Potential and use of bacterial small RNAs to combat drug resistance: a systematic review

H. pectinata (L.) Poit - Traditional uses, phytochemistry and biological-pharmacological activities in preclinical studies: A systematic review

ETHNOPHARMACOLOGICAL RELEVANCE

H. pectinata (L.) Poit, popularly known as "sambacaitá" or "canudinho", is a plant endemic to north-eastern Brazil. Its aerial parts, leaves and flowers have traditionally been used to treat gastrointestinal disorders, rhinopharyngitis, nasal congestion, bacterial and fungal infections, fever, colic, inflammation, and pain.

AIM OF THE STUDY

The aim of this review was to provide information on the botanical characteristics, ethnomedicinal uses, phytochemistry and biological-pharmacological activities of H. pectinata.

MATERIALS AND METHODS

This systematic review followed the Cochrane Handbook Collaboration and the PRISMA guidelines. The review question was what are the biological-pharmacological activities of H. pectinata presented in non-clinical studies. The search for articles was conducted in the Medline (via PubMed), Embase, Web of Science, Scopus, Virtual Health Library, SciELO, Google Scholar and the Brazilian Digital Library of Theses and Dissertations databases. Two reviewers independently selected the studies that met the inclusion criteria, extracted the data, and assessed the risk of bias of the included studies.

RESULTS

39 articles were included in this review, of which 19 reported in vitro experiments, 16 in vivo studies and 4 in vivo and in vitro experiments. H. pectinata is a plant widely used in folk medicine in north-eastern Brazil for the treatment of various ailments, such as respiratory diseases, gastrointestinal disorders, bacterial and fungal infections, and general inflammation. Supporting its popular use, several in vitro and in vivo pharmacological investigations of the essential oil and extract of H. pectinata have demonstrated their anti-inflammatory, antinociceptive, antioxidant, antidepressant, anticancer, hepatoregenerative, healing, and antimicrobial activities. H. pectinata has been reported to contain 75 bioactive constituents, comprising 9 flavonoids, 54 terpenes, and 12 other compounds.

CONCLUSION

H. pectinata is a plant commonly used in traditional medicine. Phytochemically, it contains several bioactive constituents, including terpenes and flavonoids, and has been shown to have antinociceptive, anti-inflammatory, antimicrobial and antitumour activity, as well as hepatorregenerative and healing effects, and low toxicity.

Medicinal activities of Tualang honey: a systematic review

Natural products derived from various sources, including plants, have garnered significant interest as alternative therapeutic options. Among these, Tualang honey, extracted from the nectar of Tualang trees (Koompassia excelsa (Becc.) Taub.), has a long history of traditional use due to its therapeutic properties. This review aims to examine the pharmacological activities of Tualang honey, encompassing both in vitro and in vivo studies. A systematic search was conducted in multiple databases, including PubMed, Springer, Scopus, Wiley, and Science Direct, up until December 2022 to identify relevant studies on the pharmacological activities of Tualang honey. Two independent reviewers were involved in article selection, followed by data extraction and assessment of methodological quality using Syrcle's risk of bias tool. 123 articles were included, collectively describing the pharmacological activities of Tualang honey, including antimicrobial, anticancer, anti-inflammatory, antioxidant, antinociceptive, neuroprotective effects, and others. Tualang honey has significant promise as an alternative treatment option for treating a wide range of pathological diseases due to its wide range of pharmacological properties. Tualang honey's diverse array of pharmacological actions indicates its potential for multiple medicinal uses.

Small RNA-regulated expression of efflux pump affects tigecycline resistance and heteroresistance in clinical isolates of Klebsiella pneumoniae

