Introduction
Any contact between two bacterial cells that translates to the transfer of genetic material constitutes bacterial conjugation (Casali and Preston, 2003). Transfer of the DNA through conjugation falls under horizontal gene transfer (Singh et al., 2013). The genetic information is transferred to the recipient bacterium on a plasmid, and the mechanism and proteins involved vary based on the plasmid type in the donor strain (Casali and Preston, 2003). Application of conjugation can cause transfer of disrupted genes on a self-transmissible plasmid to form a mutant strain.
Conjugation allows for bacterial biodiversity, which is significant in the sense that bacteria can acquire antibiotic resistance hence adapt to hostile environments. According to Bauer, Rösch, Itaya, and Graumann (2011), conjugation is one of the major causes of development of resistant genes in bacteria. For instance, approximately 234 bacterial conjugations have occurred in Salmonella enterica and E. coli over 100 million years. Thus, the potential of E. coli to populate previously inhabitable niches has increased (Casali and Preston, 2003).
Mechanisms of Bacterial Conjugation in Gram-Positive Bacteria
In order to understand the conjugation of gram-positive bacteria, it is vital to have a grasp of the general mechanisms that are involved in the bacterial conjugation process. According to Lujan (2008), the first phase of conjugation involves contact, which entails the physical connection that occurs between the donor bacteria and recipient cells. Secondly, mobilization occurs and this involves the preparation of the conjugative plasmid for transfer. Mobilization will cause the formation of single-stranded circular non-transfer strands and either a linear or a circular single-stranded strand. Once mobilization is done, the linear strand is moved to the recipient cytoplasm strand. Replication then occurs once the transfer of the strand is complete.
According to Srivastvaia (2013), Gram-positive bacteria have self-transmissible plasmids. Some of these include Bacillus, Staphylococcus and Streptomyces. Conjugation in the Gram-positive bacteria E. faecalis does not involve the use of sex pili (that is contact phase). Instead, it utilizes clumping of the plasmid carrying cells and plasmid-free cells. The plasmid carrying cells act as the donors while the plasmid-free cells act as the recipients cells. The conjugative plasmids are categorized into two groups. Group 1 typically involves high transferring plasmids such as pAD1 and pPD1 while group 2 involves plasmids that transfer at low rates like pACI and p5M15346 (Srivastvaia, 2013). The rate of conjugation is controlled by the production of a sex pheromone, which is essential in enhancing cell-to-cell contact (Srivastvaia, 2013). Pheromone-mediated conjugation occurs in certain species of the gram-positive bacteria. According to Singleton (2013), the pheromones are small linear peptides, which are secreted by the potential receipts of certain plasmids. The pheromones encourage the adhesion of potential conjugants.
Srivastvaia (2013) proposes a model to explain the conjugation process in pheromone coordinated conjugation. The plasmid-free recipient provides distinct two chromosomally encoded pheromones, cA and cB. For each of these pheromones, there is a specific donor cell type. The implication is that cA will stimulate conjugation process with a donor that is carrying plasmid pA and cB will stimulate conjugation with donor having the plasmid pB. The donor strains secrete a binding substance; the pheromones cA and cB then synthesize the aggregate substance by the donor cell. Once the pheromone diffuses into the donor cell, it produces responding substance in two forms, RcA and RcB, which correspond to the pheromone and facilitates the synthesis of the aggregate substance. Interaction between the binding substance and aggregate substance causes aggregation between donor and recipient and promotes conjugal transfer. Once the plasmid is received, the recipient cell stops to produce the particular hormone and the synthesis of aggregate substance stops.
References
Bauer, T., Rösch, T., Itaya, M., & Graumann, P. L. (2011). Localization Pattern of Conjugation Machinery in a Gram-Positive Bacterium. Journal of Bacteriology, 193(22), 6244-6256
Casali, N., & Preston, A. (2003). E. coli plasmid vectors: Methods and applications. Totowa, N.J: Humana Press
Lujan, S. (2008). Bacterial Conjugation and Its Inhibition: The Hows and Whys of Conjugation and what Can be Done to Control it. Miami: ProQuest.
Singleton, P. (2013). Dictionary of DNA and genome technology. Chichester, West Sussex: Wiley-Blackwell.
Singh, P. K., Ramachandran, G., Ramos-Ruiz, R., Peiró-Pastor, R., Abia, D., Wu, L. J., & Meijer, W. J. (2013). Mobility of the Native Bacillus subtilis Conjugative Plasmid pLS20 Is Regulated by Intercellular Signaling. Plos Genetics, 9(10), 1-13.
Srivastava, S. (2013). Genetics of bacteria. New Delhi: Springer.
