Investigating how different concentrations of a antibiotic effects the growth of a bacterium.

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Investigating how different concentrations of a antibiotic effects the growth of a bacterium

Hypothesis- for this investigation my hypothesis is that as the concentration of the names antibiotic increases the growth of the bacterium around it will decrease.

Scientific explanation- in explaining the hypothesis that has just been made the scientific details will be divided into a number of different paragraphs. These being:

  • Antibiotics sites if affect
  • Antibiotics, how they work and what is resistance
  • The growth of bacteria
  • Penicillin


First of all, antibiotics are drugs that are used to treat and to cure infections. The antibiotics are derived from living organisms, although others such as isoniazid used to treat tuberculosis are synthetic.

                            Antibiotics are used and are as successful as they are as they interfere with some aspect of growth or metabolism of the organism. The different methods or areas of the interference are:

                                DNA         RNA Polymerase    mRNA    

Antibiotics are strong medicines that can cure many bacterial illnesses and infections. The standard definition states that an antibiotic is a substance produced by microorganisms that kill or inhibit other microorganisms. The first antibiotic, penicillin, was discovered in 1929 by Sir Alexander Fleming who observed the inhibition of staphylococci on a plate contaminated by a Penicillin mold. By the mid 1940's antibiotics were available for treatment against many bacterial infections including strep throat, pneumonia, skin infections, wound infections, scarlet fever, toxic shock syndrome and other bacterial infections. Facts concerning antibiotics are:

  • Antibiotics are only effective in the treatment of bacterial infections. They have absolutely zero impact on viral infections.
  • Antibiotics work by either killing bacteria or by inhibiting growth.
  • Antibiotics don't have any impact on viruses such as colds, flu, bronchitis, or other viral infections. Only your doctor can determine if you have a viral or bacterial infection.
  • Each time we take antibiotics, sensitive bacteria are killed, but resistant ones may be left to grow and multiply. Repeated use and improper use of antibiotics are some of the main causes of the increase in resistant bacteria.

Contributors to antibiotic resistance include: 

  • Misuse and overuse of antibiotics in humans, animals, and agriculture 
  • Demand for antibiotics when antibiotics are not called for
  • Failure to finish an antibiotic prescription


As the in the section above mentioned or used Penicillin as a common antibiotic and that which is being used in the experiment is also a form of penicillin, it is important for some information concerning Penicillin.Penicillin works by inhibiting the bacteria cell from forming a cell wall. This may not seem important, but the cell wall is critical to the cell. If a cell cannot make a wall it will rupture and die. The bacteria reproduce by binary fusion. This means that a single cell divides to produce two cells. During this division the cell wall doubles in size and finally separates forming two cells. How Penicillin works is that it cannot affect an already formed cell wall, but it prevents the cell from making more of that wall and inhibits the cell from multiplying. Penicillin kills bacteria by interfering with the ability to synthesize cell wall.

The diagram to the right shows the chemical structure of Penicillin.


The next point to consider on the scientific explanation is how the bacteria grow. By research I have found that bacteria reproduce asexually by a process that has been called Binary Fission. Loosely this term “two division”. The products of the process are two new cells, which are usually equal in size. Before the cell divides, the DNA replicates itself so that the two new cells have identical sets of genes. An infolding of the plasma membrane that is thought to hold the DNA in position may help the process of DNA replication. The relevancy of the first point to this investigation is that it shows how the antibiotics that are to be used with affect the named bacterium.

In order to divide into two, the parent cell grows a new cell wall across the middle of the cell. After the division has been complete, the two daughter cells must grow to full size again by further division.

The diagram to the left hand side shows a bacterium that has went through the process of binary fission under electron micrograph, showing key stages of binary fission.

        A brief description of the commented processes is the cell wall that is produced to split the two cells.

The second, is the DNA replication that produces to identical sets of genes, these can be seen either side of the developing cell wall.

Another important area concerning the growth of the bacteria is the growth curve that is produced by the rate of growth by the bacteria or bacterium.  Looking at the graph that has been produced it shows that the growth of the bacteria goes through a number of different phases. 

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Of bacteria

        1   2         3     4

                        Time (hours)

  1. The first of these is known as the Lag phase, and at this phase the numbers of bacteria remain fairly constant as the bacteria adjust the new conditions. During this time the bacteria synthesise enzymes needed to use nutrients in the broth available/
  2. The second of these phases is known as the Log Phase, in this phase the bacteria reproduce exponentially. I.e. population continually doubles. Nutrients are plentiful and waste products have not built to the ...

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