Tigecycline and the newly Food and Drug Administration-approved tetracyclines, including eravacycline and omadacycline, are regarded as last-resort treatments for multidrug-resistant Enterobacterales. However, tigecycline resistance in Klebsiella pneumoniae has increased, especially the underlying mechanism of heteroresistance is unclear. This study aimed to elucidate the mechanisms underlying tigecycline resistance and heteroresistance in clinical K. pneumoniae isolates. A total of 153 clinical K. pneumoniae isolates were collected, and identified 15 tigecycline-resistant and three tigecycline-heteroresistant isolates using broth microdilution and population analysis profile methods, respectively. Total RNAs from K. pneumoniae ATCC13883 and the laboratory-induced tigecycline-resistant strain were extracted and sequenced on an Illumina platform. Differentially expressed genes and regulatory small RNAs (sRNAs) were analyzed and validated in clinical isolates of K. pneumoniae using quantitative real-time PCR. RNA sequencing results showed that mdtABC efflux pump genes were significantly upregulated in the tigecycline-resistant strains. Overexpression of mdtABC was observed in a clinical K. pneumoniae isolate, which increased tigecycline minimum inhibitory concentrations (MICs) and was involved in tigecycline heteroresistance. Sequencing analysis of sRNA demonstrated that candidate sRNA-120 directly interacted with the mdtABC operon and was downregulated in tigecycline-resistant strains. We generated an sRNA-120 deletion mutation strain and a complemented strain of K. pneumoniae. The sRNA-120 deletion strain displayed increased mRNA levels of mdtA, mdtB, and mdtC and an increase in MICs of tigecycline. The complemented strain of sRNA-120 restored the mRNA levels of these genes and the susceptibility to tigecycline. RNA antisense purification and parallel reaction monitoring mass spectrometry were performed to verify the interactions between sRNA-120 and mdtABC. Collectively, our study highlights that the post-transcriptional repression of mdtABC through sRNA-120 may provide an additional layer of efflux pump gene expression control, which is important for resistance and heteroresistance in clinical K. pneumoniae isolates.

Issues with the current drugs for Mycobacterium tuberculosis cure and potential of cell envelope proteins for new drug discovery

Mycobacterium tuberculosis has been the smartest pathogen ever and a challenge to drug development. Its replicative machinery is unique, so targeting the same for killing the pathogen remains a challenge. Our body typically throws out the drugs before they see the bacterium multiply. The pathogen has also learned how to remove drugs from its internal chambers and not allow them to reach their targets. Another strategy for Mtb is the mutation of the targets to reject drug binding and bypass the drug's inhibitory actions. In this review, we tried to explore possible targets on the outer side of the bacterial cell. We have also explored if those targets are promising enough and if there are drugs or inhibitors available. We also discuss the essential proteins and why they remain to be a good target. We concluded that the cell envelope has got a few proteins that can be targeted in isolation or maybe along with other machinery while making the outer environment more conducive for penetration of current drugs or newly proposed drugs.

Unravelling the flexibility of Mycobacterium tuberculosis: an escape way for the bacilli

The persistence of Mycobacterium tuberculosis makes it difficult to eradicate the associated infection from the host. The flexible nature of mycobacteria and their ability to adapt to adverse host conditions give rise to different drug-tolerant phenotypes. Granuloma formation restricts nutrient supply, limits oxygen availability and exposes bacteria to a low pH environment, resulting in non-replicating bacteria. These non-replicating mycobacteria, which need high doses and long exposure to anti-tubercular drugs, are the root cause of lengthy chemotherapy. Novel strategies, which are effective against non-replicating mycobacteria, need to be adopted to shorten tuberculosis treatment. This not only will reduce the treatment time but also will help prevent the emergence of multi-drug-resistant strains of mycobacteria.

Antitumor effect of melatonin on breast cancer in experimental models: A systematic review

Breast cancer is the most frequent malignant neoplasm in females. While conventional treatments such as chemotherapy and radiotherapy are available, they are highly invasive and toxic to oncological patients. Melatonin is a promising molecule for the treatment of breast cancer with antitumor effects on tumorigenesis and tumor progression. The aim of this systematic review was to synthesize knowledge about the antitumor effect of melatonin on breast cancer in experimental models and propose the main mechanisms of action already described in relation to the processes regulated by melatonin. PubMed, Web of Science, and Embase databases were used. The inclusion criteria were in vitro and in vivo experimental studies that used different formulations of melatonin as a treatment for breast cancer, without year or language restrictions. Risk of bias for studies was assessed using the Systematic Review Center for Laboratory Animal Experimentation (SYRCLE) tool. Data from selected articles were presented as narrative descriptions and tables. Seventy-five articles on different breast cancer cell lines and experimental models treated with melatonin alone, or in combination with other compounds were included. Melatonin showed antitumor effects on proliferative pathways related to the cell cycle and tumorigenesis, tumor death, angiogenesis, and tumor metastasis, as well as on oxidative stress and immune regulatory pathways. These effects were either dependent or independent of melatonin receptors. Herein, we clarify the antitumor action of melatonin on different tumorigenic processes in breast cancer in experimental models. Systematic review registration: PROSPERO database (CRD42022309822/https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42022309822).

