Streptomycin resistance in a streptomycin-producing microorganism

Unraveling the Nature of Antibiotics: Is It a Cure or a New Hurdle to the Patient Treatment?

Antimicrobial resistance is an increasing problem worldwide that has been exacerbated by antibiotic misuse worldwide. Growing antibiotic resistance can be attributed to as well as leads to severe infections, complications, prolonged hospital admissions, and higher mortality. One of the most important goals of administering antimicrobials is to avoid establishing antibiotic resistance during therapy. This can be done by drastically lowering worldwide antimicrobial usage, both in present and future. While current management methods to legislate antimicrobials and educate the healthcare community on the challenges are beneficial, they do not solve the problem of attaining an overall reduction in antimicrobial usage in humans. Application of rapid microbiological diagnostics for identification and antimicrobial susceptibility testing, use of inflammation markers to guide initiation and duration of therapies, reduction of standard antibiotic course durations, individualization of antibiotic treatments, and dosing considering pharmacokinetics are all possible strategies to optimize antibiotic use in everyday clinical practice and reduce the risk of inducing bacterial resistance. Furthermore, to remove any impediments to proper prescribing, strategies to improve antibiotic prescribing and antibiotic stewardship programs should enable clinical reasoning and enhance the prescribing environment. In addition, the well-established association between antimicrobial usage and resistance should motivate efforts to develop antimicrobial treatment regimens that facilitate the evolution of resistance. This review discusses the role of antibiotics, their current application in human medicine, and how the resistance has evolved to the existing antibiotics based on the existing literature.

Actinomycetes: A Never-Ending Source of Bioactive Compounds-An Overview on Antibiotics Production

The discovery of penicillin by Sir Alexander Fleming in 1928 provided us with access to a new class of compounds useful at fighting bacterial infections: antibiotics. Ever since, a number of studies were carried out to find new molecules with the same activity. Microorganisms belonging to Actinobacteria phylum, the Actinomycetes, were the most important sources of antibiotics. Bioactive compounds isolated from this order were also an important inspiration reservoir for pharmaceutical chemists who realized the synthesis of new molecules with antibiotic activity. According to the World Health Organization (WHO), antibiotic resistance is currently one of the biggest threats to global health, food security, and development. The world urgently needs to adopt measures to reduce this risk by finding new antibiotics and changing the way they are used. In this review, we describe the primary role of Actinomycetes in the history of antibiotics. Antibiotics produced by these microorganisms, their bioactivities, and how their chemical structures have inspired generations of scientists working in the synthesis of new drugs are described thoroughly.

Crossroads of Antibiotic Resistance and Biosynthesis

The biosynthesis of antibiotics and self-protection mechanisms employed by antibiotic producers are an integral part of the growing antibiotic resistance threat. The origins of clinically relevant antibiotic resistance genes found in human pathogens have been traced to ancient microbial producers of antibiotics in natural environments. Widespread and frequent antibiotic use amplifies environmental pools of antibiotic resistance genes and increases the likelihood for the selection of a resistance event in human pathogens. This perspective will provide an overview of the origins of antibiotic resistance to highlight the crossroads of antibiotic biosynthesis and producer self-protection that result in clinically relevant resistance mechanisms. Some case studies of synergistic antibiotic combinations, adjuvants, and hybrid antibiotics will also be presented to show how native antibiotic producers manage the emergence of antibiotic resistance.

Autoregulatory factors of secondary metabolism and morphogenesis in actinomycetes

The Gram-positive bacterial genus Streptomyces possesses interesting biological aspects, such as the ability to produce a wide variety of secondary metabolites and a mycelial form of growth that culminates in sporulation. A close relationship of secondary metabolism and cell differentiation has been well recognized; secondary metabolism might be a physiological expression of cell differentiation. A variety of diffusible low-molecular-weight chemical substances have been found to function in general as regulatory factors, like "hormones" in eukaryotes, for secondary metabolism and cell differentiation. Among these factors, A-factor has been most extensively studied. This review summarizes recent research on the chemical structures, functions, biosyntheses, and mode of action of these regulatory factors.

Self-resistance of the nourseothricin-producing strain Streptomyces noursei

The nourseothricin producer Streptomyces noursei is resistant to its own antibiotic in submerged as well as in surface culture. The strain shows no cross-resistance to miscoding inducing aminoglycoside antibiotics. Cell free extracts of Streptomyces noursei inactivate nourseothricin by enzymatic acetylation. The pattern of cross-resistance of Streptomyces noursei correlates well with the substrate specificity of the nourseothricin acetyltransferase. Furthermore, the acetyltransferase activity parallels the resistance level in nourseothricin-producing strains and nonproducing mutants. The results suggest that the nourseothricin acetyltransferase is important in the self-defence strategy of the nourseothricin-producing strain.

