The emergence and dissemination of antimicrobial resistance (AMR) represent a dynamic biological phenomenon in which microorganisms acquire, preserve, and redistribute genetic determinants that compromise the effectiveness of antimicrobial agents. Rather than occurring solely through the gradual accumulation of mutations within individual microbial lineages, resistance can spread rapidly when genetic information moves between cells. This capacity for genetic exchange allows resistance traits to cross strain, species, and sometimes genus boundaries, creating an important evolutionary advantage for microorganisms exposed to antimicrobial selection pressure.
Resistance genes may be located on bacterial chromosomes or on mobile genetic elements such as plasmids, transposons, integrons, and insertion sequences. Their mobility creates several routes through which resistance determinants can be acquired and subsequently expressed by recipient cells. The principal mechanisms of horizontal gene transfer are conjugation, transformation, and transduction (Figure 1). Conjugation involves direct cell-to-cell transfer of genetic material, frequently mediated by plasmids carrying resistance determinants. Transformation enables competent bacteria to take up free extracellular DNA released from other cells, while transduction relies on bacteriophages to transport bacterial DNA between hosts. These mechanisms differ in their molecular requirements and ecological settings, but collectively provide microorganisms with an efficient means of expanding their genetic repertoire.
The transfer process is further shaped by mobile DNA structures that can capture, rearrange, and redistribute resistance genes. Integrons, for example, can assemble multiple resistance gene cassettes, whereas transposable elements facilitate movement of genetic sequences within and between DNA molecules. Resistance genes may become incorporated into transferable plasmids or other mobile platforms, allowing several resistance determinants to disseminate together. Such genetic associations contribute to the emergence of organisms displaying resistance to multiple antimicrobial classes.
The study of resistance-gene transfer therefore extends beyond identifying individual resistance mechanisms. It requires examination of the molecular pathways that permit genetic information to move, the mobile elements that facilitate this movement, and the environmental conditions that favor successful transmission. Characterizing these mechanisms provides a framework for understanding how resistance emerges and spreads through microbial communities and establishes the biological basis for developing strategies aimed at limiting the propagation of antimicrobial resistance.
Conjugation
Conjugation is a highly efficient mechanism of horizontal gene transfer through which bacteria exchange genetic information by establishing direct physical contact between a donor cell and a recipient cell. Unlike vertical inheritance, in which genetic traits are transmitted from a parent cell to its descendants during cell division, conjugation permits genetic determinants to move between existing bacterial cells. This process has considerable significance in the dissemination of antimicrobial resistance because resistance-associated DNA can be transferred to bacteria that were previously susceptible to one or more antimicrobial agents. Through this mechanism, a resistance trait can move rapidly within a microbial population and, under favorable ecological conditions, extend beyond closely related bacterial lineages.
A central component of conjugative transfer is the plasmid. Plasmid is a circular, extrachromosomal DNA molecule capable of autonomous replication within a suitable bacterial host. Plasmids are not generally essential for the basic survival of their host under ordinary conditions; however, they may carry genes that provide substantial adaptive advantages. These genes can encode antimicrobial resistance, virulence-associated traits, metabolic functions, or other characteristics that improve bacterial fitness under particular environmental pressures. Of particular concern are conjugative and mobilizable plasmids carrying resistance determinants, because their genetic organization enables resistance genes to become mobile within and between bacterial populations.
The conjugation process begins when a donor bacterium containing a transferable plasmid encounters a compatible recipient cell. In many Gram-negative bacteria, recognition and initial attachment involve extracellular filamentous structures known as sex pili or conjugative pili. These structures facilitate contact between the donor and recipient and contribute to the formation of a specialized cell-to-cell connection. The pilus itself should not be viewed simply as a hollow tube through which an intact plasmid passes. Rather, it participates in bringing the cells together, after which a multiprotein type IV secretion system or conjugative transfer apparatus establishes the functional pathway through which DNA is delivered to the recipient.