The Acinetobacter baumannii model can explain the role of small non-coding RNAs as potential mediators of host-pathogen interactions

Bacterial small RNAs (sRNAs) research has accelerated over the past decade, boosted by advances in RNA-seq technologies and methodologies for capturing both protein-RNA and RNA-RNA interactions. The emerging picture is that these regulatory sRNAs play important roles in controlling complex physiological processes and are required to survive the antimicrobial challenge. In recent years, the RNA content of OMVs/EVs has also gained increasing attention, particularly in the context of infection. Secreted RNAs from several bacterial pathogens have been characterized but the exact mechanisms promoting pathogenicity remain elusive. In this review, we briefly discuss how secreted sRNAs interact with targets in infected cells, thus representing a novel perspective of host cell manipulation during bacterial infection. During the last decade, Acinetobacter baumannii became clinically relevant emerging pathogens responsible for nosocomial and community-acquired infections. Therefore, we also summarize recent findings of regulation by sRNAs in A. baumannii and discuss how this emerging bacterium utilizes many of these sRNAs to adapt to its niche and become successful human pathogen.

Functions of Small Non-Coding RNAs in Salmonella–Host Interactions

Simple Summary

In the process of infecting the host, Salmonella senses and adapts to the environment within the host, breaks through the host’s defense system, and survives and multiplies in the host cell. As a class of universal regulators encoded in intergenic space, an increasing number of small non-coding RNAs (sRNAs) have been found to be involved in a series of processes during Salmonella infection, and they play an important role in interactions with the host cell. In this review, we discuss how sRNAs help Salmonella resist acidic environmental stress by regulating acid resistance genes and modulate adhesion and invasion to non-phagocytic cells by regulating virulent genes such as fimbrial subunits and outer membrane proteins. In addition, sRNAs help Salmonella adapt to oxidative stress within host cells and promote survival within macrophages. Although the function of a variety of sRNAs has been studied during host–Salmonella interactions, many of sRNAs’ functions remain to be discovered.

Abstract

Salmonella species infect hosts by entering phagocytic and non-phagocytic cells, causing diverse disease symptoms, such as fever, gastroenteritis, and even death. Therefore, Salmonella has attracted much attention. Many factors are involved in pathogenesis, for example, the capsule, enterotoxins, Salmonella pathogenicity islands (SPIs), and corresponding regulators. These factors are all traditional proteins associated with virulence and regulation. Recently, small non-coding RNAs (sRNAs) have also been reported to function as critical regulators. Salmonella has become a model organism for studying sRNAs. sRNAs regulate gene expression by imperfect base-pairing with targets at the post-transcriptional level. sRNAs are involved in diverse biological processes, such as virulence, substance metabolism, and adaptation to stress environments. Although some studies have reported the crucial roles of sRNAs in regulating host–pathogen interactions, the function of sRNAs in host–Salmonella interactions has rarely been reviewed. Here, we review the functions of sRNAs during the infection of host cells by Salmonella, aiming to deepen our understanding of sRNA functions and the pathogenic mechanism of Salmonella.

Carvone and its pharmacological activities: A systematic review

Natural products from plants have gained prominence in the search for therapeutic alternatives. Monoterpenes, such as carvone, are suggested as candidates for the treatment of several diseases. Therefore, the objective of this study is to review the pharmacological activities of carvone in experimental models in vitro and in vivo. For this, the searches were carried out in May 2020 (upgraded in July 2021) in the databases of PubMed, Web of Science and Scopus and gathered studies on the pharmacological activities of carvone. Two independent reviewers performed the selection of articles using the Rayyan application, extracted the relevant data and assessed the methodological quality of the selected studies using Syrcle's risk of bias tool. Ninety-one articles were selected that described 10 pharmacological activities of carvone, such as antimicrobial, antispasmodic, anti-inflammatory, antioxidant, antinociceptive, anticonvulsant, among others. The evaluation of the methodological quality presented an uncertain risk of bias for most studies. In light of that, carvone stands out as a viable and promising alternative in the treatment of several pathological conditions. However, carrying out studies to evaluate possible mechanisms of action and the safety of this monoterpene is recommended.

