Question

1. Describe the basic features of the bacterium Staphylococcus aureus (S. aureus) and its modes of transmission. Explain why people can harbor S. aureus and not be infected by it.

2. Describe how beta-lactam antibiotics work. Outline the steps in bacterial reproduction. Now, discuss the consequences of binary fission for the accumulation of mutations in a bacterial population over time. Next, describe gene transfer. Finally, illustrate and describe how binary fission, mutation, and gene swapping, or gene transfer, interact to create diverse bacterial populations, some of which are resistant to an antibiotic.

Answer 1

Q1)

Staphylococcus aureus are gram positive cocci found in grape like clusters, they are non- moving small round shaped bacteria, they do not form spores, they are catalase positive, and oxidase negative and are facultatively anaerobic (a facultative aerobe is an organism which makes ATP by aerobic respiration if oxygen is present, but is capable to switch to fermentation or anaerobic respiration if oxygen is absent).

Transmission of S.aureus occurs by air droplets or aerosol, when an infected person coughs or sneezes it produces many small droplets of saliva which remains suspended in air when an healthy person inhales it gets infected. Another mode of transmission is by direct contact with the objects contaminated by the bacteria or by bites by infected persons or animals.

People can have S.aureus on their body but not infected by it because, S.aureus infects the person when it enters the body and comes in contact with the blood stream, it starts multiplying and leads to infection.

People having a week immune system are more vulnerable to get infected by S.aureus. Also in people in whom there is regular insertion of medical equipment into the body are more prone to get the infection.

Q2) Beta lactam antibiotics are broad spectrum antibiotics, they are usually penicillin derivatives, contain a beta lactam ring at the core, they act by inhibiting the peptidoglycan layer of cell wall of bacteria. Beta lactam antibiotics attack the penicillin-binding proteins, these are a group of enzymes found attached to the cell membrane, they help in the cross-linking of the bacterial cell wall. The beta-lactam ring portion of this group of antibiotics binds to the different PBPs, as a result they are not able to synthesize the cell wall. Due to this the death of the bacterial cell occurs.

Bacterial reproduction:- In bacteria genetic information is in the form of a single loop of DNA, they reproduce by binary fission. Bacteria is a single cell, binary fission divides the bacteria into two identical daughter cells, it begins when the DNA divides into two, bacterial cell elongates and divides into two daughter cells each daughter cell having a DNA identical to parental cell. Each daughter cell is a replica of parent cell.

There are various types of mutations beneficial and harmful and neutral, harmful mutations are eliminated through death of the organism’s, but beneficial mutations are vertically transmitted to the organism’s daughter cells by binary fission.

Gene transfer:- It is the insertion of foreign gene into the genome of target cell, this method is usually used for treatment of diseases to supply patients with therapeutic genes.

In bacteria DNA can be transferred from one organism to another by the horizontal gene transfer mechanism, the DNA transferred by horizontal method is stably incorporated in the recipient, and it does permanent changes in the genetic composition of the recipient. It is done by three methods conjugation, transduction and transformation.

Conjugation- In this the transfer of genes between cells takes place when they are in physical contact with each another.

Transduction- In this case the transfer of genes from one cell to another takes place by bacteriophage.

Transformation- Here transfer of cell free DNA takes place from one cell to another.

Horizontal gene transfer is a main mechanism for the spread of antibiotic resistance in bacteria, and plays an important role in the evolution of bacteria, which can degrade compounds like pesticides and helps in the evolution, maintenance, and transmission of virulence of bacteria. Genes responsible for antibiotic resistance in one species of bacteria can be transferred to another species of bacteria by various mechanisms like by F-pilus.

Binary fission makes clones, or genetically identical copies, of the parent bacterium. In bacteria, reproduction is very fast, a generation takes very short time, this short generation time, along with random mutations and the mechanisms of genetic recombination helps the bacteria to evolve very quickly. Horizontal gene transfer makes the bacteria antibiotic resistant. As there is rapid evolution, it makes the bacteria to adapt to environmental changes, like the introduction of an antibiotic, very quickly, as a result the bacteria grows but it is harmful to the human population in many cases, and the antibiotic resistance also makes it more vulnerable towards mankind.