Once stable contact has been established, the plasmid undergoes a carefully coordinated transfer process. A specialized enzyme complex recognizes a defined region of the transferable DNA, commonly associated with the origin of transfer (oriT). One strand of the plasmid is enzymatically nicked and prepared for movement into the recipient cell. The displaced DNA strand is transferred through the conjugative secretion machinery, while complementary DNA synthesis occurs in both the donor and recipient cells. The donor generally retains a complete copy of the plasmid, while the recipient reconstructs a double-stranded plasmid molecule. Successful completion of this process converts the recipient into a potential new donor when the transferred plasmid contains the genetic machinery required for subsequent conjugative transfer.
Conjugation is controlled by certain type of genes known as the F plasmids. Normally, a donor bacterial cell (containing the F+ plasmid) converts another bacterial cell (known as an F— cell) into an F+ cell that encodes a gene of interest particularly antibiotic resistance gene (Figure 2). The plasmid DNA being transferred is replicated through a type of replication known as the rolling circle replication mechanism. Many plasmids carry genes that confer resistance to antibiotics. When two bacterial cells are in close proximity to each other, a hollow bridge like structure known as the “pilus” forms between the two cells. This allows a copy of the plasmid as it is duplicated to be transferred from one bacterium (the donor cell) to another (the recipient cell). This process called conjugation, which is a plasmid-encoded mechanism of gene transfer, enables a susceptible bacterium to acquire resistance genes for a particular antibiotic.
The importance of conjugation in AMR lies in the ability of plasmids to accumulate and disseminate multiple resistance determinants. A single transferable plasmid may carry genes conferring resistance to several antimicrobial classes, allowing their simultaneous movement between bacterial cells. In addition, resistance genes located on transposons and integrons can become associated with conjugative plasmids, creating highly mobile genetic assemblies. Integrons are particularly important because they can capture and express gene cassettes, including antimicrobial resistance genes, while conjugative plasmids can provide an efficient vehicle for their movement between cells. The interaction between these mobile genetic elements therefore increases the potential for the rapid assembly and dissemination of multidrug-resistance profiles.
Conjugation is not restricted to a single bacterial species or to organisms occupying the same ecological niche. Transfer can occur in diverse environments, including clinical settings, wastewater, agricultural systems, food-associated environments, soil, and the gastrointestinal tract. High bacterial density, close cellular association, antimicrobial selection pressure, and the presence of compatible mobile genetic elements can create conditions favorable for transmission. Importantly, the recipient does not need to develop resistance through mutation before acquiring the trait; it can obtain a pre-existing resistance determinant directly from another bacterium.
The biological consequence is therefore more substantial than the transfer of an isolated DNA molecule. Conjugation can rapidly alter the genetic composition of bacterial communities by introducing resistance determinants into previously susceptible populations. Once acquired, these determinants may be maintained when they provide a selective advantage or when the associated plasmid persists through other mechanisms. This capacity for rapid genetic exchange makes conjugation a major pathway through which antimicrobial resistance can propagate across bacterial populations.
The coordinated activity of donor-recipient recognition, plasmid processing, DNA transport, and replication transforms conjugation into an efficient genetic exchange system. Its capacity to mobilize resistance genes, particularly when plasmids are linked with integrons and transposable elements, provides an important molecular basis for the emergence and expansion of antimicrobial-resistant bacteria.
Transduction
Transduction is a form of horizontal gene transfer in which bacteriophages facilitate the movement of bacterial genetic material from one cell to another. Bacteriophages, commonly referred to as phages, are viruses that specifically infect bacterial cells and exploit their cellular machinery to produce progeny phage particles. Although phages primarily function as infectious agents, their replication cycles can also create opportunities for the accidental capture and redistribution of bacterial genes. When the transferred DNA contains an antimicrobial resistance determinant, transduction can contribute to the acquisition and dissemination of resistance within bacterial populations. This mechanism is particularly significant because genetic material can be transported between bacterial cells without requiring direct cell-to-cell contact as is the case in conjugation.
