Lower autolytic activity in a homogeneous methicillin-resistant Staphylococcus aureus strain compared to derived heterogeneous-resistant and susceptible strains

Antimicrobial heteroresistance: an emerging field in need of clarity

"Heteroresistance" describes a phenomenon where subpopulations of seemingly isogenic bacteria exhibit a range of susceptibilities to a particular antibiotic. Unfortunately, a lack of standard methods to determine heteroresistance has led to inappropriate use of this term. Heteroresistance has been recognized since at least 1947 and occurs in Gram-positive and Gram-negative bacteria. Its clinical relevance may be considerable, since more resistant subpopulations may be selected during antimicrobial therapy. However, the use of nonstandard methods to define heteroresistance, which are costly and involve considerable labor and resources, precludes evaluating the clinical magnitude and severity of this phenomenon. We review the available literature on antibiotic heteroresistance and propose recommendations for definitions and determination criteria for heteroresistant bacteria. This will help in assessing the global clinical impact of heteroresistance and developing uniform guidelines for improved therapeutic outcomes.

A model of isoniazid treatment of tuberculosis

A mathematical model is presented of the growth and death of bacilli in a granuloma. The granuloma is treated with isoniazid (INH), a drug that inhibits the synthesis of mycolic acids (MA). Since MA is an essential component of cell walls, the organisms fail to reach maturity if deficient in MA. Cell wall turnover is a well-known feature of bacteria, at the exterior surface material sloughs off to foil attacks by hosts or other organisms, simultaneously synthesizing products for new cell wall assembly. Thus cell wall thickness is maintained in a dynamic equilibrium (Doyle et al., 1988). Presumably cell death is a result of loss in cell wall due to autolysis in combination with stinted replenishing. The mathematical model presented here uses differential equations to predict the effects of intracellular INH on cell wall thickness and cell viability. This analysis purposely distinguishes intracellular INH concentration from the concentration in the plasma. The concentration in the plasma depends only on the dosing. The intracellular INH concentration, however, depends on diffusion through the cell walls of the bacteria. This paper addresses the complex interactions between intracellular INH, cell wall thickness, and the rate of cell wall synthesis.

An antibiotic that inhibits a late step in wall teichoic acid biosynthesis induces the cell wall stress stimulon in Staphylococcus aureus

Wall teichoic acids (WTAs) are phosphate-rich, sugar-based polymers attached to the cell walls of most Gram-positive bacteria. In Staphylococcus aureus, these anionic polymers regulate cell division, protect cells from osmotic stress, mediate host colonization, and mask enzymatically susceptible peptidoglycan bonds. Although WTAs are not required for survival in vitro, blocking the pathway at a late stage of synthesis is lethal. We recently discovered a novel antibiotic, targocil, that inhibits a late acting step in the WTA pathway. Its target is TarG, the transmembrane component of the ABC transporter (TarGH) that exports WTAs to the cell surface. We examined here the effects of targocil on S. aureus using transmission electron microscopy and gene expression profiling. We report that targocil treatment leads to multicellular clusters containing swollen cells displaying evidence of osmotic stress, strongly induces the cell wall stress stimulon, and reduces the expression of key virulence genes, including dltABCD and capsule genes. We conclude that WTA inhibitors that act at a late stage of the biosynthetic pathway may be useful as antibiotics, and we present evidence that they could be particularly useful in combination with beta-lactams.

Mechanisms of in-vitro-selected daptomycin-non-susceptibility in Staphylococcus aureus

Daptomycin is highly active against Staphylococcus aureus, including multidrug-resistant strains and those with reduced susceptibility to vancomycin. However, daptomycin-non-susceptible (Dap(NS)) strains [minimum inhibitory concentration (MIC) >1mg/L] have been derived clinically and in vitro. The mechanism(s) by which this occurs is incompletely understood, but existing data indicate that it is multifactorial. Dap(NS) derivatives of one laboratory and three clinical strains of S. aureus produced using gradient plates were evaluated. The Dap(NS) phenotype included increases in glycopeptide and nisin MICs and in some instances defective autolysis and reduced susceptibility to lysostaphin lysis. Amino acid substitutions in MprF, YycG (WalK), or both, were identified in all Dap(NS) strains. Reduced cytochrome c binding and ability of daptomycin to depolarise whole cells correlated with the Dap(NS) phenotype, consistent with an increase in cell surface positivity. Gene expression data revealed increased expression of vraS, one member of a two-component system involved in the regulation of cell wall biosynthesis, in three of five Dap(NS) strains. The pathway to the Dap(NS) phenotype is not linear, as variable genetic and phenotypic changes may result in identical increases in MICs.

