DNA probe for identification of Streptococcus pneumoniae

A total of 287 clinical isolates of Streptococcus pneumoniae (pneumococcus) were tested for their ability to undergo autolysis when treated with sodium deoxycholate. The test was positive for all but one isolate, strain DOC-1. This autolysis required the activity of an enzyme which is unique and characteristic of S. pneumoniae: a choline-dependent N-acetylmuramoyl-L-alanine amidase, the gene product of the lytA gene. We used lytA as a DNA probe to test the distribution of the autolysin gene among clinical isolates of S. pneumoniae. In dot blot hybridization experiments our probe reacted with the DNA of 60 of 60 strains tested, including the autolysis-deficient clinical isolate DOC-1. No hybridization occurred when strains of Streptococcus sanguis, Streptococcus mutans, Streptococcus pyogenes, Streptococcus (Enterococcus) faecalis, Streptococcus (Enterococcus) faecium, Streptococcus agalactiae, and Streptococcus bovis were tested. The lytA gene appears to be an ideal candidate for use as a DNA probe for the identification of S. pneumoniae.

Involvement of multiple genetic determinants in high-level methicillin resistance in Staphylococcus aureus

A methicillin-susceptible, novobiocin-resistant strain of Staphylococcus aureus (RN2677; methicillin MIC, 0.8 micrograms/ml) was transformed with DNA prepared from highly and homogeneously methicillin-resistant S. aureus strains (methicillin MIC, greater than or equal to 400 micrograms/ml) or from heterogeneous strains in which the majority of cells had a low level of resistance (methicillin MIC, 6.3 micrograms/ml). All methicillin-resistant transformants showed low and heterogeneous resistance (methicillin MIC, 3.1 micrograms/ml) irrespective of the resistance level of DNA donors. All transformants examined produced normal amounts of the low-affinity penicillin-binding protein (PBP) 2a, and methicillin resistance and the capacity to produce PBP 2a showed the same degree of genetic linkage to the novobiocin resistance marker with both homogeneous and heterogeneous DNA donors. Next, we isolated a methicillin-susceptible mutant from a highly and homogeneously resistant strain which had a Tn551 insertion near or within the PBP 2a gene and thus did not produce PBP 2a. With this mutant used as the recipient, genetic transformation of the methicillin resistance gene was repeated with DNA isolated either from highly and homogeneously resistant strains or from heterogeneous (low-resistance) strains. All transformants obtained expressed high and homogeneous resistance and produced PBP 2a irrespective of the resistance level of the DNA donors. Our findings suggest that (i) the methicillin resistance locus is identical to the structural gene for PBP 2a, (ii) although the ability to produce PBP 2a is essential for resistance, the MICs for the majority of cells are not related to the cellular concentration of PBP 2a, and (iii) high MICs and homogeneous expression of resistance require the products of other distinct genetic elements as well.

Penicillin-binding protein families: evidence for the clonal nature of penicillin resistance in clinical isolates of pneumococci

In view of the worldwide emergence of penicillin-resistant pneumococci among clinical isolates it was of importance to examine a large number of strains to test the uniformity of the resistance mechanism. Among 160 clinical isolates of pneumococci (minimum inhibitory concentration [MIC], 0.005-16 micrograms/mL), susceptible strains showed a common pattern of five penicillin-binding proteins (PBPs) with high penicillin affinities (PBP 3 greater than 1A greater than or equal to 2A greater than 1B greater than 2B). PBPs 1A, 2A, and 2B (but not PBP 3) each showed distinct stepwise decreases in penicillin affinities parallel with increasing levels of antibiotic resistance. The number and molecular sizes of PBPs became variable in strains with MIC values greater than 1.0 microgram/mL; among 39 strains with a MIC of greater than or equal to 1.0 microgram/mL, 11 distinct and stable PBP patterns could be identified. Using PBP profiles, serotypes, and antibiotic resistance patterns, as well as data on isolation dates and sites, we identified at least three groups of resistant strains that showed clear indication of clonal origin.

Mechanism of pneumococcal cell wall degradation in vitro and in vivo

We compared the products of autolytic amidase-catalyzed wall degradation in vivo (in penicillin-induced lysis) and in vitro. Pneumococci labeled in their cell wall stem peptides by radioactive lysine were treated with penicillin, and the nature of wall degradation products released to the medium during lysis of the bacteria was determined. At early times of lysis (20% loss of wall label), virtually all the radioactive peptides released (greater than 94%) were of high molecular size and were still attached to glycan and teichoic acid. At times of more extensive bacterial lysis (56%), progressively larger and larger fractions of the released peptides became free, i.e., detached from glycan and teichoic acid. Analysis of the nondegraded residual wall material by high-resolution high-pressure liquid chromatography revealed that this in vivo-triggered autolysis did not involve selective hydrolysis of some of the chemically distinct stem peptides. Parallel in vitro experiments yielded completely different results. Purified pneumococcal cell walls labeled with radioactive lysine were treated in vitro with low concentrations of pure amidase, and the nature of wall degradation products released during limited hydrolysis and after more extensive degradation was determined. In sharp contrast to the in vivo experiments, the main products of in vitro hydrolysis were free peptides. After a short treatment with amidase (resulting in a 20% loss of label), the material released was enriched for the monomeric stem peptides. At all times of hydrolysis (including the time of extensive degradation), only a relatively small fraction of the released wall peptides was covalently attached to glycan and teichoic acid components (17% as compared with 40% in the intact cell wall). We propose that the in vivo-triggered amidase activity first attacks the amide bonds in some strategically located (or unprotected) stem peptides that hold large segments of cell wall material together. The observations indicate that the in vivo activity of the pneumococcal autolysin is under topographic constraints.

Insertional inactivation of the major autolysin gene of Streptococcus pneumoniae

The lytA gene encoding the major pneumococcal autolysin (N-acetylmuramoyl-L-alanine amidase) was inactivated by inserting the 2-kilobase MspI fragment of pE194 containing the staphylococcal ermC gene. Stable autolysis-deficient (Lyt-) mutants and their isogenic Lyt+ parents were used in experiments designed to test possible physiological functions of the amidase. No autolysis could be induced in the mutants grown at 37 degrees C by deoxycholate, by incubation in stationary phase, or by treatment with penicillin. On the other hand, the Lyt- mutants exhibited normal growth rates and yields and normal adaptive responses during shifts from one growth temperature or nutritional condition to another. There was no evidence for impeded cell separation (chain formation). Colonies of Lyt- insertional mutants produced normal hemolytic zones on blood agar; they showed normal (high) levels of competence for genetic transformation. Lyt- mutants were also able to produce type 3 and 6 capsular polysaccharides, and such strains showed the same degree of virulence in mice as did the isogenic Lyt+ parent. The physiological function(s) of the amidase remains a puzzle.