The process begins when a bacteriophage recognizes and attaches to specific receptors located on the surface of a susceptible bacterial cell. Following adsorption, the phage introduces its nucleic acid into the bacterial cell, while the proteinaceous phage structure generally remains outside. The subsequent events depend on the biological strategy of the phage. In the lytic cycle, phage genetic material redirects the host’s biosynthetic machinery toward the production of phage components. The bacterial chromosome may become fragmented during this process, generating DNA fragments that can occasionally be incorporated into newly assembled phage particles. This incorporation is not normally intentional; rather, it results from errors during phage assembly and packaging.
A newly formed phage particle containing bacterial DNA instead of, or in addition to, the appropriate phage genome can subsequently encounter another susceptible bacterial cell. When this particle attaches to the recipient, it injects the packaged bacterial DNA into that cell. If the transferred DNA contains a functional AMR gene and is maintained or incorporated into the recipient’s genetic material, the recipient may acquire the corresponding resistance phenotype. The newly introduced DNA may undergo homologous recombination with the recipient chromosome when sufficient sequence similarity exists, allowing the resistance determinant to become a stable component of the recipient’s genome. In other circumstances, transferred genetic material may be associated with mobile elements that facilitate its subsequent persistence or movement.
Transduction is broadly classified into generalized and specialized transduction, according to how bacterial genes are captured and transferred. In generalized transduction, phage-mediated packaging errors can result in virtually any region of the bacterial chromosome being incorporated into a phage particle. Different bacterial genes, including resistance determinants, may potentially be transferred during different infection events. The process is therefore relatively unrestricted with respect to the location of the transferred bacterial DNA. Generalized transduction can occur during the lytic replication of certain phages and provides a mechanism through which genetic variation can be distributed among compatible bacterial populations.
Specialized transduction follows a more selective pathway and is associated primarily with temperate bacteriophages, which are capable of establishing a lysogenic relationship with their bacterial hosts. During lysogeny, the phage genome becomes integrated into the bacterial chromosome as a prophage or is maintained in another stable state within the host. When the prophage is subsequently induced to enter a productive replication cycle, an imprecise excision event may remove adjacent bacterial DNA together with the phage genome. The resulting phage particle can therefore carry specific bacterial genes located near the prophage integration site. After infection of another susceptible bacterium, these genes may be introduced into the recipient and potentially become established through recombination. Unlike generalized transduction, specialized transduction is consequently restricted to particular bacterial genes positioned close to the phage integration region.
The contribution of transduction to AMR is influenced by several factors, including phage host range, receptor availability, the presence and location of resistance determinants, and the capacity of transferred DNA to persist in the recipient. Phages may encounter bacterial communities in diverse environments, including clinical settings, wastewater, food-associated ecosystems, soil, and natural aquatic systems. These environments can provide repeated opportunities for interactions among phages, susceptible bacteria, and resistant populations. Phage-mediated gene transfer may function as a genetic bridge connecting bacterial populations that occupy the same ecological niche.
An important feature of transduction is that it does not depend on the physical proximity required for conjugative transfer. A phage particle can act as a biological carrier, protecting bacterial DNA during its passage between host cells and delivering it to a new bacterial recipient. This characteristic allows genetic information to circulate within microbial communities under suitable ecological conditions. Furthermore, resistance genes may occur within broader genetic structures, such as transposons or integrons, meaning that phage-mediated movement of DNA can potentially contribute to the wider redistribution of complex resistance determinants.
