EFFECT OF ANTIBIOTICS AND INHIBITORS ON M PROTEIN SYNTHESIS

Dissecting the Nucleoside Antibiotics as Universal Translation Inhibitors

Without question, natural products have provided the lion share of leads, if not drugs themselves, for the treatment of bacterial infections. The bacterial arms race, fueled by selection and survival pressures has delivered a natural arsenal of small molecules targeting the most essential of life processes. Antibiotics that target these critical intracellular processes face the formidable defense of both penetrating a bacterial cell membrane and avoiding efflux to exert their effect. These challenges are especially effective in Gram-negative (Gram-(-)) bacteria, which have a double membrane structure and efficient efflux systems from the combination of outer-membrane porins and inner membrane proton pumps. In this landscape of offense and defense, our clinically used antibiotics have only successfully targeted three intracellular processes for therapeutic intervention in Gram-(-) bacteria: dihydrofolate biosynthesis, transcription, and translation. Not surprisingly, such critical survival machinery is a popular target for bacterial warfare, and eight of our 14 classes of commonly used antibiotics target translation with the bacterial ribosome remaining one the most vetted targets for antimicrobial therapy. On the plus side, its anionic character attracts cationic inhibitors, which are generally more capable of penetrating the bacterial cell wall, and clinical resistance rates are usually manageable as mutation of such a highly evolved machine is difficult. On the down side, this highly evolved machine renders it difficult to inhibit selectively, and the inhibition of prokaryotic translation versus both eukaryotic cellular and mitochondrial translation is critical for clinical development and minimization of undesired toxicities.A class of natural products known as the "nucleoside antibiotics" have historically been recognized as universal inhibitors of the ribosome and can inhibit translation in prokaryotes, eukaryotes, and archaea. While they have served an essential role in dissecting the biochemical underpinnings of the enzymatic functions of the ribosome, they have not proven therapeutically useful as they target the highly conserved rRNA in the P-site and are toxic to mammalian cells. In this Account, we describe our studies on the natural product amicetin, a nucleoside antibiotic that we have demonstrated to break the rule of being a universal translation inhibitor. While the cytosine of amicetin mimics C75 of the 3'-CCA tail of the P-site tRNA akin to other nucleoside antibiotics, we advance a hypothesis that amicetin's unique interaction with the ribosomal protein uL16 exploits an untapped mechanism for selectively targeting the bacterial ribosome. A complex molecule comprised of a nucleoside, carbohydrates and amino acids, amicetin is also chemically unstable. Our initial attempts to stabilize and simplify this scaffold are presented with the ultimate goal of rebuilding the compound with improved penetrance to bacterial cells. If successful, this scaffold would demonstrate a path forward for a new class of antibiotics capable of selectively targeting the ribosomal P-site.

Mandated restrictions on the use of medically important antibiotics in broiler chicken production in Canada: implications, emerging challenges, and opportunities for bolstering gastrointestinal function and health– A review

Chicken Farmers of Canada has been progressively phasing out prophylactic use of antibiotics in broiler chicken production. Consequently, hatcheries, veterinarians, and nutritionists have been mandated to contend with less reliance on use of preventive antibiotics. A topical concern is the increased risk of proliferation of enteric pathogens leading to poor performance, increased mortality and compromised welfare. Moreover, the gut harbors several taxa such as Campylobacter and Salmonella capable of causing significant illnesses in humans via contaminated poultry products. This has created opportunity for research and development of dietary strategies designed to modulate gastrointestinal environment for enhanced performance and food safety. Albeit with inconsistent responses, literature data suggests that dietary strategies such as feed enzymes, probiotics/prebiotics and phytogenic feed additives can bolster gut health and function in broiler chickens. However, much of the efficacy data was generated at controlled research settings that vary significantly with the complex commercial broiler production operations due to variation in dietary, health and environmental conditions. This review will summarize implications of mandated restrictions on the preventative use of antibiotics and emerging Canadian broiler production programs to meet processor specifications. Challenges and opportunities for integrating alternative dietary strategies in commercial broiler production settings will be highlighted.

Assay of type-specific M antigens on whole group A streptococci

A novel radioimmunoassay of type-specific M antigens on whole group A streptococcal cells is described. Absorbed rabbit anti-M antisera directed against M types 12 and 49 were used for determining M antigens on intact bacterial organisms. Staphylococcal protein A labelled with 125I was used as an anti-antibody reagent. The absorbed antisera were tested against ten homologous and 48 heterologous serotypes. All homologous serotypes gave an unequivocal reaction distinct from the weaker reaction with the heterologous serotypes. The type-specificity of the reaction was confirmed by the removal of type-specific antibodies after absorption to purified M protein coupled to Sepharose 4B. The results indicate that the described method is a simple and reliable technique for the recognition of M types of group A streptococci and offers a valuable tool for studies of M antigen in situ.