Penicillin resistance and defective lysis in clinical isolates of pneumococci: evidence for two kinds of antibiotic pressure operating in the clinical environment

Seventy percent of clinical isolates of penicillin-resistant pneumococci also exhibit defective lysis when treated with penicillin exceeding the minimal inhibitory concentration (MIC). To provide a possible explanation for the frequent association of these two traits, we exposed penicillin-susceptible pneumococci to two kinds of antibiotic pressures in the laboratory. Treatment of cultures with cycles of high concentrations of penicillin (20 X MIC) followed by growth of the survivors in drug-free medium selected for lysis-defective mutants that died only slowly during antibiotic treatment but had unchanged MICs. Exposure to sustained, low levels of penicillin produced resistant mutants, with elevated MICs, that lysed normally with penicillin. We suggest that the cyclic antibiotic exposure generally used in the clinical setting may select primarily for enhanced survival. From these survivors a second type of antibiotic exposure--sustained antibiotic concentrations just above the MIC (concentrations that may be restricted to the tail-end trough of a dosing interval)--selects for penicillin-resistant mutants.

Altered peptidoglycan structure in a pneumococcal transformant resistant to penicillin

A series of isogenic pneumococcal transformants differing in their levels of penicillin resistance and containing altered penicillin-binding proteins were compared for their cell wall structures by using a recently developed technique that can resolve the peptidoglycan stem peptides of Pneumococcus strains to over 40 components (J. F. Garcia-Bustos, B. T. Chait, and A. Tomasz, J. Biol. Chem. 32:15400-15405). The stem peptides from the highly resistant transformants differed strikingly from those of the susceptible recipient strain, and the peptide patterns were almost identical to that of the DNA donor. Four peptides representing the major components in the walls of susceptible cells were replaced by six new peptides that were only minor components of susceptible cell walls. A remarkable common feature of these new species was their high alanine content. Amino acid analysis, sequencing, and mass spectrometry allowed the assignment of the extra alanine residues to dialanine or alanylserine cross bridges in the six new stem peptides. The common feature of the four peptide species that were present as major components in the susceptible walls, but became minor species in the resistant cells, was the absence of a cross bridge in at least one of the stem peptide components. We suggest that the extensive remodelling of cell wall synthetic enzymes that accompanies acquisition of penicillin resistance eventually also alters the reactivity of these proteins towards their natural substrates in cell wall synthesis. As a result, highly penicillin-resistant pneumococci will shift from the use of wall precursors with linear stem peptides to a preferential use of precursors containing the more-hydrophobic peptides carrying dialanyl or alanylserine cross bridges.

A sandwich adhesion on Streptococcus pneumoniae attaching to human oropharyngeal epithelial cells in vitro

Streptococcus pneumoniae attach to human pharyngeal epithelial cells through the specific interaction of bacterial surface adhesins with glycoconjugate receptors. The present study defines the adhesin as a molecule bridging between an anchoring site in the bacterial cell wall and the epithelial cell receptor. The nature of the adhesin was defined in three ways: First, the attachment of whole bacteria was reduced by trypsin, periodate and heat. Second, heat treatment of whole bacteria was shown to release material, which was able to reconstitute the adherence. The heat extract bound to epithelial cells, as shown by fluorescence labelling, and agglutinated latex beads covalently coupled with receptor oligosaccharide. Active material could be extracted by heat from both high and low adhering strains, but could reconstitute only attaching strains. Third, the bacterial component binding the adhesin was localized to protoplasts and cell wall fractions obtained by mechanical or deoxycholate induced lysis of pneumococci. Isolated pneumococcal surface components, which did not inhibit attachment, included peptidoglycan, C polysaccharide, Forssmann antigen, capsular polysaccharide and a phenol extract produced in analogy to streptococcal lipoteichoic acid. The procedure used to extract the adhesin was previously used to prepare the competence factor. The competence deficient mutant RA7- attached poorly compared to the competent R6 parent. The possible relatedness of attachment to competence is discussed.

Autolysis-resistant peptidoglycan of anomalous composition in amino-acid-starved Escherichia coli

Nongrowing Escherichia coli deprived of an essential amino acid continued to produce peptidoglycan at a rate approximately 30% of that of growing cells. The composition of this peptidoglycan was very different from that of growing cells and resembled that of peptidoglycan left undegraded during partial autolysis of the bacteria. Synthesis of this peptidoglycan of anomalous composition began at once upon the removal of the amino acid from the medium. Fifteen minutes of amino acid deprivation was sufficient to virtually completely prevent penicillin-induced autolytic wall degradation in vivo. During this time, although the specific activities of soluble and membrane-bound hydrolytic transglycosylases and endopeptidases remained high, the peptidoglycan produced showed decreased sensitivity to degradation in vitro. After more extensive (2-h) starvation, triggering of autolysis by chaotropic agents was also blocked. Autolysis in growing cells may be selective for peptidoglycan representing the cylindrical portion of the sacculus. It is suggested that at least part of the mechanism of the well-known lysis resistance of nongrowing E. coli is related to the deposition of structurally anomalous and relatively autolysin-resistant peptidoglycan at some strategically located sites on the bacterial surface.

The contribution of pneumococcal cell wall to the pathogenesis of experimental otitis media

We studied the contribution of pneumococcal cell wall to the pathogenesis of otitis media in chinchillas after middle ear inoculation of killed, encapsulated type 7F Streptococcus pneumoniae; killed, unencapsulated R6 S. pneumoniae; and isolated R6 pneumococcal cell wall. Ears inoculated with encapsulated and unencapsulated pneumococci had significantly higher concentrations of polymorphonuclear and mononuclear leukocytes and lysozyme in middle ear fluid and developed more epithelial metaplasia and granulation tissue than did saline-inoculated ears. The mean concentration of lysozyme in middle ear fluid was higher in ears inoculated with killed, unencapsulated than encapsulated pneumococci. The middle ear mucoperiosteum of ears inoculated with pneumococcal cell wall showed significantly more polymorphonuclear leukocytes, epithelial metaplasia, subepithelial congestion, and granulation tissue than did control ears. Because nonviable, unencapsulated pneumococci and pneumococcal cell wall caused middle ear inflammation in the chinchilla model of otitis media, it is possible that cell envelope and cell wall components released during bacterial lysis may contribute to chronic otitis media with effusion in humans.