A model of the complex response of Staphylococcus aureus to methicillin

It is widely accepted that beta-lactam antimicrobials cause cell death through a mechanism that interferes with cell wall synthesis. Later studies have also revealed that beta-lactams modify the autolysis function (the natural process of self-exfoliation of the cell wall) of cells. The dynamic equilibrium between growth and autolysis is perturbed by the presence of the antimicrobial. Studies with Staphylococcus aureus to determine the minimum inhibitory concentration (MIC) have revealed complex responses to methicillin exposure. The organism exhibits four qualitatively different responses: homogeneous sensitivity, homogeneous resistance, heterogeneous resistance and the so-called 'Eagle-effect'. A mathematical model is presented that links antimicrobial action on the molecular level with the overall response of the cell population to antimicrobial exposure. The cell population is modeled as a probability density function F(x,t) that depends on cell wall thickness x and time t. The function F(x,t) is the solution to a Fokker-Planck equation. The fixed point solutions are perturbed by the antimicrobial load and the advection of F(x,t) depends on the rates of cell wall synthesis, autolysis and the antimicrobial concentration. Solutions of the Fokker-Planck model are presented for all four qualitative responses of S. aureus to methicillin exposure.

Cephalosporins in clinical development

Bacterial resistance to established classes of antibiotics in clinical use is continuing to increase, making the need for new agents that can be used to treat the newly multi-drug resistant organisms steadily more urgent. Cephalosporins have been a successful group of antibiotics since they were first introduced to combat drug-resistant organisms, including staphylococci. The history of cephalosporins has emphasised an improvement of their stability towards beta-lactamases, thus expanding their spectrum of activity against important Gram-negative pathogens. New cephalosporins that have potent activity against multi-resistant Gram-positive bacteria, including methicillin-resistant staphylococci and penicillin-resistant pneumococci have recently entered clinical development. At least two of these, BAL-5788 and S-3578, also have Gram-negative activity, which is comparable to that of the third-and fourth-generation cephalosporins, making them broad-spectrum agents that could be used in hospital infections where methicillin-resistant staphylococci is likely to be present.

Increase of methicillin resistance in Staphylococcus aureus caused by deletion of a gene whose product is homologous to lytic enzymes

A spontaneous high-level methicillin-resistant mutant, SRM1648, for which the MIC of methicillin is 1,600 microg/ml, was isolated on a plate containing 400 microg of the antibiotic/ml on which had been cultured the low-level methicillin-resistant Staphylococcus aureus SR17238, for which the MIC is 6.3 microg/ml. Analysis of the chromosomal DNAs of the mutant and the parental strains by the restriction landmark genomic scanning method with two-dimensional electrophoresis of restriction fragments revealed a 1.6-kb deletion in the chromosome of the mutant. The HindIII fragment of 2.5 kb containing this deleted region was cloned into a plasmid vector and introduced into the parental strain. A deletion mutant reconstructed in the presence of a low concentration of methicillin by integration and excision of the recombinant plasmid exhibited a high level of resistance (methicillin MIC, 1,600 microg/ml), confirming that the deletion had caused the elevation of the resistance level. Sequence analysis indicated that the deletion occurred in three consecutive open reading frames (ORFs). The predicted amino acid sequence of the first ORF showed high homology with both RelA and SpoT of Escherichia coli, which are involved in the synthesis and hydrolysis of guanosine 5',3'-polyphosphate, and that of the third ORF showed a relatively high homology to the lytic enzyme encoded by the lytC gene of Bacillus subtilis. We also isolated another high-level resistant mutant with a deletion within the third ORF, which suggested that inactivation of some lytic enzyme resulted in the increased resistance.

Methicillin resistance in staphylococci: molecular and biochemical basis and clinical implications

Methicillin resistance in staphylococci is determined by mec, composed of 50 kb or more of DNA found only in methicillin-resistant strains. mec contains mecA, the gene for penicillin-binding protein 2a (PBP 2a); mecI and mecR1, regulatory genes controlling mecA expression; and numerous other elements and resistance determinants. A distinctive feature of methicillin resistance is its heterogeneous expression. Borderline resistance, a low-level type of resistance to methicillin exhibited by strains lacking mecA, is associated with modifications in native PBPs, beta-lactamase hyperproduction, or possibly a methicillinase. The resistance phenotype is influenced by numerous factors, including mec and beta-lactamase (bla) regulatory elements, fem factors, and yet to be identified chromosomal loci. The heterogeneous nature of methicillin resistance confounds susceptibility testing. Methodologies based on the detection of mecA are the most accurate. Vancomycin is the drug of choice for treatment of infection caused by methicillin-resistant strains. PBP 2a confers cross-resistance to most currently available beta-lactam antibiotics. Investigational agents that bind PBP 2a at low concentrations appear promising but have not been tested in humans. Alternatives to vancomycin are few due to the multiple drug resistances typical of methicillin-resistant staphylococci.