The Application of Nucleic Acids and Nucleic Acid Materials in Antimicrobial Research

Due to the misuse of antibiotics, multiple drug-resistant pathogenic bacteria have increasingly emerged. This has increased the difficulty of treatment as these bacteria directly affect public health by diminishing the potency of existing antibiotics. Developing alternative therapeutic strategies is the urgent need to reduce the mortality and morbidity related to drug-resistant bacterial infections. In the past 10 to 20 years, nanomedicines have been widely studied and applied as an antibacterial agent. They have become a novel tool for fighting resistant bacteria. The most common innovative substances, metal and metal oxide nanoparticles (NPs), have been widely reported. Until recently, DNA nanostructures were used alone or functionalized with specific DNA sequences by many scholars for antimicrobial purposes which were alternatively selected as therapy for severe bacterial infections. These are a potential candidate for treatments and have a considerable role in killing antibiotic-resistant bacteria. This review involves the dimensions of multidrug resistance and the mechanism of bacteria developing drug resistance. The importance of this article is that we summarized the current study of nano-materials based on nucleic acids in antimicrobial use. Meanwhile, the current progress and the present obstacles for their antibacterial and therapeutic use and special function of stem cells in this field are also discussed.

Antitumor Effects of Carvacrol and Thymol: A Systematic Review

Background: It is estimated that one in five people worldwide faces a diagnosis of a malignant neoplasm during their lifetime. Carvacrol and its isomer, thymol, are natural compounds that act against several diseases, including cancer. Thus, this systematic review aimed to examine and synthesize the knowledge on the antitumor effects of carvacrol and thymol.

Methods: A systematic literature search was carried out in the PubMed, Web of Science, Scopus and Lilacs databases in April 2020 (updated in March 2021) based on the PRISMA 2020 guidelines. The following combination of health descriptors, MeSH terms and their synonyms were used: carvacrol, thymol, antitumor, antineoplastic, anticancer, cytotoxicity, apoptosis, cell proliferation, in vitro and in vivo. To assess the risk of bias in in vivo studies, the SYRCLE Risk of Bias tool was used, and for in vitro studies, a modified version was used.

Results: A total of 1,170 records were identified, with 77 meeting the established criteria. The studies were published between 2003 and 2021, with 69 being in vitro and 10 in vivo. Forty-three used carvacrol, 19 thymol, and 15 studies tested both monoterpenes. It was attested that carvacrol and thymol induced apoptosis, cytotoxicity, cell cycle arrest, antimetastatic activity, and also displayed different antiproliferative effects and inhibition of signaling pathways (MAPKs and PI3K/AKT/mTOR).

Conclusions: Carvacrol and thymol exhibited antitumor and antiproliferative activity through several signaling pathways. In vitro, carvacrol appears to be more potent than thymol. However, further in vivo studies with robust methodology are required to define a standard and safe dose, determine their toxic or side effects, and clarify its exact mechanisms of action.

This systematic review was registered in the PROSPERO database (CRD42020176736) and the protocol is available at https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=176736.

Amino acid-derived defense metabolites from plants: A potential source to facilitate novel antimicrobial development

For millennia, humanity has relied on plants for its medicines, and modern pharmacology continues to reexamine and mine plant metabolites for novel compounds and to guide improvements in biological activity, bioavailability, and chemical stability. The critical problem of antibiotic resistance and increasing exposure to viral and parasitic diseases has spurred renewed interest into drug treatments for infectious diseases. In this context, an urgent revival of natural product discovery is globally underway with special attention directed toward the numerous and chemically diverse plant defensive compounds such as phytoalexins and phytoanticipins that combat herbivores, microbial pathogens, or competing plants. Moreover, advancements in “omics,” chemistry, and heterologous expression systems have facilitated the purification and characterization of plant metabolites and the identification of possible therapeutic targets. In this review, we describe several important amino acid–derived classes of plant defensive compounds, including antimicrobial peptides (e.g., defensins, thionins, and knottins), alkaloids, nonproteogenic amino acids, and phenylpropanoids as potential drug leads, examining their mechanisms of action, therapeutic targets, and structure–function relationships. Given their potent antibacterial, antifungal, antiparasitic, and antiviral properties, which can be superior to existing drugs, phytoalexins and phytoanticipins are an excellent resource to facilitate the rational design and development of antimicrobial drugs.