Structure of the peptide network of pneumococcal peptidoglycan

The peptide network of Streptococcus pneumoniae cell walls was solubilized using the pneumococcal autolytic amidase (N-acetylmuramoyl-L-alanine amidase, EC 3.5.1.28). The peptide material was fractionated into size classes by gel filtration followed by reverse-phase high-performance liquid chromatography which resolved the peptide population into over 40 fractions. About 40% of the lysines present participate in cross-links between stem peptides. The main components (3 monomers, 5 dimers, and 2 trimers), accounting for 77% of all the wall peptides, were purified. Their structures were determined using a combination of amino acid and end-group analysis, mass spectrometry, and gas-phase sequencing. Two different types of cross-links between stem peptides were found. In the most abundant type there is an alanylserine cross-bridge between the alanine in position 4 of the donor stem peptide and the lysine at position 3 of the acceptor peptide, as in type A3 peptidoglycan. In the second type of cross-link there is no intervening cross-bridge, as in the type A1 peptidoglycan of Gram-negative bacteria. The data indicate that pneumococcal peptidoglycan has a structural complexity comparable to that recently shown in some Gram-negative species.

A mutant of Streptococcus pneumoniae that exhibits thermosensitive penicillin tolerance and the paradoxical effect

Mutants of Streptococcus pneumoniae that contain active autolysin and yet cannot be induced to lyse during treatment with penicillin (Lyt+Tol+ mutants) have been described. We have now shown that these mutants are temperature dependent (32 degrees C); at 37 degrees C these bacteria underwent penicillin-induced lysis. In addition, mutants at the lysis-permissive temperature showed the so-called 'paradoxical response' to penicillin. Temperature shift experiments indicated that the change from tolerant to lytic response or vice versa is a fast process. No differences were detected in autolysin specific activity or in the kinetics of inhibition of protein, peptidoglycan and teichoic acid syntheses in cells treated with penicillin at 32 and 37 degrees C. The results of genetic crosses indicated that the thermosensitivity of penicillin-induced autolysis in the Lyt+Tol+ mutants is not a property of the autolytic enzyme itself. The observations suggest that the thermosensitive process in the mutants represents either a step(s) in autolysin regulation or involves some difference in the structure of the cell walls produced at 32 degrees C versus 37 degrees C.

Nonsteroidal anti-inflammatory agents in the therapy for experimental pneumococcal meningitis

An increased inflammatory mass in the subarachnoid space during bacterial meningitis may correlate with a poor outcome of disease. Using a rabbit model of pneumococcal meningitis, we sought to reduce this inflammatory process. The ability of the pneumococcal cell wall to cause death and to generate leukocytosis and abnormal chemistry in cerebrospinal fluid was prevented when animals were treated with inhibitors of cyclooxygenase pathway of arachidonate metabolism. Bacterial lysis by ampicillin led to release of cell wall that caused a significant, transient increase in meningeal inflammation. This inflammatory burst was also prevented by administering cyclooxygenase inhibitors concurrently with the antibiotic.

Teichoic acid-containing muropeptides from Streptococcus pneumoniae as substrates for the pneumococcal autolysin

Pneumococcal cell walls in which the normal phosphorylcholine component of the wall teichoic acids is replaced with phosphorylethanolamine cannot absorb the homologous autolytic enzyme and are completely resistant to autolytic degradation (S. Giudicelli and A. Tomasz, J. Bacteriol. 158:1188-1190, 1984). We have now isolated and characterized soluble teichoic acid-containing muropeptides from such cell walls and tested them as substrates for the pneumococcal autolytic enzyme. Both choline- and ethanolamine-containing muropeptides were hydrolyzed to the same extent by the enzyme. Furthermore, free choline concentrations that totally inhibited the digestion of pneumococcal cell walls in vivo and in vitro were without effect when the soluble substrates were used.

Construction of a penicillin-tolerant laboratory mutant of Staphylococcus aureus

Numerous conflicting reports describing clinical penicillin tolerant Staphylococcus aureus isolates have raised questions concerning the reproducibility and genetic homogeneity of such isolates. The purpose of our study was to construct a tolerant mutant which would then serve as a reference strain and could also be used for testing the potential significance of antibiotic tolerance during chemotherapy of staphylococcal infections. A penicillin sensitive laboratory mutant strain was mutagenized and exposed to cycles of penicillin selection. A culture was then obtained which lost about 1 log of viability during exposure to up to 1000 times the MIC of penicillin. The same conditions caused a 3-4 log decline in the viability of the parent cell. The tolerance property was stable for at least 150 generations of nonselective growth and was reproducible in randomly picked colonies. The superior survival during treatment was also demonstrable in cultures grown in rabbit serum or cerebrospinal fluid. Penicillin tolerant cultures could also be obtained by selection without mutagenesis, suggesting that such a mutant may indeed be selected for in the clinical environment.

Conversion of a homogeneously methicillin-resistant strain of Staphylococcus aureus to heterogeneous resistance by Tn551-mediated insertional inactivation

Plasmid pRN3208, thermosensitive for replication, and carrying the erythromycin transposon Tn551, was used for insertional inactivation of methicillin resistance in a highly and homogeneously resistant strain of Staphylococcus aureus. Two kinds of insertionally inactivated cells were obtained. Cultures of the major class contained highly methicillin resistant cells with a frequency of about 10(-3) to 10(-4), produced DNA with methicillin resistance transforming activity, and also produced penicillin binding protein 2a, the 78 kd low affinity penicillin binding protein characteristic of methicillin resistant Staphylococcus aureus, in apparently normal quantities. The single member of class B had no detectable methicillin resistant cells (less than 10(-8)) with an MIC greater than 1 micrograms/ml, contained no DNA with methicillin resistant transforming activity and no penicillin binding protein 2a. The data suggest that in the class A cells insertional inactivation did not affect the structural gene(s) of methicillin resistance but a regulatory locus or loci needed for the homogeneous expression of resistance.