Localization of penicillin-binding proteins to the splitting system of Staphylococcus aureus septa by using a mercury-penicillin V derivative

Precise localization of penicillin-binding protein (PBP)-antibiotic complexes in a methicillin-sensitive Staphylococcus aureus strain (BB255), its isogenic heterogeneous methicillin-resistant transductant (BB270), and a homogeneous methicillin-resistant strain (Col) was investigated by high-resolution electron microscopy. A mercury-penicillin V (Hg-pen V) derivative was used as a heavy metal-labeled, electron-dense probe for accurately localizing PBPs in situ in single bacterial cells during growth. The most striking feature of thin sections was the presence of an abnormally large (17 to 24 nm in width) splitting system within the thick cross walls or septa of Hg-pen V-treated bacteria of all strains. Untreated control cells possessed a thin, condensed splitting system, 7 to 9 nm in width. A thick splitting system was also distinguishable in unstained thin sections, thereby confirming that the electron contrast of this structure was not attributed to binding of bulky heavy metal stains usually used for electron microscopy. Biochemical analyses demonstrated that Hg-pen V bound to isolated plasma membranes as well as sodium dodecyl sulfate-treated cell walls and that two or more PBPs in each strain bound to this antibiotic. In contrast, the splitting system in penicillin V-treated bacteria was rarely visible after 30 min in the presence of antibiotic. These findings suggest that while most PBPs were associated with the plasma membrane, a proportion of PBPs were located within the fabric of the cell wall, in particular, in the splitting system. Inhibition of one or more high-M(r) PBPs by beta-lactam antibiotics modified the splitting system and cross-wall structure, therefore supporting a role for these PBPs in the synthesis and architectural design of these structures in S. aureus.

Cloning and characterization of a gene affecting the methicillin resistance level and the autolysis rate in Staphylococcus aureus

Tn918 mutagenesis of a high-level methicillin-resistant Staphylococcus aureus (methicillin MIC, 800 micrograms/ml) led to the isolation of a low-resistance mutant. The Tn918 insert was transferred back to the parent to produce strain SRM563 (methicillin MIC, 12.5 micrograms/ml), which showed heterogeneous resistance. Twenty-two clinical isolates of methicillin-resistant S. aureus were transformed with DNA of SRM563. In the transformants of most strains, instances of reduced resistance were observed with concomitant increases of autolysis rate induced by Triton X-100 and were generally more profound in high-resistance strains. Two transformants exhibited a decrease of the autolysis rate and little reduction of resistance. In the transformant of methicillin-susceptible strain RN2677, an increase of the autolysis rate and little reduction of resistance were observed. The production of low-affinity penicillin-binding protein (PBP2') did not significantly decrease in the mutants. Insertion of Tn918 occurred within the 3'-terminal region of a novel gene designated llm, which was cloned and sequenced. RNA blot analysis demonstrated that the gene was transcribed. The encoded protein was composed of 351 amino acid residues with a molecular weight of 38,512 and was hydrophobic, suggesting its location on the membrane. The gene was detected by PCR in all S. aureus strains tested but not in the other 26 staphylococcal species. Comparison of the 3'-terminal sequences of the gene among several S. aureus strains showed that, whereas nucleotide substitutions occurred at the third position in seven of eight 3'-terminal codons, only C-terminal amino acid variation of glutamate or aspartate was observed.

Suppression of autolysis and cell wall turnover in heterogeneous Tn551 mutants of a methicillin-resistant Staphylococcus aureus strain

Isogenic Tn551 mutants of a highly and uniformly methicillin-resistant strain of Staphylococcus aureus were tested for their rates of autolysis and cell wall degradation in buffer and for cell wall turnover during growth. The normal (relatively fast) autolysis and turnover rates of the parent strain were retained in a Tn551 mutant in which the insert was located within the mec gene and which produced undetectable levels of penicillin-binding protein 2A. On the other hand, autolysis and cell wall turnover rates were greatly reduced in auxiliary mutants, i.e., mutants in which the transposon caused conversion of the high-level and uniform resistance of the parent strain to a variety of distinct heterogeneous expression types and greatly decreased resistance levels. All of these mutants contained an intact mec gene and produced normal amounts of penicillin-binding protein 2A, and one of the mutations was located in the femA region of the staphylococcal chromosome (B. Berger-Bachi, L. Barberis-Maino, A. Strassle, and F. H. Kayser, Mol. Gen. Genet. 219:263-269, 1989). Autolysis rates were related to the degree of residual methicillin resistance and to the sites of Tn551 insertion. Fast cell wall turnover may help expression of high-level methicillin resistance by providing a mechanism for the excision of abnormal (and potentially lethal) structural elements of the cell wall synthesized by the bacteria in the presence of methicillin.

Basic features of the staphylococcal heat shock response

The major heat shock proteins of Staphylococcus aureus had apparent Mrs of 84,000, 76,000, and 60,000, and other prominent proteins of Mrs 66,000, 51,000, 43,000 and 24,000 were also induced. Staphylococcus epidermidis showed a similar response. These proteins were also induced by CdCl2, ethanol and apparently osmotic stress (1.71 M NaCl or 2.25 M sucrose). Most of the proteins sedimented with the membrane fraction, but the Mr 60,000 protein remained in the cytoplasm.