Fluorescence assay for studying the ability of macrolides to induce production of ribosomal methylase

Streptococcus pyogenes infects human endometrium by limiting the innate immune response

Group A Streptococcus (GAS), a Gram-positive human-specific pathogen, yields 517,000 deaths annually worldwide, including 163,000 due to invasive infections and among them puerperal fever. Before efficient prophylactic measures were introduced, the mortality rate for mothers during childbirth was approximately 10%; puerperal fever still accounts for over 75,000 maternal deaths annually. Yet, little is known regarding the factors and mechanisms of GAS invasion and establishment in postpartum infection. We characterized the early steps of infection in an ex vivo infection model of the human decidua, the puerperal fever portal of entry. Coordinate analysis of GAS behavior and the immune response led us to demonstrate that (a) GAS growth was stimulated by tissue products; (b) GAS invaded tissue and killed approximately 50% of host cells within 2 hours, and these processes required SpeB protease and streptolysin O (SLO) activities, respectively; and (c) GAS impaired the tissue immune response. Immune impairment occurred both at the RNA level, with only partial induction of the innate immune response, and protein level, in an SLO- and SpeB-dependent manner. Our study indicates that efficient GAS invasion of the decidua and the restricted host immune response favored its propensity to develop rapid invasive infections in a gynecological-obstetrical context.

Regulation of gene expression by macrolide-induced ribosomal frameshifting

The expression of many genes is controlled by upstream ORFs (uORFs). Typically, the progression of the ribosome through a regulatory uORF, which depends on the physiological state of the cell, influences the expression of the downstream gene. In the classic mechanism of induction of macrolide resistance genes, antibiotics promote translation arrest within the uORF, and the static ribosome induces a conformational change in mRNA, resulting in the activation of translation of the resistance cistron. We show that ketolide antibiotics, which do not induce ribosome stalling at the uORF of the ermC resistance gene, trigger its expression via a unique mechanism. Ketolides promote frameshifting at the uORF, allowing the translating ribosome to invade the intergenic spacer. The dynamic unfolding of the mRNA structure leads to the activation of resistance. Conceptually similar mechanisms may control other cellular genes. The identified property of ketolides to reduce the fidelity of reading frame maintenance may have medical implications.

Attenuation-based dual-fluorescent-protein reporter for screening translation inhibitors

A reporter construct was created on the basis of the transcription attenuator region of the Escherichia coli tryptophan operon. Dual-fluorescent-protein genes for red fluorescent protein and cerulean fluorescent protein were used as a sensor and internal control of gene expression. The sequence of the attenuator was modified to avoid tryptophan sensitivity while preserving sensitivity to ribosome stalling. Antimicrobial compounds which cause translation arrest at the stage of elongation induce the reporter both in liquid culture and on an agar plate. This reporter could be used for high-throughput screening of translation inhibitors.

Programmed drug-dependent ribosome stalling

The ribosome has the intrinsic capacity to monitor the sequence and structure of the nascent peptide. This fundamental property of the ribosome is often exploited in regulation of gene expression, in particular, for activation of expression of genes conferring resistance to ribosome-targeting antibiotics. Induction of expression of these genes is controlled by the programmed stalling of the ribosome at a regulatory open reading frame located upstream of the resistance cistron. Formation of the stalled translation complex depends on the presence of an antibiotic in the ribosome exit tunnel and the sequence of the nascent peptide. In this review, we summarize our current understanding of the molecular mechanisms of drug- and nascent peptide-dependent ribosome stalling.

Induction of erm(C) expression by noninducing antibiotics

Ketolides, which represent the newest macrolide antibiotics, are generally perceived to be noninducers of inducible erm genes. In the study described in this paper we investigated the effects of several macrolide and ketolide compounds on the expression of the inducible erm(C) gene by Escherichia coli cells. Exposure to 14-member-ring macrolide drugs and to azithromycin led to a rapid and pronounced increase in the extent of dimethylation of Erm(C) target residue A2058 in 23S rRNA. When cells were incubated with subinhibitory concentrations of ketolides, the extent of A2058 dimethylation was also increased, albeit to a lower level and with kinetics slower than those observed with macrolides. The induction of erm(C) expression by ketolides was further confirmed by using a reporter construct which allows the colorimetric detection of induction in a disc diffusion assay. Most of the ketolides tested, including the clinically relevant compounds telithromycin and cethromycin, were able to induce the reporter expression, even though the induction occurred within a more narrow range of concentrations compared to the concentration range at which induction was achieved with the inducing macrolide antibiotics. No induction of the reporter expression was observed with 16-member-ring macrolide antibiotics or with a control drug, chloramphenicol. The deletion of three codons of the erm(C) leader peptide eliminated macrolide-dependent induction but left ketolide-dependent induction unchanged. We conclude that ketolides are generally capable of inducing erm genes. The narrow range of ketolide inducing concentrations, coupled with the slow rate of induction and the lower steady-state level of ribosome methylation, may mask this effect in MIC assays.