Modulation of bacteriolysis by cooperative effects of penicillin-binding proteins 1a and 3 in Escherichia coli

Escherichia coli characteristically lyses upon treatment with most beta-lactam antimicrobial agents. In contrast, an investigational aminothiazole cephem, CGP 31523A, produced a new combination of antibacterial effects: it was highly bactericidal without causing cell wall degradation or lysis. Killing was associated with the formation of vacuolated filaments. Because the compound bound to penicillin-binding proteins (PBPs) 1a and 3, we investigated the role of PBP 3 in modulation of lysis caused by inhibition of PBP 1a. A temperature-sensitive mutant with a nonfunctional PBP 3 lysed when treated with CGP 31523A. The combination of a PBP 1 inhibitor (cefsulodin) and a PBP 3 inhibitor (aztreonam) also caused filamentation and death without lysis of wild-type cells over a narrow concentration range. We conclude that cooperative effects between PBPs in E. coli can lead to a dissociation of bacterial killing and lysis.

Penicillin-binding proteins of penicillin-susceptible and -resistant pneumococci: immunological relatedness of altered proteins and changes in peptides carrying the beta-lactam binding site

There are several major differences between the penicillin-binding proteins (PBPs) of highly penicillin-resistant and -susceptible strains of pneumococci. The highest-molecular-size PBP 1a (98 kilodaltons [kDa]) of susceptible pneumococci is not detectable in resistant bacteria. Instead, resistant strains contain a PBP of smaller size: 92 and 94 kDa in South African strains 8249 and A95210, respectively, and 96 kDa in New Guinea strain 2955 (S. Zighelboim and A. Tomasz, Antimicrob. Agents Chemother. 17:434-442, 1980). Using antibodies prepared against PBP 1a of penicillin-susceptible pneumococci, we demonstrated that these anomalous-sized proteins in the resistant strains are immunologically related to PBP 1a of penicillin-susceptible bacteria. A second difference between the PBP patterns of strain 8249 and the susceptible pneumococci is that the 78-kDa PBP 2b is not detectable by the radioactive penicillin binding assay in the resistant strain. Using antibodies prepared against PBP 2b of susceptible cells, we demonstrated the presence of PBP 2b in membrane preparations from strain 8249 cells. Thus, the poor detection of this PBP appears to be related to its greatly decreased affinity for the antibiotic molecule. We also compared the patterns of penicillin-labeled peptides derived from PBPs of resistant and susceptible cells during partial proteolysis by V8 protease. Several changes were observable in small peptides carrying the beta-lactam binding site generated from the high Mr (PBP 1a-related) binding proteins. In contrast, no differences in the pattern of penicillin-labeled peptides were seen when the pattern of PBP 2a of susceptible pneumococci was compared with the peptide pattern of PBP 2a from resistant strains. One of the resistant isolates (strain 2955) also had a PBP 3 with a higher-than-normal molecular weight. This protein gave strong positive reaction with antibodies against PBP 3 of susceptible cells. Examination of the pattern of penicilloyl peptides generated from the susceptible and resistant PBP 3s during partial proteolysis revealed only differences which seem to reside distant from the beta-lactam binding site.

Beta-lactam-specific resistant mutants of Staphylococcus aureus

In an approach to understanding the origin of methicillin resistance in clinical isolates of staphylococci, a series of Staphylococcus aureus mutants resistant to various beta-lactam antibiotics were isolated in the laboratory by antibiotic selection. Mutants with low- and intermediate-level resistance showed considerable specificity for the particular antibiotic used in the selection process (methicillin, cefotaxime, cephalexin, and amdinocillin), and resistance in such mutants also showed alterations in the antibiotic binding capacities of penicillin-binding proteins (PBPs). In each case the isolation of mutants resistant to high concentrations of antibiotics required sequential passage in gradually increasing concentrations of the drug. The acquisition of increasing levels of methicillin resistance was paralleled by a gradual decrease in the binding capacities of PBPs 2, 3, and, possibly, 1. In a highly methicillin-resistant mutant (MIC, 150 micrograms/ml), PBPs 2 and 3 were no longer detectable by the penicillin binding assay. Instead, a new PBP of poor binding capacity and anomalous molecular size (about 78 kilodaltons [kDa]) appeared in these cells. This corresponds to the molecular size of PBP 2a, the unique PBP that appears to be the biochemical correlate of resistance in clinical isolates of methicillin-resistant S. aureus. Also, similar to the case of resistant clinical isolates, high-level beta-lactam resistance was highly pH dependent in the laboratory mutants. We compared the patterns of radioactive peptides generated by partial proteolysis from the penicillin-labeled PBP 2 of antibiotic-susceptible staphylococci and from the 78-kDa PBP 2a of a resistant clinical strain. Although the patterns were clearly different, seven of the eight characteristic peptides generated from PBP 2 of the susceptible strain were also detectable among the peptides released from PBP 2a. The results suggest that the 78-kDa PBP 2a of the resistant clinical strain evolved from PBP 2 of antibiotic-susceptible staphylococci and that in PBP 2a of the clinical isolate mutational changes have resulted in extensive alterations near the beta-lactam binding site.

Transglycosylase and endopeptidase participate in the degradation of murein during autolysis of Escherichia coli

The cell wall degradation products released from Escherichia coli during autolysis triggered by cephaloridine or trichloroacetic acid were isolated and characterized. Murein was selectively lost from the disaccharide tetrapeptides and the bisdisaccharide tetrapeptide components. Two major autolytic products accounted for more than 85% of the released material. Compound 1 (60 to 80% of released material) was a disaccharide tetrapeptide monomer containing a 1,6-anhydromuramic acid residue. Compound 2 (15 to 30% of released material) was a mixture of a tritripeptide and a tritetrapeptide without hexosamines. Taken together the findings suggest that autolytic cell wall degradation in E. coli is selective and involves the activity of both the hydrolytic transglycosylase and an endopeptidase. Upon release, at least some of the wall components were also exposed to the activity of the N-acetylmuramic acid-L-alanine amidase.

Induction of autolysis in nongrowing Escherichia coli

Unless relaxation of the stringent response is achieved, all nongrowing bacteria rapidly develop resistance to autolysis induced by a variety of agents, including all classes of cell wall synthesis inhibitors. We now describe inhibitors of cell wall synthesis which were unusual in that they could continue to effectively induce autolysis in relA+ Escherichia coli even after prolonged amino acid starvation. The process of cell wall degradation seems to be catalyzed by similar hydrolytic enzymes in nongrowing and growing cells, yet the activity of these new agents capable of inducing autolysis in the nongrowing relA+ cells did not involve relaxation of RNA or peptidoglycan synthesis. We propose that the suppression of autolysis characteristic of nongrowing cells can be bypassed by a novel mechanism of autolytic triggering which is independent of the relA locus.