M proteins of group A streptococci

Images

Correlation of M protein production with those factors found to influence growth and substrate utilization of Streptococcus pyogenes

In the absence of proteinase formation, factors reported to influence the growth or fermentation by streptococci have been evaluated to determine their quantitative effect upon the production of M protein during the growth of Streptococcus pyogenes. Buffers, amino acids, peptides, gross organic additions, and carbohydrate substrates were tested under a variety of cultural conditions. The M protein content was remarkably constant throughout the late logarithmic period of growth, i.e., when the cell population doubled, the M protein doubled. However, several factors affected the M protein content per milligram of cells (dry weight). When types 1, 12, and 22 were grown aerobically in a semidefined medium, the M protein content of the cell population essentially doubled; in Todd-Hewitt broth, this aerobic effect on M protein synthesis was not observed. When cells grown on Todd-Hewitt broth were transferred to medium containing 0.1% starch and no added glucose, the M protein content per milligram of cells (dry weight) increased as much as fourfold. When growth was initiated in glucose, the rate of M protein formation was at a maximum in the early logarithmic phase of growth and was comparatively greater than the rate of cellular multiplication. When the amount of substrate fermented was greater than 0.2%, increased M protein was not observed. An evaluation of the effects of medium or conditions of growth showed the units of M per milligram of cells (dry weight) were not influenced by a shift in the stoichiometry of either the anaerobic or aerobic fermentation, substrate used, or adenosine triphosphate utilized for growth. These results show that M protein synthesis is subject to limited glucose repression or substrate catabolite repression.

Evaluation of methods used to purify acid-extracted group A streptococcal M protein

The literature includes descriptions of both acid-soluble and acid-insoluble M protein in the preparation of "hot acid-extracted group A streptococcal M protein." We present evidence for the contamination of crude type 1 acid-insoluble M protein. The purification of preparations of crude and partially purified acid-soluble type 1 and type 12 M protein is described. Our quantitative criteria for purification were recovery of M precipitin activity, improvement in specific activity, and removal of carbohydrate. Exclusion of nucleic acid is also discussed. Greater purification in a single passage was found with a carboxymethylcellulose column (with acidic elution) than with hydroxyapatite, diethylaminoethyl-Sephadex, or carboxymethyl-cellulose (with neutral elution) columns or with ammonium sulfate fractional precipitation. Carboxymethylcellulose with acidic elution was found to be a satisfactory standard laboratory procedure for the preparation of purified acid-extracted (acid-soluble) group A streptococcal M protein.

Effect of culture medium composition and pH on the production of M protein and proteinase by group A Streptococci

The effects of pH, yeast extract, and neopeptone on the production of extracellular proteinase and M protein by group A streptococci were studied with a type 1 strain capable of producing both M protein and proteinase. The strain DS 2036-66 grew moderately well in a semisynthetic broth. M protein was produced without adding peptides to the medium. When added to a medium with 1% glucose, yeast extract (0.1%) was found to stimulate both growth and proteinase formation. Limiting the glucose to 0.25% prevented a drop in pH below 6.7 and prevented proteinase formation. Although less growth occurred with limited glucose, M protein of high specific activity was produced with an actual increase in acid-extractable M protein during the stationary phase of growth. When the medium was buffered at pH 7.85 with tris(hydroxymethyl)aminomethane buffer, 0.5% neopeptone prevented proteinase formation. This was true even in the presence of 1% glucose and 0.1% yeast extract, which resulted in a fall in pH to about 4.8 by 48 hr. Growth was greater than in Todd Hewitt broth, but the specific activity of M protein was considerably less than that found in the medium with glucose limited to 0.25%. Neopeptone was found to have little direct action on crude streptococcal proteinase. Instead, the evidence suggested that neopeptone somehow prevents proteinase elaboration. Yeast extract, on the other hand, appears to stimulate proteinase elaboration. To prevent proteinase formation, neopeptone must be added early, during the logarithmic phase of growth or at the start. In contrast, when yeast extract was added as late as 24 hr, it resulted in the elaboration of extracellular proteinase and in the decline of M protein. When 38 M nontypable strains from the diagnostic laboratory were tested for proteinase activity under conditions similar to those used in the diagnostic laboratory, only six produced much proteinase.

Quantitative aspects of the M protein capillary precipitin test

A capillary procedure for quantitatively determining M protein is described. Capillaries were filled with measured amounts of serum and streptococcal extract. The capillaries were incubated, and then centrifuged to pack the precipitates. The relative sizes of the precipates were compared by a determination of the weights of their paper images (obtained by reflection from a microscope). Meaningful dilution curves were determined by this method. Variations of pH from 6 to 8 had little effect on the M protein precipitin test, and the test was not seriously affected by variations of the NaCl concentration from 0.85 to 4.67%. The addition of divalent ions (Ca(++) and Mg(++)) did not influence the results. This test can be used to make quantitative comparisons of M protein preparations and to titrate type-specific antisera.

Symposium on the fine structure and replication of bacteria and their parts. 3. Bacterial cell-wall replication followed by immunofluorescence

Images

INDUCIBLE RESISTANCE TO ERYTHROMYCIN IN STAPHYLOCOCCUS AUREUS

Weaver, Judith R. (Iowa State University, Ames), and P. A. Pattee . Inducible resistance to erythromycin in Staphylococcus aureus . J. Bacteriol. 88: 574–580. 1964.—The dissociated resistance of Staphylococcus aureus to erythromycin was examined and was found to possess the characteristics of an inducible enzyme. The induction of resistance to high concentrations of erythromycin in S. aureus occurred only after prior exposure to subinhibitory concentrations of erythromycin. The only macrolide antibiotic examined which induced resistance was erythromycin, and the resistance of induced populations was rapidly lost when they were grown in the absence of this antibiotic. Induction did not occur when protein synthesis was inhibited by either chloramphenicol or histidine starvation of a histidine auxotroph. The macrolide antibiotics inhibited the induction of resistance at the same minimal concentrations required to inhibit growth and induced synthesis of β-galactosidase. Therefore, the mode of action of the macrolide antibiotics is to inhibit protein synthesis, and the induction of resistance overcomes this inhibition in some manner which is associated with the synthesis of new protein.