BA, BSc, and MSc Degree Examinations

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1 Examination Candidate Number: Desk Number: BA, BSc, and MSc Degree Examinations Department : BIOLOGY Title of Exam: Microbiology Time Allowed: 1 hour and 30 minutes Marking Scheme: Total marks available for this paper: 50 Instructions: Answer all ques ons in the spaces provided on the examina on paper The marks available for each ques on are indicated on the paper Materials Supplied: CALCULATOR For marker use only: Total as % Office use only: DO NOT WRITE ON THIS BOOKLET BEFORE THE EXAM BEGINS DO NOT TURN OVER THIS PAGE UNTIL INSTRUCTED TO DO SO BY AN INVIGILATOR page 1 of 8

2 Answer all questions in the spaces provided 1. a) You wish to determine the number of bacteria in an actively growing broth culture of E. coli. To do this you remove 1.0 ml of the culture from the flask and dilute this in 9 ml of nutrient broth to obtain a 10-1 dilution. You then serially dilute the sample further until you obtain a range of dilutions between 10-2 and From each dilution you then spread plate 0.1 ml of the suspension onto nutrient agar and incubate overnight. The next morning you have the following results Dilution Neat Colonies on plate Too many to cont 10-1 Too many to count 10-2 Too many to count Determine how many bacteria per ml there were in the original sample taken from the overnight culture. Show your work. The correct dilution must be selected. Students have to select a plate that has between 30 and 300 colonies (in this case the 10-3 dilution) (1 mark). This would be 280 colonies produced from 0,1 ml of culture spread onto the plate. This would then need converting to the number per ml you in that dilution (x10) which is 2800 (1 mark). It is then a case of multiplying this figure by the dilution factor (10 3 ) to determine the original concentration. This works out at 2800x10 3 or 2.8x10 6 per ml of culture. (1 mark) b) You adjust the density of the cell suspension so that there are 1x10 6 bacteria per ml in broth and add 1 ml of this to a new culture flask. Assuming exponential growth and a doubling time of 30 minutes, how many bacteria will be present in the flask after 5 hours? Show your work. page 2 of 8

3 Students need to use the equation (1 mark) N = N 0 2 n Where N = final cell number N 0 = initial cell number n = number of generations In this case N 0 = 1x10 6 and n = 10. Answer is N = 1x10 6 x 2 10 (which is 1024) = or 1.024x10 9 (full two marks) Learning outcome covered - Use aseptic techniques and perform a number of assays aimed at identifying different bacterial species 2. a) Draw a simple labeled diagram that shows the major differences between the cell envelopes of a Gram positive and Gram negative bacteria. (5 marks) Answer is diagram Two marks for Gram positive. Three marks for Gram negative. b) How do the cell envelope of thermophilic Archaea differ from those of bacteria. The cell membrane of archaea is made up of a monolayer of biphytanyl rather than a phospholipid bilayer (1 mark). This is then surrounded by a layer of protein or pseudomurein (1 mark). page 3 of 8

4 Learning outcome covered - Describe the specialised molecular structures found in different microorganisms including prokaryotes (bacteria and archaea), viruses and single cell eukaryotes (algae, fungi and protozoa) and correlate the structures to their function. 3. Explain how the FtsZ and the MinCDE systems in bacteria interact to ensure that septum formation during cell division occurs at the middle of the dividing cell. (4 marks) FtsZ forms a ring (Z ring) at midcell and recruits all the other cell division proteins to that site that create the cell division (1 mark). The MinCDE system makes sure that FtsZ forms the Z ring in the correct place (1 mark). The MinCDE system repeatedly builds up and is broken down in polar parts of cell (1 mark). Because Min C prevents Z ring formation and concentration of MinC is higher in poles that at midcell the Z ring can only form in middle (1 mark). Learning outcome covered - Describe the specialised molecular structures found in different microorganisms including prokaryotes (bacteria and archaea), viruses and single cell eukaryotes (algae, fungi and protozoa) and correlate the structures to their function. 4. a) For the production of penicillin and many antibiotics it is necessary to operate a fermenter in batch culture. Why is this? Most antibiotics are secondary metabolites (1 Mark). They are only produced when the culture goes into the stationary phase of growth (1 mark). Microbes grown under batch culture conditions will experience the necessary depletion of resources needed to shift them into producing secondary metabolites.(1 mark) b) Why would continuous culture be favoured over batch culture for the production of microbial biomass? Batch culture is relatively inefficient (1 mark). The conditions in the vessel change over time as nutrients are depleted. This means that the microbial culture is not exposed to optimum growth conditions over time. The operation of fermenters under batch conditions also requires repeated stop / start processes that are not optimum for industrial processes, which are easier to operate continuously (1 mark). Continuous culture allows the maintenance of optimum conditions over time to maximise microbial growth rates (1 mark) Learning outcome covered - Recall the practical applications of microorganisms in the production of chemicals such as antibiotics and high value metabolites page 4 of 8