Alterations in kinetic properties of penicillin-binding proteins of penicillin-resistant Streptococcus pneumoniae

Earlier studies have shown that the highly penicillin-resistant South African Strains of pneumococci contain altered penicillin-binding proteins (PBPs) (S. Zighelboim and A. Tomasz, Antimicrob. Agents Chemother. 17:434-442, 1980). We now describe a detailed quantitative characterization of the reaction of radioactively labeled penicillin with the PBPs of the penicillin-susceptible and penicillin-resistant pneumococci and several intermediate-resistance-level genetic transformants as well. The altered binding of the antibiotic by the PBPs of resistant cells appears to be due to a combination of two factors: lower drug affinity and change in the cellular amounts of PBPs. No alteration in the rates of deacylation of the penicilloyl-PBPs of the resistant cells was detected.

Penicillin-binding proteins and the antibacterial effectiveness of beta-lactam antibiotics

It is currently believed that both the antibacterial potency and the nature of the antibacterial effects of beta-lactam antibiotics are ultimately dependent on the inhibition (acylation) of one or more of the bacterial penicillin-binding proteins (PBPs). Nevertheless, bacterial factors (e.g., autolysins) that do not directly react with the antibiotic molecule also profoundly influence the fate (inhibition of growth or lysis) of the antibiotic-treated bacterial cell. The quantitative relationship between the minimal inhibitory concentration of a beta-lactam antibiotic and its reactivity with certain PBPs is not well understood. Also poorly understood is the mechanism by which inhibition of PBP function causes triggering of suicidal autolytic activity. Much remains to be done before the structural basis of the highly selective PBP affinities observed with some beta-lactam antibiotics is understood. A new form of antibiotic resistance involving mutational alteration of PBPs (in the direction of lower antibiotic affinity) has emerged among clinical isolates of most of the major human pathogens.

The rate of killing of Escherichia coli by beta-lactam antibiotics is strictly proportional to the rate of bacterial growth

Nongrowing bacteria evade the bactericidal activity of beta-lactam antibiotics. We sought to determine if slow growth rate also alters bactericidal activity. The bactericidal activity of two beta-lactams on Escherichia coli grown in glucose limited chemostats was compared for generation times ranging from 0.7 to 12 h. The degree of killing varied with drug structure and with E. coli strain. However, all killing rates were a constant function of the bacterial generation time: slowly growing bacteria became progressively more phenotypically tolerant to beta-lactam antibiotics as the generation time was extended.

Evaluation of the bactericidal activity of beta-lactam antibiotics on slowly growing bacteria cultured in the chemostat

The bactericidal activity of 23 beta-lactam antibiotics was compared in slowly growing bacteria cultured in a chemostat. In an attempt to mimic possible in vivo conditions, slowly growing cultures were produced by limitation of iron, glucose, phosphate, or magnesium. Only select antibiotics remained effectively bactericidal against slowly growing cells. For these compounds, the rate of antibiotic-induced loss of viability was a constant when killing was expressed per generation (in contrast to absolute time) in that slowly growing bacteria were killed proportionately more slowly. Individual antibiotics differed greatly, however, in their specific bactericidal activities against slowly growing cells, i.e., in the absolute degree of killing elicited during exposure of the bacteria to MIC equivalents of the drugs. Specific bactericidal activities varied not only with drug structure but also with the bacterial strains and, to a lesser extent, with the nature of the growth-limiting nutrient. In slowly growing cultures exposure to the low drug concentrations studied here (near MIC) caused killing without detectable lysis. Antibiotics with high specific bactericidal activities were capable of rapidly killing cultures of slowly growing pathogens despite extremely long generation times approaching those reported for in vivo growth rates.

Mutants of Streptococcus pneumoniae that contain a temperature-sensitive autolysin

Two mutants of Streptococcus pneumoniae deficient in autolysin activity produced a protein that showed immunological identity with the N-acetyl-muramyl-L-alanyl-amidase present in the wild-type strain, when tested with antiserum obtained against this enzyme. The protein was produced by the mutant cultures grown either at 37 degrees C or at 30 degrees C, although only the cell extracts obtained at 30 degrees C showed significant cell wall hydrolysing activity. In contrast to the lysis resistance of these bacteria grown at 37 degrees C, mutant cultures grown at 30 degrees C exhibited significant degrees of autolysis when treated with detergent or cell wall inhibitors. Extracts of the mutant cultures contained a cell wall hydrolysing activity that was rapidly inactivated during incubation at 37 degrees C.

The role of complement in inflammation during experimental pneumococcal meningitis

The mechanism whereby an effective bactericidal inflammatory reaction develops in the subarachnoid space is not clearly defined. While normal cerebrospinal fluid is deficient in complement, immunoglobulin and leukocytes, these serum components appear in cerebrospinal fluid (CSF) during the course of bacterial meningitis. Using a rabbit model of pneumococcal meningitis we examined the role of the alternate complement pathway in three early events important to the defense of the subarachnoid space: leukocyte chemotaxis, phagocyte mediated bacterial killing, and clearance of bacterial components from the cerebrospinal fluid space. Rabbits treated with cobra venom factor to deplete complement were inoculated intracisternally with encapsulated (type II or XIX) pneumococci. Following complement depletion, there was a dramatic (at least 100-fold) decrease in the LD50 for these strains. Nevertheless, complement depletion did not affect the magnitude of CSF leucocytosis or the rate of clearance of bacterial particles from CSF. A short delay in the appearance of leukocytes in CSF was found in the absence of complement. The major effect of complement depletion, however, was to diminish the efficiency of leukocyte mediated killing of encapsulated bacteria in the CSF. Although the short delay in the onset of leukocytosis in the complement depleted animals is consistent with a chemotactic role of complement in the normal animal, the quantitatively normal leukocytosis in the complement depleted rabbits clearly indicates that important chemotaxins other than complement function in CSF. Inhibition of leukocytosis by indomethacin and diclofenac suggests that metabolite(s) of the arachidonic acid pathway may perform such a chemotactic role. A major role of complement in the defense of the subarachnoid space appears to be as an opsonin needed for the effective bactericidal activity of leukocytes. It is the lack of this function that best explains the greatly decreased LD50 value of encapsulated pneumococci in the complement depleted animal.