Modes and modulations of antibiotic resistance gene expression

Since antibiotic resistance usually affords a gain of function, there is an associated biological cost resulting in a loss of fitness of the bacterial host. Considering that antibiotic resistance is most often only transiently advantageous to bacteria, an efficient and elegant way for them to escape the lethal action of drugs is the alteration of resistance gene expression. It appears that expression of bacterial resistance to antibiotics is frequently regulated, which indicates that modulation of gene expression probably reflects a good compromise between energy saving and adjustment to a rapidly evolving environment. Modulation of gene expression can occur at the transcriptional or translational level following mutations or the movement of mobile genetic elements and may involve induction by the antibiotic. In the latter case, the antibiotic can have a triple activity: as an antibacterial agent, as an inducer of resistance to itself, and as an inducer of the dissemination of resistance determinants. We will review certain mechanisms, all reversible, that bacteria have elaborated to achieve antibiotic resistance by the fine-tuning of the expression of genetic information.

[Antibiotherapy for acute CAP in adults]

Community acquired pneumonia is one of the most frequent infections. With time, bacterial epidemiology and bacterial resistance evolve and new antibiotics become available. So an up-date on adequate antibiotic use is necessary. We reviewed the epidemiology of pneumonia and the evolution of bacterial resistance. We also collected data on new antibiotics which can be used for this infection such as levofloxacin, moxifloxacin, telithromycin, and pristinamycin. All these drugs are effective on bacteria involved in pneumonia. At this time, only few Streptococcus pneumoniae strains have developed resistance to these drugs. However, resistance to fluoroquinolones is not easily detected with common laboratory techniques. There is no effectiveness difference between the 2 new fluoroquinolones (levofloxacin, moxifloxacin) in clinical studies. However, in bacteriological and pharmacological studies, moxifloxacin seems to be more effective than levofloxacin (500 mg/day). For the treatment of pneumonia due to Legionella pneumophila, fluoroquinolones are now widely recommended. For Streptococcus pneumonia, amoxicillin remain the drug of choice, even for bacteria with a decreased susceptibility to penicillin. The importance of treating atypical pathogens remains to be documented.

The antimicrobial armamentarium: evaluating current and future treatment options

The development and introduction of new antibiotics has, unfortunately, not kept pace with the development of bacterial resistance, and the need for new agents is becoming acute. Although some currently marketed agents remain valuable tools in the treatment of infectious diseases, few new drugs have reached the market in the last decade. In recent years, antibiotics with activity against certain problematic resistant bacteria such as methicillin-resistant Staphylococcus aureus, including linezolid and daptomycin, have been approved for clinical practice. Recently, tigecycline, a minocycline derivative, received approval by the United States Food and Drug Administration for treatment of complicated skin and skin structure and intraabdominal infections; the agent is also active against a variety of multidrug-resistant bacteria. Of the other agents in phase III development, ceftobiprole--a cephalosporin, and faropenem and doripenem--both carbapenems, have wide antibacterial spectra. Antimicrobial agents in the pipeline with marked gram-positive activity include dalbavancin, telavancin, and oritavancin.

Telithromycin: A Ketolide Antibiotic for Treatment of Respiratory Tract Infections

Telithromycin, a recently approved ketolide antibiotic derived from 14-membered macrolides, is active against erythromycin-resistant pneumococci. Telithromycin has enhanced activity in vitro because it binds not only to domain V of ribosomal RNA (like macrolides do) but also to domain II. However, it is not active against streptococci and staphylococci with constitutive macrolide, lincosamide, and streptogramin B resistance. Telithromycin, available in an oral formulation, is approved by the US Food and Drug Administration for use in adults for treatment of (1) community-acquired pneumonia due to Streptococcus pneumoniae (including multidrug-resistant isolates), Haemophilus influenzae, Moraxella catarrhalis, Chlamydia pneumoniae, or Mycoplasma pneumoniae; (2) acute exacerbation of chronic bronchitis due to S. pneumoniae, H. influenzae, or M. catarrhalis; or (3) acute bacterial sinusitis due to S. pneumoniae, H. influenzae, M. catarrhalis, or methicillin- and erythromycin-susceptible Streptococcus aureus. It is not approved for treatment of tonsillitis, pharyngitis, or severe pneumococcal pneumonia. Unique visual adverse effects occurred in 0.27%-2.1% of patients receiving telithromycin therapy. Its enhanced activity against some common respiratory pathogens makes it a valuable addition to the available macrolides.

Impact of Ketolides on Resistance Selection and Ecologic Effects during Treatment for Respiratory Tract Infections

Ketolides are a new class of antibacterials that have been specifically developed for the treatment of community-acquired respiratory tract infections in an era of increasing resistance among major etiologic pathogens. These agents possess several unique structural features, including a 3-keto function and a large aromatic side chain, that confer not only a mode of action that differentiates them from the macrolide class but also a reduced potential to induce--or select for--resistant strains. Studies also suggest that ketolides such as telithromycin have a lower ecologic impact on the body's microflora than agents such as clarithromycin and amoxicillin-clavulanate, potentially reducing the risk of emergence of resistant strains and the spread of such resistance to pathogenic species. Therefore, available evidence suggests that ketolides may not only provide important new treatment options in an era of increasing resistance but may also contribute to reducing the pressure for development of further resistance. Clearly, further studies are required to confirm this low resistance potential once the ketolide agents become more widely used in routine practice.