Self-cloning in Streptomyces griseus of an str gene cluster for streptomycin biosynthesis and streptomycin resistance

An str gene cluster containing at least four genes (strR, strA, strB, and strC) involved in streptomycin biosynthesis or streptomycin resistance or both was self-cloned in Streptomyces griseus by using plasmid pOA154. The strA gene was verified to encode streptomycin 6-phosphotransferase, a streptomycin resistance factor in S. griseus, by examining the gene product expressed in Escherichia coli. The other three genes were determined by complementation tests with streptomycin-nonproducing mutants whose biochemical lesions were clearly identified. strR complemented streptomycin-sensitive mutant SM196 which exhibited impaired activity of both streptomycin 6-phosphotransferase and amidinotransferase (one of the streptomycin biosynthetic enzymes) due to a regulatory mutation; strB complemented strain SD141, which was specifically deficient in amidinotransferase; and strC complemented strain SD245, which was deficient in linkage between streptidine 6-phosphate and dihydrostreptose. By deletion analysis of plasmids with appropriate restriction endonucleases, the order of the four genes was determined to be strR-strA-strB-strC. Transformation of S. griseus with plasmids carrying both strR and strB genes enhanced amidinotransferase activity in the transformed cells. Based on the gene dosage effect and the biological characteristics of the mutants complemented by strR and strB, it was concluded that strB encodes amidinotransferase and strR encodes a positive effector required for the full expression of strA and strB genes. Furthermore, it was found that amplification of a specific 0.7-kilobase region of the cloned DNA on a plasmid inhibited streptomycin biosynthesis of the transformants. This DNA region might contain a regulatory apparatus that participates in the control of streptomycin biosynthesis.

Molecular cloning of tetracycline resistance genes from Streptomyces rimosus in Streptomyces griseus and characterization of the cloned genes

Two tetracycline resistance genes of Streptomyces rimosus, an oxytetracycline producer, were cloned in Streptomyces griseus by using pOA15 as a vector plasmid. Expression of the cloned genes, designated as tetA and tetB was inducible in S. griseus as well as in the donor strain. The tetracycline resistance directed by tetA and tetB was characterized by examining the uptake of tetracycline and in vitro polyphenylalanine synthesis by the sensitive host and transformants with the resultant hybrid plasmids. Polyphenylalanine synthesis with crude ribosomes and the S150 fraction from S. griseus carrying the tetA plasmid was resistant to tetracycline, and, by a cross-test of ribosomes and S150 fraction coming from both the sensitive host and the resistant transformant, the resistance directed by tetA was revealed to reside mainly in crude ribosomes and slightly in the S150 fraction. However, the resistance in the crude ribosomes disappeared when they were washed with 1 M ammonium chloride. These results suggest that tetA specified the tetracycline resistance of the machinery for protein synthesis not through ribosomal subunits, but via an unidentified cytoplasmic factor. In contrast, S. griseus carrying the tetB plasmid accumulated less intracellular tetracycline than did the host, and the protein synthesis by reconstituting the ribosomes and S150 fraction was sensitive to the drug. Therefore, it is conceivable that tetB coded a tetracycline resistance determinant responsible for the reduced accumulation of tetracycline.

Involvement of 16S ribosomal RNA in resistance of the aminoglycoside-producers Streptomyces tenjimariensis, Streptomyces tenebrarius and Micromonospora purpurea

Resistance to aminoglycoside antibiotics in Micromonospora purpurea (the producer of gentamicin C complex), Streptomyces tenebrarius (the nebramycin producer) and Streptomyces tenjimariensis (which makes istamycin) occurs at the level of the ribosome. Reconstitution analysis has revealed, in each case, that 16S rRNA plays a critical role in determining such resistance.

Some insights into the possible development of a biosynthetic pathway and biological function for anthramycin in Streptomyces refuineus

Using anthramycin, a potent antitumor antibiotic produced by Streptomyces refuineus, as an example, we have developed a rational model for the evolution of the capability of this microorganism to produce, tolerate and retain the genetic information needed to make this extremely potent secondary metabolite. The concepts and ideas outlined in this article have also been applied in a more general way to other antibiotics with the hope that this might stimulate research designed to test some of these concepts.

Ribosomal resistance in the gentamicin producer organism Micromonospora purpurea

The mechanism of resistance of the gentamicin-producing organism Micromonospora purpurea was analyzed. Determination of minimal inhibitory concentrations revealed high resistance to the 4,6-substituted deoxystreptamine aminoglycosides amikacin, gentamicin, kanamycin, netilmicin, sisomicin, and tobramycin and also to lividomycin A and hygromycin B, but susceptibility to streptomycin, dihydrostreptomycin, paromomycin, and neomycin during all phases of the growth cycle. The nonproducing, closely related Micromonospora melanosporea was susceptible to these compounds. In agreement with results from previous studies (R. Benveniste and J. Davies, Proc. Natl. Acad. Sci. U.S.A. 70:2276-2280, 1973), extracts from M. purpurea showed no activity of enzymes specifically modifying gentamicin. 70S ribosomes from M. purpurea but not from M. melanosporea were resistant to inhibition by gentamicin, kanamycin, tobramycin, and lividomycin in a polyuridylic acid-dependent polyphenylalanine synthesis system and susceptible to those compounds which were inhibitory in vivo. The former antibiotics were also unable to induce misreading. Subunit exchange experiments between M. purpurea and M. melanosporea showed that the main site for inhibition and induction of misreading is the 30S subunit (up to gentamicin concentrations of 10 micrograms/ml).