Question

Horizontal Gene Transfer

Q:1. A strain of living Streptococcus pneumoniae that cannot make a capsule is injected into mice and has no adverse effect. This strain is then mixed with a culture of heat-killed Streptococcus pneumoniae that when alive was able to make a capsule and kill mice. After a period of time, this mixture is injected into mice and kills them. In terms of horizontal gene transfer, describe what might account for this.

Q:2. A Gram-negative bacterium that was susceptible to most common antibiotics suddenly becomes resistant to several of them. It also appears to be spreading this resistance to others of its kind. Describe the mechanism that most likely accounts for this.

Answer 1

Horizontal Gene Transfer

Ans1. When a strain of living Streptococcus pneumonia that cannot make a capsule was injected into mice, the mice survived as the bacterial cell cannot produce capsule which is responsible for virulency of the bacteria. When this non virulent strain is mixed with heat killed Streptococcus pneumoniae that can make the capsule and kill the mice, transformation of genetic material occurs from the heat killed virulent one to the non-virulent type.

Transformation is a phenomenon in which the bacterial cells take up the genetic material (DNA) from the environment (outside the cell) and incorporate it into its own genetic material leading to alteration in the cell. In this case the non-virulent strain takes up the DNA fragment (DNA fragment that help in capsule formation) of the heat killed virulent strain, and incorporated it into its own genome leading to formation of capsule thus the cells that received the exogenous genetic material (DNA dna fragment) get transformed from non-virulent type to virulent type.

Ans2. The mechanism that most likely accounts for this is conjugation.

Conjugation is the transfer of genetic material between two bacterial cells by the mean of pilus. In this process the donor cell i.e. the cell having the antibiotic resistance gene (in its plasmid) form the pilus which get attached to the recipient cell, once the bridge if formed the plasmid is transferred from the donor cell to the recipient cell and the recipient cell also acquire the antibiotic resistance gene and become donor (for the antibiotic resistance gene, R-factor). In this way the spreading of the resistance occurs between these bacteria.