5 5. a) Describe the different growth forms that fungi may take. Fungi can grow in two major growth forms, hyphal and unicellular (1 mark). Hyphal forms can be then further subdivided into septate (divided into cells each with a nucleus) or aseptate (coenocytic) with several nuclei per super cell (1 mark). b) Explain briefly three ways that fungal pathogens cause disease in humans. host Hypersensitivity - The fungal spores initiate an allergic reaction in the Mycosis - Direct infection of tissue, either superficially or deep into tissue leading to cell death. Mycotoxins - Interfere with cellular processes resulting in seizures spasms or hallucinations. No marks for just naming and not explaining. Learning outcomes covered - Describe microorganisms as agents of disease and the molecular mechanisms responsible for different pathologies - Describe the specialised molecular structures found in different microorganisms including prokaryotes (bacteria and archaea), viruses and single cell eukaryotes (algae, fungi and protozoa) and correlate the structures to their function. 6. a) What organelle differentiates algae from other protists? (1 mark) Chloroplast (1 mark) b) Below are pictures of a diatom and a coccolithophore page 5 of 8

6 i) Which is which (1 mark), and what materials are their shells made of? i. Coccolithophore, Calcium carbonate (CaCO 3 ) ii. Diatom, Silica (SiO 2 ) 1 mark for correct assignation of names, 1 mark for each correct material c) Briefly outline two possible functional roles of the shells of coccolithophores? Protection from viral attack (1 mark), grazing reduction (1 mark), ballast to help modify position in water column (1 mark), light modulation (1 mark), enhancing photosynthesis by CO 2 release during calcification (1 mark) Learning outcome covered - Describe the specialised molecular structures found in different microorganisms including prokaryotes (bacteria and archaea), viruses and single cell eukaryotes (algae, fungi and protozoa) and correlate the structures to their function. 7. a) Describe two groups of protozoans which cause disease with an example of each. How is each transmitted to human hosts? (6 marks) (i) Amoebas (e.g. Entamoeba; dysentary or amebiasis ) - contaminated page 6 of 8

7 food/drink. (ii) Flagellates (e.g. Trypanosoma brucei spp.; HAT, Leishmania spp.; leishmaniasis, Trypanosoma cruzi; Chagas disease) Insect vectors: tsetse fly, sandfly or reduviit bug (contaminated drinks/ organ transplant also OK) (iii) Apicomplexans (e.g. Plasmodium falciparum ; causes malaria, Toxoplasma gondii ; toxoplasmosis) mosquito bite; cat feces or contaminated food. b) Give an example of a protozoan not pathogenic to humans. (1 mark) Trypanosoma brucei brucei (non-primate), Paramecium, all correct answers accepted. Learning outcome covered - Describe the specialised molecular structures found in different microorganisms including prokaryotes (bacteria and archaea), viruses and single cell eukaryotes (algae, fungi and protozoa) and correlate the structures to their function. Learning outcomes covered - Describe microorganisms as agents of disease and the molecular mechanisms responsible for different pathologies 8. Describe three specific features of the smallpox virus that enabled its eradication. (i) Exclusive to humans - no hidden reservoirs of wild animals. Easy to contain and treat infected people. (ii) No hidden, asymptomatic carriers - easier to spot to treat. Characteristic disease phenotype was obvious and easily diagnosed. (iii)only one serotype - stable and not variable, easier to design vaccine. (iv) Vaccination 100% successful with few side effects. Easy to get people to agree to it. Learning outcomes covered - Describe microorganisms as agents of disease and the molecular mechanisms responsible for different pathologies page 7 of 8

8 9. a) Give one example of a primary lymphoid organ and one example of a secondary lymphoid organ. Primary: Thymus, bone marrow Secondary: Spleen, Lymph node, will also accept various mucosal sites (e.g. tonsils, Peyer s patches, not taught) b) State two ways in which antibodies can protect against pathogens. Answers from: 1. Direct neutralisation of toxins/essential molecules 2. Pathogen opsonisation and uptake by phagocyte 3. Complement activation c) How do epitopes recognised by T cells and B cells differ? T cells can recognise only linear epitopes (1 mark) B cells can recognise both linear and conformational epitopes (1 mark) Learning outcomes covered - Describe microorganisms as agents of disease and the molecular mechanisms responsible for different pathologies page 8 of 8