Biology · Culturing microorganisms
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3real answers to the same question, each marked for real by MarkMatch’s AI marker — not invented scores.
A student investigated whether ultraviolet light could reduce contamination by microorganisms on a work surface. The student used sterile nutrient agar in three Petri dishes. Dish A was opened near the work surface for 10 minutes, dish B was opened near the work surface for 10 minutes and then exposed to ultraviolet light, and dish C was kept sealed. All three dishes were incubated at 25 °C for 48 hours. The student counted the colonies that grew. Dish A had 46 colonies, dish B had 12 colonies and dish C had no colonies. Explain what these results show about contamination and how the investigation could be improved to produce more reliable evidence.
Level 3 (5-6): Relevant points (reasons/causes) are identified, given in detail and logically linked to form a clear account. Level 2 (3-4): Relevant points (reasons/causes) are identified, and there are attempts at logically linking. The resulting account is not fully clear. Level 1 (1-2): Points are identified and stated simply, but their relevance is not clear and there is no attempt at logical linking. No relevant content (0).
The work surface was probably dirty because dish A grew colonies. The UV light helped dish B because it had fewer colonies. Dish C was clean, so UV killed the germs.
What this answer missed
- •Dish C is a negative control: its lack of colonies shows that the sterile agar and sealed dish were not contaminated before the investigation, rather than showing that ultraviolet light killed the microorganisms.
- •Use several replicate dishes for each treatment and calculate a mean colony count to make the evidence more reliable.
- •Repeat the investigation on different days or at different work surfaces, while keeping variables such as exposure time, distance from the ultraviolet source, agar volume, incubation temperature and incubation time the same.
Microorganisms were present near the work surface because 46 colonies grew in dish A. Dish B had fewer colonies, so ultraviolet light reduced contamination, but it did not get rid of it all.
What this answer missed
- •Use several replicate dishes for each treatment and calculate a mean number of colonies, so random variation is reduced and the evidence is more reliable.
The 46 colonies in dish A show that the work surface or surrounding air contained microorganisms. Dish B had 12 colonies, so the UV treatment reduced the contamination, although it did not destroy every microorganism. Dish C was a negative control and had no growth, suggesting the dishes and agar were not contaminated before the test. More dishes should be used for each treatment and a mean should be calculated. The same incubation temperature and time, UV exposure time and distance, and amount of agar should be used each time.
- •Dish A's 46 colonies show that microorganisms were present on the work surface or in the surrounding air.
- •Dish B had fewer colonies than dish A, showing that ultraviolet light reduced the number of microorganisms able to grow.
- •The 12 colonies in dish B show that the ultraviolet treatment did not remove or inactivate all the microorganisms.
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