Expression of methicillin resistance in heterogeneous strains of Staphylococcus aureus

The phenotypic expression of methicillin resistance was studied in a number of clinical isolates and laboratory strains of Staphylococcus aureus. The methicillin-resistant S. aureus strains could be divided into three classes, homogeneous, heterogeneous, and thermosensitive heterogeneous methicillin-resistant S. aureus, on the basis of their plating efficiencies at 30 or 37 degrees C on methicillin-containing agar plates. Heterogeneous strains of methicillin-resistant S. aureus were composed of two subpopulations: a small minority of cells (10(-5) to 10(-3); MIC, 600 to 1,000 micrograms/ml) that expressed resistance to high concentrations of methicillin at 37 degrees C, and a majority of cells (MIC, 5 micrograms/ml) that remained susceptible to the drug at 37 degrees C. Cultures of a thermosensitive heterogeneous strain were able to grow in the presence of high concentrations of methicillin, provided that the growth temperature was 30 degrees C. Such cultures lost their phenotypic resistance within 30 min (i.e., in less than one doubling time) after the growth temperature was shifted to the nonpermissive 37 degrees C. Shift of the temperature of the culture in the reverse direction (37 to 30 degrees C) resulted in an equally rapid expression of phenotypic resistance. The majority of the cells in such heterogeneous strains may be considered heat (or salt) conditional in their phenotypic expression of methicillin resistance. Both heterogeneous and thermosensitive heterogeneous strains, irrespective of their temperature of cultivation and degree of phenotypic resistance, contained detectable quantities of the 78-kilodalton penicillin-binding protein 2a (PBP 2a) that previous studies have suggested is a biochemical correlate of methicillin resistance in homogeneous strains of methicillin-resistant S. aureus. However, in contrast to the homogeneous stains, in heterogeneous and thermosensitive heterogeneous isolates the ability to synthesize PBP 2a is apparently not sufficient to provide a resistant phenotype. In these strains some additional, as yet undefined factor(s) is also needed for the expression of methicillin resistance.

Alterations in penicillin-binding proteins of clinical and laboratory isolates of pathogenic Streptococcus pneumoniae with low levels of penicillin resistance

Several recent surveys of clinical isolates have indicated that substantial fractions of naturally occurring populations of Streptococcus pneumoniae have undergone a distinct upward move in the required minimal inhibitory concentration (MIC) of benzylpenicillin (from a range of 0.006-0.008 to 0.03-0.05 microgram/ml). Evidence is presented that in clinical pneumococcal isolates, penicillin-binding proteins (PBPs) groups 1 and 2 have a decreased affinity for radioactive benzylpenicillin as compared with penicillin-sensitive isolates from the same locale. Exposure of a penicillin-sensitive type 2 strain (MIC, 0.006 microgram/ml) to sequentially increasing concentrations of penicillin allowed the isolation of spontaneous resistant mutants with stepwise increases in the MIC of penicillin required (0.01-0.02, 0.025-0.05, and 0.1 microgram/ml), and in these laboratory isolates too, PBP groups 1 and 2 showed decreased affinity for labeled benzylpenicillin. DNA from the low-level resistant clinical or laboratory isolates could be used to transform the appropriate levels of penicillin resistance into penicillin-sensitive laboratory isolates. These findings suggest that significant fractions of natural pneumococcal populations may have acquired one or two of the low-level penicillin resistance genes.

Inhibition of cell wall synthesis and acylation of the penicillin binding proteins during prolonged exposure of growing Streptococcus pneumoniae to benzylpenicillin

Growing cultures of an autolysis-defective pneumococcal mutant were exposed to [3H]benzylpenicillin at various multiples of the minimal inhibitory concentration and incubated until the growth of the cultures was halted. During the process of growth inhibition, we determined the rates and degree of acylation of the five penicillin-binding proteins (PBPs) and the rates of peptidoglycan incorporation, protein synthesis, and turbidity increase. The time required for the onset of the inhibitory effects of benzylpenicillin was inversely related to the concentration of the antibiotic, and inhibition of peptidoglycan incorporation always preceded inhibition of protein synthesis and growth. When cultures first started to show the onset of growth inhibition, the same characteristic fraction of each PBP was in the acylated form in all cases, irrespective of the antibiotic concentration. Apparently, saturation of one or more PBPs with the antibiotic beyond these threshold levels is needed to bring about interference with normal peptidoglycan production and cellular growth. Although it was not possible to correlate the inhibition of cell wall synthesis or cell growth with the degree of acylation (percentage saturation) of any single PBP, there was a correlation between the amount of peptidoglycan synthesized and the actual amount of PBP 2b that was not acylated. In cultures exposed to benzylpenicillin concentrations greater than eight times the minimal inhibitory concentration, the rates of peptidoglycan incorporation underwent a rapid decline when bacterial growth stopped. However, in cultures exposed to lower concentrations of benzylpenicillin (one to six times the minimal inhibitory concentration) peptidoglycan synthesis continued at constant rate for prolonged periods, after the turbidity had ceased to increase. We conclude that inhibition of bacterial growth does not require a complete inhibition or even a major decline in the rate of peptidoglycan incorporation. Rather, inhibition of growth must be caused by an as yet undefined process that stops cell division when the rate of incorporation of peptidoglycan (or synthesis of protein) falls below a critical value.

Penicillin tolerance in multiply drug-resistant natural isolates of Streptococcus pneumoniae

Five of six multiply drug-resistant clinical isolates of Streptococcus pneumoniae from South Africa demonstrated penicillin tolerance. In contrast to the common wild-type strains of pneumococci, treatment of the tolerant strains with penicillin above the minimum inhibitory concentration did not induce cell wall degradation, lysis, or leakage of intracellular components, and the rate of loss of viability was reduced compared with that of nontolerant strains. While these South African strains contained lower specific activity of autolytic enzyme than did nontolerant strains, the residual autolytic activity (15%-26% of the nontolerant wild type) was much more than that found in lysis-defective laboratory mutants of pneumococci (less than or equal to 1%); the rate of penicillin-induced lysis did not correlate with the specific activity of residual autolysin. Also, in contrast to the complete lysis resistance of lysis-defective mutants to all lytic agents, the tolerant South African strains were resistant primarily to lysis by beta-lactam antibiotics but could still be lysed by other cell wall inhibitors (e.g., cycloserine) and detergents. The penicillin resistance and penicillin tolerance traits could be separated by genetic transformation. We suggest that the drug-specific tolerance of the South African pneumococcal strains is related to some alteration in the control of autolysin activity.

