Tetracyclines were discovered in the 1940s and exhibited activity against a wide range of microorganisms including gram-positive and gram-negative bacteria, chlamydiae, mycoplasmas, rickettsiae, and protozoan parasites. They are inexpensive antibiotics, which have been used extensively in the prophlylaxis and therapy of human and animal infections and also at subtherapeutic levels in animal feed as growth promoters. The first tetracycline-resistant bacterium, Shigella dysenteriae, was isolated in 1953. Tetracycline resistance now occurs in an increasing number of pathogenic, opportunistic, and commensal bacteria. The presence of tetracycline-resistant pathogens limits the use of these agents in treatment of disease. Tetracycline resistance is often due to the acquisition of new genes, which code for energy-dependent efflux of tetracyclines or for a protein that protects bacterial ribosomes from the action of tetracyclines. Many of these genes are associated with mobile plasmids or transposons and can be distinguished from each other using molecular methods including DNA-DNA hybridization with oligonucleotide probes and DNA sequencing. A limited number of bacteria acquire resistance by mutations, which alter the permeability of the outer membrane porins and/or lipopolysaccharides in the outer membrane, change the regulation of innate efflux systems, or alter the 16S rRNA. New tetracycline derivatives are being examined, although their role in treatment is not clear. Changing the use of tetracyclines in human and animal health as well as in food production is needed if we are to continue to use this class of broad-spectrum antimicrobials through the present century.

Tetracycline Antibiotics: Mode of Action, Applications, Molecular Biology, and Epidemiology of Bacterial Resistance

In 1998 we updated earlier descriptions of the largest family of secondary transport carriers found in living organisms, the major facilitator superfamily (MFS). Seventeen families of transport proteins were shown to comprise this superfamily. We here report expansion of the MFS to include 29 established families as well as five probable families. Structural, functional, and mechanistic features of the constituent permeases are described, and each newly identified family is shown to exhibit specificity for a single class of substrates. Phylogenetic analyses define the evolutionary relationships of the members of each family to each other, and multiple alignments allow definition of family-specific signature sequences as well as all wellconserved sequence motifs. The work described serves to update previous publications and allows extrapolation of structural, functional and mechanistic information obtained with any one member of the superfamily to other members with limitations determined by the degrees of sequence divergence.

The major facilitator superfamily.

Dangerous, antibiotic resistant bacteria have been observed with increasing frequency over the past several decades. In this review the factors that have been linked to this phenomenon are addressed. Profiles of bacterial species that are deemed to be particularly concerning at the present time are illustrated. Factors including economic impact, intrinsic and acquired drug resistance, morbidity and mortality rates, and means of infection are taken into account. Synchronously with the waxing of bacterial resistance there has been waning antibiotic development. The approaches that scientists are employing in the pursuit of new antibacterial agents are briefly described. The standings of established antibiotic classes as well as potentially emerging classes are assessed with an emphasis on molecules that have been clinically approved or are in advanced stages of development. Historical perspectives, mechanisms of action and resistance, spectrum of activity, and preeminent members of each class are discussed.

Antibiotics and Bacterial Resistance in the 21st Century

A primary goal in organ transplantation is the prevention or effective treatment of infection, the most common life-threatening complication of long-term immunosuppressive therapy. The challenges involved in achieving this goal are several: a broad range of potential sources of infection ranging from latent viruses to pathogens of both community and hospital origin; immunosuppression-induced impairment of the inflammatory response, which attenuates the signs and symptoms of invasive infection; and the adverse effects of the antimicrobial drugs used for prophylaxis and therapy, which result both from the duration of therapy required and from interactions with the immunosuppressive drugs cyclosporine and tacrolimus. Our . . .

Infection in Organ-Transplant Recipients

The influence of natural products upon drug discovery.

Ring contraction of the neutral oleandrose sugar in the 14-membered-ring macrolide antibiotic oleandomycin (2) has been accomplished using [(methoxycarbonyl)sulfamoyl]triethylammonium hydroxide inner salt (1). The product of this interesting rearrangement, after methanolic hydrolysis of the 2'-acetate, is the 11-acetyl-3-O-(3"-methoxy-4"-vinylfuranosyl)oleandomycin (12). The in vitro activity of furanoside 12 is only moderately less than that of 11-acetyloleandomycin (13).

Ring contraction of oleandrose on the macrolide antibiotic oleandomycin with [(Methoxycarbonyl)sulfamoyl]triethylammonium hydroxide inner salt.

Three derivatives of oleandomycin in which the C"-4 hydroxyl moiety was replaced for the first time with a nitrogen functionality have been compared with erythromycin base and oleandomycin base. The minimum inhibitory concentrations of these derivatives for 90% of a group of clinical isolates of Staphylococcus aureus were one-half to one-fourth those of erythromycin. The minimum inhibitory concentrations of the experimental macrolides for 50% of a group of S. aureus isolates resistant to greater than 12.5 micrograms of erythromycin per ml ranged from 0.2 to 0.39 micrograms/ml. The activities of these experimental compounds were equivalent to the activities of erythromycin against Staphylococcus epidermidis, Bacteroides fragilis, and Haemophilus influenzae isolates. In general, erythromycin was more active against Streptococcus species. Each experimental macrolide was superior to erythromycin in inhibiting RNA-directed, cell-free polypeptide synthesis. The three experimental compounds were markedly more active than erythromycin base after oral administration to mice infected with S. aureus. The 50% protective doses of the experimental compounds ranged from 27.4 to 45.7 mg/kg; that of erythromycin was approximately 100 mg/kg.

