Bacterial Conjugation

Bacterial conjugation is when DNA is introduced into bacteria through direct contact. This is one of the most extensive gene transfers between bacteria, and it is responsible for the spread and diversification of genes, particularly antimicrobial resistance genes. Bacterial conjugating is not only important in nature, but also widely studied and applied in genetic engineering. In this article, we will study the definitions, types, methods, mechanisms, and real-world case studies of bacterial conjugating.

What is Bacterial Conjugation?

Bacterial conjugation means that DNA (most commonly, plasmids) is transferred from one bacteria to another by direct interaction between the bacteria and a molecular chain reaction. It is similar to the "switching of genes", except that bacterial conjugation (unlike viral transmission or natural mutation) is based on physical contact between bacteria and gene transfer.

The benefit of bacterial conjugation is that it facilitates bacteria in acquiring new genes, particularly antimicrobial resistance gene to adapt quickly to changes in the environment. This process is critical to bacterial evolution and flexibility and one of the main causes of genetic variation in bacteria.

Fig.1 Illustration of bacterial conjugation. (Brolund, Alma., 2014)

Table.1 Custom Bioconjugation Services at BOC Sciences.

Bacterial Conjugation Mechanism

There are generally four types of bacterial conjugation, depending on the kind of genetic content to be added and the form of delivery:

Plasmid Conjugation: Plasmid is a small circular DNA molecule inside the bacteria cell, usually it contains some unnecessarily generated genes like antimicrobial resistance genes, heavy metal tolerance genes, etc. Plasmid expressing bacteria (donor bacteria) transmit the plasmid to absent bacteria (recipient bacteria). One of the most popular forms of conjugation is plasmid conjugation, where bacteria quickly pick up new properties.

Chromosome Conjugation: Chromosome conjugation is very complicated, where bacterial chromosomes are transferred. Sometimes, an atom of the bacterium's chromosome is transferred from the donor bacteria to the recipient bacteria, and sometimes it needs special apparatus to do so. The F factor (Fertility factor) for instance, is a key chromosome conjugation mediator that can encourage an end of the chromosome to pass into the receptive bacteria.

Antibiotic Resistance Gene Conjugation: Transmission of resistance genes is one of the most important activities in bacterial conjugation. By conjugating bacteria can transmit antimicrobial resistance genes from one bacterium to another, expanding antimicrobial resistant strains. This conjugation is clinically very troubling as it can lead to the development of antibiotic resistance issues.

Conjugation of Transposon: Transposons are DNA molecules that can pass through the genome and when they are conjugated, they can be transmitted from the donor to the recipient bacteria. Transposons may encode antimicrobial resistance genes or other genes of relevance, so its propagation is critical for the flexibility and survival of bacteria.

Table.2 Microbial-related Conjugation Services at BOC Sciences.

Types of Bacterial Conjugation

Bacterial conjugation is a complex and highly coordinated process that can generally be divided into the following stages:

Preparation of the donor bacteria: At the beginning of conjugation, the donor bacteria need to have a vector capable of providing genetic material, usually a plasmid or a specific factor on a chromosome. For example, in F-factor-mediated plasmid conjugation, the donor bacterium carries the F plasmid, which is a genetic factor that allows bacteria to conjugate. The donor bacteria synthesize the pilus for contact with the recipient bacteria through a specific gene on the F factor.

Donor-Recipient Contact: The donor bacteria come into contact with the recipient bacteria through contact filaments on their surface. These contact filaments can stretch, connect, and form a tight bridge between the donor and recipient bacteria. Contact filaments not only help bacteria get closer, but may also involve the exchange of signals so that gene transfer between bacteria can proceed smoothly.

Transmission of genetic material: Once a connection is established between the donor and recipient bacteria, the genetic material of the donor bacteria (usually a plasmid) begins to transfer to the recipient bacteria via a "sexual bridge". The plasmid replicates itself through a mechanism called rolling circle replication and delivers the replicated single-stranded DNA to the recipient bacteria. Recipient bacteria integrate this genetic material into their own genomes by synthesizing complementary strands.

Completion of conjugation: Once the genetic material has been successfully transferred into the recipient bacteria, the donor bacteria and the recipient bacteria each resynthesize the intact plasmid or chromosome. At this time, the recipient bacterium has acquired new genetic information and is able to inherit certain characteristics of the donor bacterium. After this process, both the donor and recipient bacteria become bacteria containing plasmids or associated genes that can continue to be conjugated or otherwise genetically transmitted.

Process of Conjugation in Bacteria

The mechanism of bacterial conjugation involves the synergistic action of multiple molecules and structures, mainly including the following aspects:

F-factor and contact filaments: In plasmid conjugation, F-factor is one of the most important. Factor F is usually located on bacterial plasmids and is responsible for regulating the occurrence of conjugation. There are multiple genes encoding different proteins on the F factor, the most critical of which are the pilA, pilB, and tra genes, which are involved in the formation of contact filaments and the transfer of genetic material.

Pilus: A contact filament is a protein structure on the surface of a donor bacterium that is able to bind to the surface of the recipient bacterium and form a direct channel between the bacteria. Through this channel, the donor bacteria can pass the genetic material to the recipient bacteria.

Rolling Circle Replication: During plasmid conjugation, the plasmid on the donor bacteria will copy the DNA fragments through the rolling replication mechanism and pass the replicated single-stranded DNA to the recipient bacteria. The recipient bacteria use their own enzyme system to synthesize a supplement strand, resulting in the formation of an intact plasmid.

Transmission of resistance genes: In resistance gene conjugation, resistance genes are transmitted through plasmids or transposons, resulting in the recipient bacteria gaining resistance. This transmission mechanism has an important impact on the spread of antibiotic-resistant bacteria, especially in healthcare settings, and poses a significant challenge to antibiotic resistance.

Conjugation in Bacteria Example

A classic example of bacterial conjugation is factor F conjugation in Escherichia coli. Factor F is a bacterial plasmid that can deliver itself from one E. coli cell to another by conjugation. E. coli that carry the F-factor are called F-positive bacteria, and they are able to carry out gene transfer with F-negative bacteria (bacteria without the F-factor) through contact filaments. The transferred genetic material is usually plasmid DNA, and the F-factor replicates and integrates in the recipient bacteria, causing the recipient bacteria to also become F-positive.

In addition, conjugation of resistance genes is also common in Escherichia coli and other pathogens. Some bacteria transmit antibiotic resistance genes to other bacteria through conjugation, a phenomenon that is particularly pronounced in antibiotic treatment, resulting in the emergence of "superbugs".

Summary

Bacterial conjugation is an important way for genetic material exchange between bacteria, through which bacteria are able to quickly acquire new genes, thereby improving their adaptability and viability. Whether it's plasmid conjugation, chromosomal conjugation, or the transmission of resistance genes, bacterial conjugation plays a crucial role in the evolution of microorganisms and the spread of antimicrobial resistance. With the increasing problem of antimicrobial resistance, studying the bacterial conjugation mechanism is not only helpful to understand the adaptive evolution of bacteria, but also provides an important theoretical basis for the development of new antimicrobial strategies.