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Streptococcus faecalis

1. Streptococcal chains are generally shorter in the recipient cultures than in donor cultures.

2. Light microscopy studies indicates the pairs remain in contact for approximately 15 minutes.

3. Since it reaches 20% - 25% after 2 hours of conjugal mating, the frequency of intracellular connection between streptococcal chain experience and elevation and statisfies the reasonable approximation of that predicted from the genetic studies.

4. It also results with the HFR.

5. When studying the conjugal mating mixture, we observe rapidly oscillating pairs of streptococcal chains.

6. There was an obvious qualitative difference between the motion observed in the mating mixture of S. faecalis and these in control.

7. Similarly, the number of rapidly oscillating pairs of streptococcal chains was significantly greater in the mating mixture than in either donor or recipient control.

8. The studies indicates that the conjugal transfer of plasmid between certain strains of enterococci is associated with direct intercellular connections between streptococcal chains in the mating mixture.

9. It also clarifies that conjugation does not only relies on neither pili nor fibria present in the donor and recipient strains but they are associated in the conjugal transfer via wall to wall contact.

10. The frequency of conjugal transfer in S. faecalis is reported was one of the highest known among gram positive organisms.

11. Some strains of S.faecalis excrete pheromone-like compounds that can stimulate mating with donor cells.

12. These pheromones are small hydrophobic peptides (7-8 amino acids) that produced by recipient cells and expression of tra gene in the plasmids of neighbouring  bacteria, there by inducing aggregation and mating.

13. Plasmid pAD1 from Streptococcus faecalis is the best studied  pheromone-induced plasmid.

14. About half of its genome (30kb) is devoted to conjugation and related functions whose expression is induced by the pheromone cAD1.

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This mechanism requires the presence of a special plasmid called the F plasmid. Plasmids are small, circular pieces of DNA that are separate and replicate indepentently. The F plasmid, for example, facilites conjugation. Some plasmids can integrate reversibly into the bacterial chromosome. An integrated plasmid is called an episome.

Bacteria that have a F plasmid are referred to as as F⁺ or male while those that do not have an F plasmid are F⁻ of female. The F plasmid consists of 25 genes that mostly code for production of sex pilli. A conjugation event occurs when the male cell extends his sex pili and one attaches to the female.

This attached pilus is a temporary cytoplasmic bridge through which a replicating F plasmid is transferred from the male to the female. When transfer is complete, the result is two male cells.

The F plasmid can behave as an episome. When the F⁺ plasmid is integrated within the bacterial chromosome, the cell is called an HFR cell (high frequency of recombination cell).

The HFR cell still behaves as a F⁺ cell, transferring F genes to a F- cell, but now it can take some of the bacterial chromosome with it. A cross over event can occur between homologous genes on the HFR fragment and the F⁻ DNA. Pieces of DNA not recombined will be degraded or lost in cell division. Thus, the recombinant genome can be passed on to future generations.

The genes required for transfer are called tra genes.

tra genes can be divided into two components

-Dtr component (DNA Transfer and Replication)
-Mpf component (Mating-pair formation).

Mpf system includes :

-The pilus
-The channel
-Coupling proteins

The Dtr is involved in preparing the plasmid DNA for transfer. A number of protein make up this component  such as Relaxase and Primase.

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1. Gram positive donor bacteria produce adhesins that cause them to aggregate with recipient cells.

2. Conjugal transfer of a plasmid in gram positive bacteria was associated with direct intercellular connections between chains of streptococci in the absence of either pili/fimbriae.

3. Plamids such as plasmids that determining antibiotics resistance in gram positive are transferred during conjugal transfer from donor to recepient strains.

4. Transformation is excluded because DNAse did not decrease the frequency of transfer.

5. Plasmids are not the only DNA elements in bacteria that are capable of transferring themselves by conjugation, also some intergrating elements of gram positive bacteria are self transmissible (conjugative transposone). they can excise and transfer themselves into other cells eventhough they cannot replicate.

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F+ x F- crosses

1. Gram negative donor bacteria produce bridge-like pilus that cause them to attaches to recipient cell and brings the two cells together.

2. The plasmid is nicked at a specific site, replicated by a rolling circle mechanism,  a single strand of DNA passes through the conjugation bridge to the recipient.

Hfr x F- crosses

1. Formation of conjugation bridge.

2. The F factor that intergrated with the chromosomal DNA is transferred after the entire chromosome is transferred then the F factor will be transferred to the recipient.

F' x F- crosses

1. Formation of conjugation bridge.

2. Intergrated F plasmid that leaves the chromosome by homologous recombination. it will take a short genetic sequence from the chromosome.

3. The intergrated F plasmid is transferred to the recipient.

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                                                                   Conjugation

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                                                                   Conjugation

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Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells.

In 1946, a simple experimental work of Joshua Lederberg and Edward Tatum studied two strains of Escherichia coli with different nutritional requirements.

It could be suggested that the cells of the two strains do not really exchange genes but instead leak substances that the other cells can absorb and use for growing. This possibility of “cross feeding” was ruled out by Bernard Davis. He constructed a U-tube in which the two arms were separated by a fine filter. The pores of the filter were too small to allow bacteria to pass through but large enough to allow easy passage of the fluid medium and any dissolved substances.

(sources : https://www.ncbi.nlm.nih.gov/books/NBK21942/)