Increased amounts of a novel penicillin-binding protein in a strain of methicillin-resistant Staphylococcus aureus exposed to nafcillin

In addition to the four typical penicillin-binding proteins (PBPs), a strain of heterogeneously methicillin-resistant Staphylococcus aureus produced an extra 78-kD PBP (PBP 2a) that had a low affinity for nafcillin and penicillin. Addition of nafcillin to cultures of this strain caused a rapid increase in the amount of this PBP in cell membranes. This increase occurred at subinhibitory concentrations of drug within minutes of exposure, and was blocked by inhibitors of protein and RNA synthesis. This suggests that the synthesis of PBP 2a can be stimulated by exposure to beta-lactam antibiotics. This process may, in part, explain the heterogeneity in methicillin-resistant S. aureus.

Peptidoglycan cross-linking and teichoic acid attachment in Streptococcus pneumoniae

Autolysin-defective pneumococci continue to synthesize both peptidoglycan and teichoic acid polymers (Fischer and Tomasz, J. Bacteriol. 157:507-513, 1984). Most of these peptidoglycan polymers are released into the surrounding medium, and a smaller portion becomes attached to the preexisting cell wall. We report here studies on the degree of cross-linking, teichoic acid substitution, and chemical composition of these peptidoglycan polymers and compare them with normal cell walls. peptidoglycan chains released from the penicillin-treated pneumococci contained no attached teichoic acids. The released peptidoglycan was hydrolyzed by M1 muramidase; over 90% of this material adsorbed to vancomycin-Sepharose and behaved like disaccharide-peptide monomers during chromatography, indicating that the released peptidoglycan contained un-cross-linked stem peptides, most of which carried the carboxy-terminal D-alanyl-D-alanine. The N-terminal residue of the released peptidoglycan was alanine, with only a minor contribution from lysine. In addition to the usual stem peptide components of pneumococcal cell walls (alanine, lysine, and glutamic acid), chemical analysis revealed the presence of significant amounts of serine, aspartate, and glycine and a high amount of alanine and glutamate as well. We suggest that these latter amino acids and the excess alanine and glutamate are present as interpeptide bridges. Heterogeneity of these was suggested by the observation that digestion of the released peptidoglycan with the pneumococcal murein hydrolase (amidase) produced peptides that were resolved by ion-exchange chromatography into two distinct peaks; the more highly mobile of these was enriched with glycine and aspartate. The peptidoglycan chains that became attached to the preexisting cell wall in the presence of penicillin contained fewer peptide cross-links and proportionally fewer attached teichoic acids than did their normal counterparts. The normal cell wall was heavily cross-linked, and the cross-linked peptides were distributed equally between the teichoic acid-linked and teichoic acid-free fragments.

Factors affecting sensitivity of group B streptococci to an exogenous murein hydrolase

Group B streptococci treated with cell wall synthesis inhibitors (penicillin or vancomycin) or by a variety of membrane-acting agents are sensitized to the lytic action of exogenous M1 muramidase. Muramidase without a sensitizing agent caused rupture of bacterial chains only, accompanied by the release of a small amount of cell wall peptidoglycan label and an increase of the number of colony-forming units. In combination with sensitizing agents the exogenous muramidase appeared to initiate hydrolysis of biosynthetically new peptidoglycan. Treatment of the cells with chloramphenicol or starvation for nutritionally required amino acids suppressed the rate of cell lysis and peptidoglycan hydrolysis during subsequent sensitization and muramidase treatment of the bacteria. Purified cell walls prepared from the amino acid starved cells were also hydrolyzed with a slower rate by muramidase. It is suggested that agents sensitizing the bacteria to the exogenous muramidase act by perturbing or removing some nonmurein components of the cell envelope which protect the peptidoglycan from the activity of exogenous enzyme. Agents increasing resistance against exogenous muramidase may also cause some alteration in peptidoglycan structure.

The induction of meningeal inflammation by components of the pneumococcal cell wall

Pneumococcal cell wall induces meningeal inflammation in rabbits injected intracisternally with greater than 10(5) cell equivalents. Both of the major cell wall components, teichoic acid and peptidoglycan, contribute to this inflammatory activity although responses differ depending on the chemical nature, size, and complexity of these fractions. Challenge with teichoic acid (membrane or wall associated) results in greater inflammation at 5 hr than at 24 hr. Degraded teichoic acid is inactive. In contrast, the inflammation caused by whole cell wall or high-molecular-weight peptidoglycan-containing fractions increases in intensity from 5 to 24 hr. Peptidoglycan fractions lose activity at 24 hr when hydrolyzed to disaccharide-stem peptide moieties. Generation of free cell wall components in cerebrospinal fluid as, for example, during treatment with antibiotics that are bacteriolytic as well as bactericidal, could contribute to increased inflammation in the subarachnoid space.

The relative role of bacterial cell wall and capsule in the induction of inflammation in pneumococcal meningitis

The relative contribution of bacterial components to the induction of inflammation during Streptococcus pneumoniae meningitis is unknown. Several strains of pneumococci with differences in cell surface characteristics (capsule or cell wall) were compared for the effect on the inflammatory response evoked during infection of the cerebrospinal fluid (CSF) in vivo. The presence of bacterial capsular polysaccharide was not necessary for bacterial growth in CSF in vivo but correlated with greater CSF bacterial density. CSF inflammatory changes began to appear when the bacterial concentration exceeded 10(5) cfu/ml, regardless of the pneumococcal strain. CSF inflammatory changes could be invoked by cisternal instillation of 10(5)-10(6) cell equivalents of whole, heat-killed unencapsulated strains or their isolated cell walls but not by similar concentrations of heat-killed encapsulated strains or isolated capsular polysaccharide. Hypoglycorrhachia was observed only during inflammation caused by live bacteria. The inflammatory response characteristic of naturally acquired pneumococcal meningitis can be reproduced by challenge with both encapsulated and uncapsulated bacteria. The bacterial cell wall appears to be the most potent pneumococcal surface component in inducing CSF inflammation.

Competence-specific autolysis in Streptococcus sanguis

Streptococcus sanguis strain Wicky activated to competence for genetic transformation is known to undergo a rapid decrease in optical density upon transfer to an alkaline buffer containing reducing agents. We studied the mechanism of this autolysis-like process and made the following observations. The process was specific because preincubation of the competence inducing factor with a specific inactivating protein prevented both cellular lysis and acquisition of competence for genetic transformation. The optical density decrease of competent bacteria involved the release of a large fraction of intracellular protein, RNA and lipid. However, no hydrolysis of phospholipid and no degradation of cell wall polymers including peptidoglycan could be detected. No peptidoglycan hydrolase activity capable of degrading radiolabelled S. sanguis cell walls was detected in unfractionated S. sanguis extracts. It is suggested that autolysis of competent S. sanguis involves the activity of a novel type of murein hydrolase that introduces only a limited number of bond breaks into the peptidoglycan.