Mechanisms of Drug-Induced Tolerance in Mycobacterium tuberculosis

Successful treatment of tuberculosis (TB) can be hampered by Mycobacterium tuberculosis populations that are temporarily able to survive antibiotic pressure in the absence of drug resistance-conferring mutations, a phenomenon termed drug tolerance. We summarize findings on M. tuberculosis tolerance published in the past 20 years. Key M. tuberculosis responses to drug pressure are reduced growth rates, metabolic shifting, and the promotion of efflux pump activity. Metabolic shifts upon drug pressure mainly occur in M. tuberculosis's lipid metabolism and redox homeostasis, with reduced tricarboxylic acid cycle activity in favor of lipid anabolism. Increased lipid anabolism plays a role in cell wall thickening, which reduces sensitivity to most TB drugs. In addition to these general mechanisms, drug-specific mechanisms have been described. Upon isoniazid exposure, M. tuberculosis reprograms several pathways associated with mycolic acid biosynthesis. Upon rifampicin exposure, M. tuberculosis upregulates the expression of its drug target rpoB Upon bedaquiline exposure, ATP synthesis is stimulated, and the transcription factors Rv0324 and Rv0880 are activated. A better understanding of M. tuberculosis's responses to drug pressure will be important for the development of novel agents that prevent the development of drug tolerance following treatment initiation. Such agents could then contribute to novel TB treatment-shortening strategies.

RNA-Sequencing Analyses of Small Bacterial RNAs and their Emergence as Virulence Factors in Host-Pathogen Interactions

Proteins have long been considered to be the most prominent factors regulating so-called invasive genes involved in host-pathogen interactions. The possible role of small non-coding RNAs (sRNAs), either intracellular, secreted or packaged in outer membrane vesicles (OMVs), remained unclear until recently. The advent of high-throughput RNA-sequencing (RNA-seq) techniques has accelerated sRNA discovery. RNA-seq radically changed the paradigm on bacterial virulence and pathogenicity to the point that sRNAs are emerging as an important, distinct class of virulence factors in both gram-positive and gram-negative bacteria. The potential of OMVs, as protectors and carriers of these functional, gene regulatory sRNAs between cells, has also provided an additional layer of complexity to the dynamic host-pathogen relationship. Using a non-exhaustive approach and through examples, this review aims to discuss the involvement of sRNAs, either free or loaded in OMVs, in the mechanisms of virulence and pathogenicity during bacterial infection. We provide a brief overview of sRNA origin and importance, and describe the classical and more recent methods of identification that have enabled their discovery, with an emphasis on the theoretical lower limit of RNA sizes considered for RNA sequencing and bioinformatics analyses.

Antimicrobial resistance in mollicutes: known and newly emerging mechanisms

This review is devoted to the mechanisms of antibiotic resistance in mollicutes (class Bacilli, subclass Mollicutes), the smallest self-replicating bacteria, that can cause diseases in plants, animals and humans, and also contaminate cell cultures and vaccine preparations. Research in this area has been mainly based on the ubiquitous mollicute and the main contaminant of cell cultures, Acholeplasma laidlawii. The omics technologies applied to this and other bacteria have yielded a complex picture of responses to antimicrobials, including their removal from the cell, the acquisition of antibiotic resistance genes and mutations that potentially allow global reprogramming of many cellular processes. This review provides a brief summary of well-known resistance mechanisms that have been demonstrated in several mollicutes species and, in more detail, novel mechanisms revealed in A. laidlawii, including the least explored vesicle-mediated transfer of short RNAs with a regulatory potency. We hope that this review highlights new avenues for further studies on antimicrobial resistance in these bacteria for both a basic science and an application perspective of infection control and management in clinical and research/production settings.

Cardiolipin-Based Lipopolyplex Platform for the Delivery of Diverse Nucleic Acids into Gram-Negative Bacteria

Antibiotic resistance is a growing public health concern. Because only a few novel classes of antibiotics have been developed in the last 40 years, such as the class of oxazolidinones, new antibacterial strategies are urgently needed [1]. Nucleic acid-based antibiotics are a new type of antimicrobials. However, free nucleic acids cannot spontaneously cross the bacterial cell wall and membrane;consequently, their intracellular delivery into bacteria needs to be assisted. Here, we introduce an original lipopolyplex system named liposome polymer nucleic acid (LPN), capable of versatile nucleic acid delivery into bacteria. We characterized LPN formed with significant therapeutic nucleic acids: 11 nt antisense single-stranded (ss) DNA and double-stranded (ds) DNA of 15 and 95 base pairs (bp), 9 kbp plasmid DNA (pDNA), and 1,000 nt ssRNA. All these complexes were efficiently internalized by two different bacterial species, i.e., Escherichia coli and Pseudomonas aeruginosa, as shown by flow cytometry. Consistent with intracellular delivery, LPN prepared with an antisense oligonucleotide and directed against an essential gene, induced specific and important bacterial growth inhibition likely leading to a bactericidal effect. Our findings indicate that LPN is a versatile platform for efficient delivery of diverse nucleic acids into Gram-negative bacteria.

