The effect of antibiotics on the growth of bacteria

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The aim of this investigation is to find out what effect out of two antibiotics, penicillin and streptomycin has on the growth and multiplication of two different species of bacteria. The two different types of bacteria we will be using are E-coli and micrococcus luteus.

The Apparatus:

* Four sterilised Agar plates,

* Glass spreader,

* Masking tape,

* Wax pencil,

* Lab roll,

* A beaker with 70% alcohol,

* E- coli (Culture)

* Micrococcus luteus (culture),

* Penicillin (antibiotics),

* Streptomycin (antibiotics).

Scientific Knowledge:


E-coli are a common type of bacteria and are short for the medical term Escherichia coli. This normally lives inside your intestines, where it helps your body break down and digest the food you eat. E. coli bacteria was discovered by a German bacteriologist Theodor Escherich in the 1885.The organism can be found on a small number of cattle farms and can live in the intestines of healthy cattle. But also we need it in our bodies to keep them healthy we pretty much depend on E-Coli in our intestines to provide us with Vitamin K and B complex vitamins which play a vital role in keeping us strong and healthy.


Penicillin was the first naturally-occurring antibiotic discovered and was the first one to be used therapeutically Penicillin was discovered by Alexander Fleming in 1929. Penicillin is an antibiotic which is produced by the mould penicillium; penicillin kills only certain types of bacteria by disrupting the structure of their cell wall. As the bacterial cell is growing and making a new cell wall also dividing, a new cell wall must be made constantly to completely surround each of the dividing cells.


Streptomycin is an antibiotic that is produced by soil bacteria of the genus in streptomyces and can be active against both gram-positive and gram-negative bacteria.

Micrococcus luteus:

Micrococcus luteus can be found in many places such as the human skin, water, dust, and soil. Micrococcus luteus is generally thought as harmless bacterium, but there have been rare cases of Micrococcus infections. Also the bacteria can degrade compounds in sweat, and on the skin, into ones producing unpleasant odours. These bacteria normally grow at 37oC.

The Method:

We first of all got paired up and washed our hands, after that my partner went and colleted the lab coats and I went and collected all the equipment. Before we could carry out this investigation we placed a clean piece of lab roll on surface that we were using, then placing our equipment on the lab roll we placed one of our four agar plates in a way which we could approach it the best, we slowly and carefully opened it so that we did not contaminate the plate with other bacteria from the environment. We then cautiously spread the bacteria on the nutrient plate using a glass spreader that was dipped into 70% alcohol; we also taped the spreader on the side of the beaker so that there was on excess alcohol present.

Once the bacteria was spread evenly all across the plate so we produced an even lawn of bacteria we firmly placed our antibiotic disc in the centre. We had carried out this procedure for the rest of the plates. We placed masking tape on each end of our agar plate and using a wax pencil we placed our names and the names of the bacteria and antibiotic it contained, subsequently we placed the plates upside down in the incubator at 30c for four days also a control plate was located in to the incubator for the whole class. We used two plates with two different bacteria’s e-coli and Micrococcus luteus, we did not place antibiotics on two of the plates and after that the plates were placed into an incubator. Once we removed the plates containing bacteria from the incubator we found out that bacterium was not affected by any antibiotics due to there were no antibiotics present as we did not place any. There was also no zone of inhibition present as there was no antibiotic disc’s present on the plates.

The Aseptic Technique:

During this investigation we followed the Aseptic technique; this method was used to make sure we did not contaminate the bacteria with any fungi or bacteria that were already present in the air.

The Aseptic technique states to do some and all of the following:

* Remove or kill micro organisms from hands and objects.

* Use sterile equipment and other items.

* Decrease the person’s risk of exposure to micro organisms.

The Aseptic technique refers to washing our hands before and after carrying out the experiment this is so that no bacteria was contaminated and also after the experiment so that no bacteria was left on our hands, we also wore lab coats so that our contaminated clothes were no near the agar plates. Also refers to the use of sterilized equipment, so for us to meet this condition we used Petri dishes that were already sterilised by the means of an hot steam in an autoclave ( this works like a pressure cooker) furthermore we placed a piece of lab roll on to the table we was working on to generate a clean working area, as well as that we used sterilised glass spreaders that were dipped into 70% alcohol we also tapped the glass spread on to the side of the beaker to let of any excess alcohol.

The agar plate containing the E-coli bacteria with the Penicillin antibiotic. I think that the penicillin antibiotic did not work at all due to it have any effect on the bacteria. Looking at my results I think that I speared a lot more bacteria near the bottom of the plate as there is a lot more growth at that end. There is no zone of inhibition present. This tells me that If I had an infection or illness affected by an E-coil bacterium I would Not to cure it using streptomycin antibiotic due to it would have no affect on it. The agar plate containing E-coli bacteria and Streptomycin antibiotic.

Looking at this plate I can tell that the antibiotic did have an effect on the e-coli bacteria as I can clearly see the zone of inhibition but in addition A lot of bacterial growth too. This tells me that I can help heal an infection or disease containing the E-coli bacteria I would know to use a streptomycin antibiotic. The agar plate containing Micrococcus lutes bacteria and penicillin antibiotic. Looking at this plate I can tell that the penicillin antibiotic had effected on the Micrococcus luteus bacteria as there is a zone of inhibition present 43mm, but also there is a fair amount of bacteria growth present too.

Also to help cure an illness or infection containing Micrococcus luteus bacteria I know that I can help treat it with a penicillin antibiotic. The agar plate containing Micrococcus lutes bacteria and Streptomycin antibiotic. Looking at this plate I can say that the streptomycin antibiotic did have an effect on the micrococcus lutes bacteria as there is a zone of inhibition present 30mm, there is a lot fewer growths of bacteria present also the bacteria is mostly present at the top end of the plate and there is rare appearance of bacteria appearing near the centre.

So in addition I can say that I had an infection or disease containing the micrococcus lutes bacteria I would know to use the streptomycin antibiotic to help cure the infection or disease. The Conclusion: In conclusion I found out that my perdition was correct and that penicillin antibiotic did not have no effect on the E-coli bacteria but the Streptomycin antibiotic did have effect on the E-coli bacteria this tell me that if a person was to have illness featuring the E-coli bacteria then the penicillin antibiotic would have no effect on it where as the Streptomycin antibiotic would cure the person.

My prediction about micrococcus luteus was also correct, that there would be no effect on the bacteria from the penicillin antibiotic but as for the Streptomycin antibiotic will have effect. This also tells me that an illness containing micrococcus lutes will not be cured by the penicillin antibiotic but where as the streptomycin antibiotic we help cues the illness. The Evaluation: If I was to redo this investigation I would spread my bacteria quite close together so that the bacteria growth grew closer together to give me better readings and a clear understating of the bacteria and its effect.

Furthermore I would make sure that I do not open the agar plate fully as it may get contaminated as did mine. Due to this major error I did not get an outcome of reliable results and in addition I had to use some one with better results than me. Also the aseptic technique should be used in a suitable and watchful manor to obtain reliable and accurate results. Over all this experiment was not as very successful as I had planned it to be, as I did not achieve accurate results. But on the other hand observing my fellow classmates outcomes I have gained a clear understanding on how bacteria growth is affected by the presence of antibiotics.

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