotypic variation in the susceptibility of cells to lysis by a muramidase. The enhancement of lipoteichoic acid production by fructose and sorbitol in batch cultures was not identical in representative strains of S. mutans serotype c, nor was the effect of fructose found uniformly in representative strains of the different S. mutans serotypes.

Selective antibacterial properties of lysozyme for oral microorganisms

The antibacterial properties of lysozyme were investigated with oral microorganisms representing the seven serotypes (a through g) of Streptococcus mutans, Veillonella alcalescens, and the virulent (V) and avirulent (AV) strains of Actinomyces viscosus T14. Growth of bacteria in defined medium was monitored spectrophotometrically after the addition of various amounts (25 mug to 5 mg/ml) of enzyme. No growth inhibition of V. alcalescens was observed. Inhibition of A. viscosus T14(V) and A. viscosus T14(AV) occurred with 160 mug of lysozyme per ml. Of the S. mutans cultures tested, the serotype a and b strains were inhibited with as little as 25 mug of enzyme per ml, whereas e and f strains were most resistant to the bacteriostatic activity of lysozyme. The presence of dl-threonine or sucrose in growth medium did not significantly affect the results. A lysoplate assay was developed to rapidly survey the bacterial cultures for their susceptibility to the lytic ability of the enzyme. Lysis, as a measure of a zone of clearing in agarose plates, occurred for all microorganisms in the presence of lysozyme after the subsequent addition of NaCl or detergent. The bactericidal activity of lysozyme was determined on S. mutans BHT and S. mutans LM-7 by the pour plate technique. Preincubation of S. mutans LM-7 with as much as 1 mg of enzyme for 90 min did not affect viability or growth, whereas preincubation of S. mutans BHT with 1 mg of lysozyme resulted in no recoverable colony-forming units. An antigen containing extract of S. mutans LM-7 blocked the growth inhibitory property of lysozyme. Human lysozyme was a more effective antibacterial factor than hen egg white lysozyme. Total growth inhibition of S. mutans BHT was effected with 40 mug of human enzyme, and as little as 10 mug of human enzyme inhibited growth for greater than 20 h. The data presented indicate that different mechanisms may be responsible for the bacteriostatic, lytic, and bactericidal properties of the enzyme and that lysozyme is a selective but effective antibacterial factor for oral microorganisms.

Biology, immunology, and cariogenicity of Streptococcus mutans

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Lysis and protoplast formation of group B streptococci by mutanolysin

Group B streptococci, refractory to previously tested muralysins under physiological conditions, were successfully converted to protoplasts by use of a recently describede N-acetyl muramidase, mutanolysin, derived from a streptomycete. Purified enzyme was effective, but crude preparations, although degrading cell walls, simultaneously produced peculiar effects of cytoplasmic coagulation, retention of cell shape, loss of some intracellular enzymes, and a rise in optical density. Addition of purified mutanolysin to the array of muralysins (group C streptococcal phage-associated lysin, lysozyme), previously successful in preparing protoplasts of different streptococci, now makes possible enzymatic preparation of protoplasts of streptococci of groups A, B, C. D. G, and H.

Association of protein with the cell wall of Streptococcus mutans

Cell walls from Streptococcus mutans were prepared by conventional technique and subjected to a series of extraction procedures involving classical protein solvents. The extracted walls contained several non-peptidoglycan amino acids and were also amenable to radiolabeling with [125I]sodium iodide and chloramine T. The cell walls could be chemically modified with tetranitromethane and diazo-1H-tetrazole, suggesting the presence of tyrosine or histidine or both. Flourescence spectra of the walls revealed the presence of either tyrosine or tryptophan. Several proteases, including pronase, trypsin, subtilisin, and proteinase K, removed some of the label from the walls. In contrast, treatment of the walls with salts or denaturants did not result in the solubilization of label. When the walls were solubilized with mutanolysin and subjected to chromatography, three peaks of radioactivity with apparent molecular weights of 73,000, 39,000, and 9,600 were observed. Wall digests subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single band of radioactivity corresponding to an apparent molecular weight of 79,000. Isoelectric focusing of labeled wall digest gave rise to two major bands of radioactivity with isoelectric points of approximately 2.4 and 5.6. The results suggest that the cell wall of S. mutans contains tightly and possibley covalently bound polypeptide molecules. We propose that the cell wall polypeptides of S. mutans serve as factors in the attachment of the bacteria to smooth surfaces.