Transduction represents a biologically sophisticated route for the horizontal dissemination of AMR genes. Through phage infection, accidental capture of bacterial DNA, packaging, and subsequent delivery into another bacterial cell, resistance determinants can move between members of susceptible bacterial populations. Generalized transduction permits relatively broad transfer of chromosomal regions, whereas specialized transduction favors the movement of genes located near specific phage integration sites. These mechanisms demonstrate how bacteriophages can function not only as bacterial pathogens but also as important agents of bacterial genetic exchange, thereby influencing the evolution and distribution of antimicrobial resistance.
Transformation
Transformation is a naturally occurring mechanism of horizontal gene transfer through which bacteria acquire extracellular DNA from their surrounding environment and incorporate genetic information originating from another cell. Unlike conjugation, which generally requires direct physical contact between donor and recipient cells, transformation does not depend on a living donor cell at the moment of DNA acquisition. Instead, genetic material released into the extracellular environment can become an accessible reservoir of information that may subsequently be captured by competent bacterial cells. Through this process, bacteria can acquire new genetic characteristics without relying exclusively on inheritance from a parental cell.
The process commonly begins when bacterial cells undergo lysis as a consequence of natural senescence, environmental stress, predation, antimicrobial exposure, or other cellular damage. Rupture of the bacterial cell envelope releases intracellular DNA into the surrounding environment. Once liberated, chromosomal DNA and DNA associated with mobile genetic elements may persist for varying periods, depending on environmental conditions such as temperature, pH, nuclease activity, and the physical properties of the surrounding habitat. Consequently, extracellular DNA can function as a transient genetic reservoir within microbial communities, particularly in environments where large numbers of bacteria coexist and cellular turnover is substantial.
For transformation to occur, a recipient bacterium must possess a physiological state known as competence. Competence refers to the ability of a bacterial cell to recognize, bind, and internalize extracellular DNA. This state may occur naturally as part of the organism’s life cycle or may be induced by particular environmental conditions. Competence is not uniformly expressed among bacterial species; rather, it is regulated by species-specific genetic and environmental signals. Changes in nutrient availability, population density, stress conditions, and other ecological cues can influence the development of competence, thereby affecting the likelihood that a bacterial population will acquire environmental DNA.
Following exposure to extracellular DNA, competent cells employ specialized surface-associated proteins to recognize and bind DNA molecules. The DNA is subsequently transported across the cell envelope through a coordinated uptake system. In many bacteria, double-stranded DNA is initially encountered at the cell surface, after which one strand may be degraded while the complementary strand is transported into the cytoplasm. The internalized single-stranded DNA is then protected and processed by cellular proteins that determine whether it can be maintained, recombined, or degraded. This sequence of events makes transformation a selective biological process rather than a simple passive entry of DNA into the cell.
For a newly acquired resistance determinant to become a stable characteristic of the recipient bacterium, the incoming DNA must be successfully maintained or incorporated into the recipient’s genetic architecture. When the acquired sequence shares sufficient homology with a region of the bacterial chromosome, homologous recombination can enable its integration into the recipient genome. In this process, the incoming DNA aligns with a corresponding chromosomal sequence and replaces or modifies the existing genetic region. If the acquired DNA contains a functional resistance determinant, the resulting genetic alteration may provide the recipient with the capacity to withstand exposure to a particular antimicrobial compound.
Transformation can also contribute to the dissemination of resistance when extracellular DNA originates from bacteria carrying multiple advantageous genetic traits. DNA fragments released from lysed cells may contain resistance genes, altered target genes, regulatory sequences, or other determinants associated with antimicrobial survival. If such sequences are successfully acquired and expressed by a competent recipient, the phenotype of the recipient population can change without requiring direct contact with the original donor. The ecological significance of this process is particularly evident in densely populated microbial environments, where extracellular DNA may accumulate and encounter numerous potential recipients.
The fate of extracellular resistance DNA is nevertheless influenced by environmental stability and biological degradation. Extracellular nucleases can fragment DNA, while unfavorable physicochemical conditions may reduce its persistence or biological availability. Biofilms and other structured microbial communities can create localized environments in which extracellular DNA is retained near bacterial cells, potentially increasing opportunities for uptake. The extracellular polymeric matrix associated with biofilms may therefore provide a physical setting that favors interactions between released DNA and competent microorganisms.