Activation of the alternative complement pathway by pneumococcal lipoteichoic acid

Cell wall teichoic acids of some gram-positive bacteria are potent activators of the alternative pathway of complement. It is unclear, however, whether the other form of teichoic acid, cell membrane lipoteichoic acid (LTA), can also activate the alternative pathway. In the present study, radiolabelled pneumococcal LTA was found to bind spontaneously to sheep erythrocytes in a temperature- and time-dependent fashion. In addition, the presence of pneumococcal LTA on the erythrocyte surface was verified by the fact that they could be agglutinated by a myeloma protein (TEPC-15) specific for choline, a constituent of pneumococcal LTA. Pneumococcal LTA when fixed to the surface of erythrocytes was able to activate the alternative pathway of complement in both guinea pig serum deficient in the fourth component of complement and human serum deficient in the second component of complement, resulting in lysis of the sensitized erythrocytes. The sensitizing principle of the LTA preparation was removed before erythrocyte sensitization by immunoabsorption, using the choline-specific TEPC-15 myeloma protein. These data demonstrate that purified pneumococcal LTA will bind to sheep erythrocytes and endow them with the ability to activate the alternative pathway.

Pneumococcal Forssman antigen: enrichment in mesosomal membranes and specific binding to the autolytic enzyme of Streptococcus pneumoniae

The choline-containing pneumococcal membrane teichoic acid (Forssman antigen) can be isolated with the membrane fractions of the bacteria. The small vesicle (mesosomal) fraction generated during the formation of protoplasts seems to be highly enriched in this material. Forssman antigen was identified in cell fractions on the basis of (i) radioactive choline label, (ii) autolysin-inhibitory activity, and (iii) the sedimentation profile in sucrose density gradients with and without detergent. A membrane teichoic acid could also be isolated from pneumococci grown in medium in which choline was replaced by ethanolamine as the nutritionally required amino alcohol. This material contained radioactive ethanolamine label and behaved similarly to the choline-containing membrane teichoic acid during centrifugation in detergent-containing and detergent-free density gradients. On the other hand, the material had only low autolysin-inhibitory activity. Binding of pure pneumococcal autolysin to micelles of purified Forssman antigen could be demonstrated by mixing these components in vitro and analyzing them by sucrose density gradients and by agarose chromatography. No binding could be observed between the pneumococcal enzyme and the micellar forms of either cardiolipin or polyglycerophosphate-type lipoteichoic acid isolated from Streptococcus lactis.

Protection by D-amino acids against growth inhibition and lysis caused by beta-lactam antibiotics

D-Isomers of several amino acids completely protected growing cultures of Escherichia coli against all antibacterial effects of beta-lactam antibiotics up to approximately two to three times the MICs of the antibiotics. L-Isomers of amino acids were ineffective. Protection depended on the concentration and time of addition of the D-amino acids. This appears to be the first demonstration of natural products capable of reversing the antibacterial effects of beta-lactam antibiotics.

Attachment of pneumococcal autolysin to wall teichoic acids, an essential step in enzymatic wall degradation

High concentrations of choline and phosphorylcholine blocked the adsorption of pneumococcal autolytic enzyme to homologous cell walls and inhibited enzymatic cell wall hydrolysis in a noncompetitive manner. Enzyme adsorption had an absolute requirement for the presence of choline residues in the wall teichoic acid. Other amino alcohols and derivatives such as ethanolamine, monomethylaminoethanolamine , and phosphorylethanolamine had no effect on enzyme adsorption or hydrolytic activity. It is proposed that enzymatic hydrolysis of cell walls requires prior adsorption of enzyme molecules to the insoluble wall substrate and that cholin residues of the wall teichoic acid have the role of adsorption ligands in this process.

Low-affinity penicillin-binding protein associated with beta-lactam resistance in Staphylococcus aureus

Methicillin resistance in Staphylococcus aureus has been associated with alterations in the penicillin-binding proteins (PBPs). An intriguing property of all methicillin-resistant staphylococci is the dependence of resistance on the pH value of the growth medium. Growth of such bacteria at pH 5.2 completely suppressed the expression of methicillin resistance. We have examined the PBP patterns of methicillin-resistant staphylococci grown at pH 7.0. We detected a high-molecular-weight PBP (PBP-2a; approximate size, 78,000 daltons) that was only present in the resistant bacteria but not in the isogenic sensitive strain. In cultures grown at pH 5.2, the extra PBP was not detectable.

Production and release of peptidoglycan and wall teichoic acid polymers in pneumococci treated with beta-lactam antibiotics

Autolysin-defective pneumococci treated with inhibitory concentrations of penicillin and other beta-lactam antibiotics continued to produce non-cross-linked peptidoglycan and cell wall teichoic acid polymers, the majority of which were released into the surrounding medium. The released cell wall polymers were those synthesized by the pneumococci after the addition of the antibiotics. The peptidoglycan and wall teichoic acid chains released were not linked to one another; they could be separated by affinity chromatography on an agarose-linked phosphorylcholine-specific myeloma protein column. Omission of choline, a nutritional requirement and component of the pneumococcal teichoic acid, from the medium inhibited both teichoic acid and peptidoglycan synthesis and release. These observations are discussed in terms of plausible mechanisms for the coordination between the biosynthesis of peptidoglycan and cell wall teichoic acids.

Studies on the mechanism of intrinsic resistance to beta-lactam antibiotics in group D streptococci

Six penicillin-binding proteins (PBPs) were detected in clinical isolates of each one of three group D streptococci: Streptococcus bovis, S. faecalis and S. faecium. When examined in whole organisms, the PBPs of S. faecium, the most penicillin-resistant species of group D streptococci, generally had lower affinities for the antibiotic than those of S. faecalis (intermediate penicillin resistance), which in turn were of lower affinity than those of S. bovis (penicillin-sensitive). On the other hand, no quantitative correlation could be established between the binding of penicillin to any one PBP or group of PBPs, and the penicillin MIC value for the corresponding micro-organism. Examination of the amounts of antibiotic bound and the rates of binding to PBPs of equal numbers of protoplasts and whole bacteria of S. faecalis and S. faecium, indicated that there was no permeability barrier to benzylpenicillin in the cell walls of these species. The lower antibacterial effectiveness of cephalothin compared with ampicillin in group D streptococci was paralleled by the higher concentrations of cephalothin needed in competition assays to inhibit the lower molecular size PBPs of these bacteria.