Discovery of Small-Molecule Antibiotics against a Unique tRNA-Mediated Regulation of Transcription in Gram-Positive Bacteria

The emergence of multidrug-resistant bacteria necessitates the identification of unique targets of intervention and compounds that inhibit their function. Gram-positive bacteria use a well-conserved tRNA-responsive transcriptional regulatory element in mRNAs, known as the T-box, to regulate the transcription of multiple operons that control amino acid metabolism. T-box regulatory elements are found only in the 5'-untranslated region (UTR) of mRNAs of Gram-positive bacteria, not Gram-negative bacteria or the human host. Using the structure of the 5'UTR sequence of the Bacillus subtilis tyrosyl-tRNA synthetase mRNA T-box as a model, in silico docking of 305 000 small compounds initially yielded 700 as potential binders that could inhibit the binding of the tRNA ligand. A single family of compounds inhibited the growth of Gram-positive bacteria, but not Gram-negative bacteria, including drug-resistant clinical isolates at minimum inhibitory concentrations (MIC 16-64 μg mL^-1 ). Resistance developed at an extremely low mutational frequency (1.21×10^-10 ). At 4 μg mL^-1 , the parent compound PKZ18 significantly inhibited in vivo transcription of glycyl-tRNA synthetase mRNA. PKZ18 also inhibited in vivo translation of the S. aureus threonyl-tRNA synthetase protein. PKZ18 bound to the Specifier Loop in vitro (K_d ≈24 μm). Its core chemistry necessary for antibacterial activity has been identified. These findings support the T-box regulatory mechanism as a new target for antibiotic discovery that may impede the emergence of resistance.

Insights Into Non-coding RNAs as Novel Antimicrobial Drugs

Multidrug resistant bacteria are a serious worldwide problem, especially carbapenem-resistant Enterobacteriaceae (such as Klebsiella pneumoniae and Escherichia coli), Acinetobacter baumannii and Pseudomonas aeruginosa. Since the emergence of extensive and pan-drug resistant bacteria there are few antibiotics left to treat patients, thus novel RNA-based strategies are being considered. Here, we examine the current situation of different non-coding RNAs found in bacteria as well as their function and potential application as antimicrobial agents. Furthermore, we discuss the factors that may contribute in the efficient development of RNA-based drugs, the limitations for their implementation and the use of nanocarriers for delivery.

Drug Resistance and Gene Transfer Mechanisms in Respiratory/Oral Bacteria

Growing evidence suggests the existence of new antibiotic resistance mechanisms. Recent studies have revealed that quorum-quenching enzymes, such as MacQ, are involved in both antibiotic resistance and cell-cell communication. Furthermore, some small bacterial regulatory RNAs, classified into RNA attenuators and small RNAs, modulate the expression of resistance genes. For example, small RNA sprX, can shape bacterial resistance to glycopeptide antibiotics via specific downregulation of protein SpoVG. Moreover, some bacterial lipocalins capture antibiotics in the extracellular space, contributing to severe multidrug resistance. But this defense mechanism may be influenced by Agr-regulated toxins and liposoluble vitamins. Outer membrane porin proteins and efflux pumps can influence intracellular concentrations of antibiotics. Alterations in target enzymes or antibiotics prevent binding to targets, which act to confer high levels of resistance in respiratory/oral bacteria. As described recently, horizontal gene transfer, including conjugation, transduction and transformation, is common in respiratory/oral microflora. Many conjugative transposons and plasmids discovered to date encode antibiotic resistance proteins and can be transferred from donor bacteria to transient recipient bacteria. New classes of mobile genetic elements are also being identified. For example, nucleic acids that circulate in the bloodstream (circulating nucleic acids) can integrate into the host cell genome by up-regulation of DNA damage and repair pathways. With multidrug resistant bacteria on the rise, new drugs have been developed to combate bacterial antibiotic resistance, such as innate defense regulators, reactive oxygen species and microbial volatile compounds. This review summaries various aspects and mechanisms of antibiotic resistance in the respiratory/oral microbiota. A better understanding of these mechanisms will facilitate minimization of the emergence of antibiotic resistance.