Transformation does not necessarily result in successful resistance acquisition. Uptake alone is insufficient; the DNA must remain biologically functional, reach the appropriate intracellular location, undergo successful genetic processing, and ultimately be expressed in a manner that produces a meaningful phenotype. Selection pressure can subsequently influence the persistence of transformed cells. When antimicrobial exposure eliminates susceptible bacteria while allowing resistant transformants to survive and reproduce, the acquired resistance determinant may become increasingly represented within the population.
Transformation consequently represents an important route through which genetic information can move across bacterial populations without direct donor-recipient contact. By converting environmental DNA released from cellular debris into a potential source of heritable variation, this mechanism expands the genetic flexibility of bacterial communities. In the context of AMR, the process provides a pathway through which resistance determinants can move from previously resistant populations into susceptible or less-resistant bacteria, thereby contributing to the broader ecological circulation and persistence of resistance genes.
The transfer of AMR genes is a fundamental biological process that enables bacteria to acquire and disseminate genetic traits beyond conventional vertical inheritance. Through transformation, conjugation, and transduction, resistance determinants can move between bacterial cells and become established within new microbial populations. The involvement of mobile genetic elements, including plasmids, transposons, integrons, and insertion sequences, further increases the mobility and persistence of these determinants, allowing multiple resistance traits to accumulate and circulate within microbial communities.
These mechanisms demonstrate that AMR is not solely the consequence of spontaneous genetic mutation but can also arise from extensive genetic exchange within and between bacterial populations. Environmental conditions, microbial density, selective pressure, and the availability of extracellular or mobile DNA can influence the frequency and success of gene transfer. Thus, environments containing substantial antimicrobial exposure may favor the survival and expansion of organisms carrying transferable resistance determinants. Limiting the spread of AMR requires attention not only to resistant organisms themselves but also to the genetic pathways that enable resistance traits to move, persist, and become established across bacterial populations.
Transfection
Transfection is a molecular biology technique involving the introduction of nucleic acids, such as DNA or RNA, into a cell. The term is most commonly used to describe the artificial introduction of genetic material into eukaryotic cells, although related processes can occur in prokaryotic systems. In eukaryotic cells, transfection is commonly performed to study gene expression, protein production, gene function, or cellular responses to particular genetic sequences. Depending on the method used, nucleic acids may be introduced using chemical reagents, lipid-based carriers, electroporation, or physical methods.
Transfection should be distinguished from transformation, which traditionally refers to the uptake of naked DNA by bacterial cells. For example, bacteria can be transformed with recombinant plasmid DNA containing a gene of interest and subsequently cultured to produce large quantities of the recombinant protein. Viral-mediated introduction of genetic material into cells is more appropriately termed transduction, rather than transfection. Therefore, transfection, transformation, and transduction describe different mechanisms of introducing genetic material into cells.
In relation to AMR, the transfer of resistance genes between bacteria is primarily associated with horizontal gene transfer (HGT). HGT enables bacteria to acquire genetic traits, including antimicrobial resistance, from other bacteria rather than inheriting them solely from parent cells. As aforesaid, the three major mechanisms of HGT are conjugation, transformation, and transduction.
Transfection differs from these natural resistance-gene transfer mechanisms because it is generally an artificial laboratory technique rather than a naturally occurring bacterial process. It introduces nucleic acids into cells using experimentally controlled methods and is primarily used for research or biotechnology. In contrast, conjugation, transformation, and transduction are natural biological mechanisms through which bacteria can acquire and disseminate antimicrobial resistance genes. Thus, while transfection can experimentally introduce resistance-associated genes into cells, it should not be regarded as one of the principal natural mechanisms responsible for the spread of antimicrobial resistance among bacterial populations.
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