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.

The rapid intravenous administration to mice of pirbenicillin, carbenicillin, and ampicillin produced biexponential blood concentration-time curves when assessed by frequent blood samplings at short intervals. The pharmacokinetic behavior of pirbenicillin and the other penicillins was analyzed by the two-compartment open model. This is thought to be the first study giving detailed pharmacokinetic values of penicillins in mice. Some significant differences were noted between the pharmacokinetic values of pirbenicillin, ampicillin, and carbenicillin. These values suggest that the interchange of pirbenicillin between the central and peripheral body compartments of the mouse was slower than that of either carbenicillin or ampicillin and indicated that a greater fraction of the pirbenicillin than the ampicillin dose reached the peripheral compartment.

Pirbenicillin: Pharmacokinetic Parameters in Mice

The efficacy of sulbactam/ampicillin in the treatment of mice with fatal systemic infections produced by ampicillin-resistant Staphylococcus aureus, Haemophilus influenzae, Klebsiella pneumoniae, or Proteus vulgaris strains is well established. In this paper the demonstrations of efficacy for sulbactam/ampicillin have been extended to a number of clinically relevant models, including bacteremia and meningitis produced by H. influenzae in infant rats, experimental staphylococcal endocarditis in rabbits, localized lesions in mice, urinary tract infections in rats, and prophylaxis in a surgical wound model in mice. In these models, in which ampicillin-resistant organisms were used, sulbactam/ampicillin was either more effective than or as effective as appropriate control agents. Neither sulbactam nor ampicillin used separately displayed significant activity. The results of supportive pharmacokinetic studies, in which differential bioassays were used, demonstrated that sulbactam and ampicillin generally were delivered with equal efficiency to plasma and to extravascular fluids obtained by sampling the contents of implanted cylinders.

Activity of Sulbactam/Ampicillin in Screening and Discriminative Animal Models of Infection

In the new agent 3-(5-tetrazolyl)penam, hereafter referred to as CP-35,587, the carboxyl function at C3 in the penicillin nucleus has been replaced with the 5-tetrazolyl moiety. Marked changes in spectrum and resistance to gram-negative β-lactamases, particularly with regard to Klebsiella pneumoniae isolates, were conferred by this modification. The anti-Klebsiella activity clearly distinguishes the antibacterial spectrum of CP-35,587 from any known broad-spectrum penicillin. Compared to orally active cephalosporins, the spectrum advantage of CP-35,587 encompasses Enterobacter, Serratia marcescens, Citrobacter, Providencia, Haemophilus influenzae, and Streptococcus faecalis, both in vitro and in murine infections produced by many of the above-named microorganisms. Thus, CP-35,587 combines and extends the antibacterial activity of broad-spectrum penicillins and orally active cephalosporins.

Laboratory Evaluation of 3-(5-Tetrazolyl)Penam, a New Semisynthetic Beta-Lactam Antibacterial Agent with Extended Broad-Spectrum Activity

Susceptibility and resistance emergence studies with macrolides

(6R, 8S)-(2-Benzimidazolyl)hydroxymethylpenicillanic acids (1a-1x) are potent antibacterial agents and β-lactamase inhibitors against Gram-positive bacteria and Haemophilus influenzae. The corresponding (6R, 8R)-isomers (2a-2x), the 6, 6-spiro benzimidazole-penam alcohol (3), (7R, 9S)-(2-benzimidazolyl)hydroxymethylcephalosporanic acid (4), and 6β-(2-benzimidazolyl)aminopenicillanic acid (5) are much less active as antibacterials or β-lactamase inhibitors. The syntheses and structure-activity relationships of these compounds are discussed. Antibacterial activity and β-lactamase inhibition data are presented.

James A. Retsema

Papers

Pirbenicillin: Pharmacokinetic Parameters in Mice

Activity of Sulbactam/Ampicillin in Screening and Discriminative Animal Models of Infection

Laboratory Evaluation of 3-(5-Tetrazolyl)Penam, a New Semisynthetic Beta-Lactam Antibacterial Agent with Extended Broad-Spectrum Activity

Susceptibility and resistance emergence studies with macrolides

(6R,8S)-(2-benzimidazolyl)hydroxymethylpenicillanic acids as potent antibacterial agents and beta-lactamase inhibitors.