Evaluation of three 4"-deoxy-4"-sulfonamido-oleandomycin derivatives with erythromycin-like antibacterial potency.

Azithromycin has been shown to preferentially distribute to infection loci. Due to the potential contribution of phagocytes as transporters of drug to these sites, there has been some concern that immunosuppression of the cellular arm of the host defence system would greatly reduce the delivery of azithromycin to sites of infection and hence impair efficacy. Therefore, we evaluated the pharmacokinetics and pharmacodynamics of azithromycin in a Staphylococcus aureus intramuscular infection model in normal and immunosuppressed mice, employing therapeutic and prophylactic regimens. Immunosuppression was induced by daily doses of cyclophosphamide that culminated in leucopenia with an underlying granulocytopenic condition, with circulating peripheral granulocytes numbering from < or = 0.1-0.3 x 10(9)/L. Azithromycin tissue levels were not reduced in infection loci in granulocytopenic mice but moderate increases in Cmax and AUC values were observed, relative to similar tissues from normal mice. The tissue half-life of azithromycin in infected tissues in a therapeutic mode (75 h) was three-fold longer than in a prophylactic mode (25 h); this correlated with the degree of inflammation (therapy was withheld until inflammation was evident; i.e., prophylaxis reduced inflammation). Histological examination of infected tissues from normal and leucopenic mice was indistinguishable despite a 70%-85% reduction in circulating granulocytes. Compared with untreated infected controls, bactericidal activity was noted following prophylaxis with azithromycin and bacteraemia was suppressed in mice receiving azithromycin therapeutically. In summary, these data indicate that azithromycin delivery and efficacy in a moderately immunosuppressed animal model are unimpaired.

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Influence of immunosuppression on the pharmacokinetics and pharmacodynamics of azithromycin in infected mouse tissues

The rate of 14 C-benzylpenicillin (penicillin G) binding to Staphylococcus aureus Oxford cells increased with increasing hydrogen ion concentration. The extent of inhibition of 14 C-penicillin G binding caused by a competing 12 C-β-lactam antibiotic is a function of hydrogen ion concentration and can be correlated both with net charge of a competing 12 C-molecule and net charge of the S. aureus cell at a given p H. The ability of a β-lactam antibiotic to compete for 14 C-penicillin G-binding sites can generally be correlated with its hydrophobic nature. It is proposed that, although semisynthetic cephalosporins are chemically less reactive than penicillins, they are superior to benzylpenicillin in their ability to permeate the outer surface of the Staphylococcus cell wall and irreversibly bind to reactive sites.

Correlation Between the Binding of β-Lactam Antibiotics to Staphylococcus aureus and Their Physical-Chemical Properties

Two cyclic homopentapeptides, CP-101,680 and CP-163,234 [6a-(3',4'-dichlorophenylamino) analogs of viomycin and capreomycin, respectively], were identified as novel antibacterial agents for the treatment of animal disease, especially for livestock respiratory disease. The in vitro microbiological characterization of both CP-101,680 and CP-163,234 was carried out using their parent compounds, viomycin and capreomycin, as controls. This characterization included antibacterial spectrum, influence of media, inoculum size, pH, EDTA, polymixin B nonapeptide (PMBN), serum, cell-free protein synthesis inhibition, and time-kill kinetics. Our results indicated that the capreomycin analog, CP-163,234, showed slightly improved in vitro potency over the viomycin analog, CP-101,680. Both analogs showed very potent cell-free protein synthesis inhibition activity and were bactericidal against Pasteurella haemolytica, P. multocida and Actinobacillus pleuropneumoniae at the level of 4 times and 8 times MICs. CP-163,234 was bactericidal at the level of 4x and 8x MIC against E. coli, but re-growth was observed after 24 hours incubation at both concentrations of CP-101,680.

James A. Retsema

Papers

Ring contraction of oleandrose on the macrolide antibiotic oleandomycin with [(Methoxycarbonyl)sulfamoyl]triethylammonium hydroxide inner salt.

Evaluation of three 4"-deoxy-4"-sulfonamido-oleandomycin derivatives with erythromycin-like antibacterial potency.

Influence of immunosuppression on the pharmacokinetics and pharmacodynamics of azithromycin in infected mouse tissues

Correlation Between the Binding of β-Lactam Antibiotics to Staphylococcus aureus and Their Physical-Chemical Properties

In vitro microbiological characterization of novel cyclic homopentapeptides, CP-101,680 and CP-163,234, for animal health use.