Calculate the Average Number of Colony Forming Units Per Cm3 in the Homogenate

6. An 8 gram sample of cream cheese was homogenised in sterile Ringer’s solution to give a total volume of 20 cm3. The homogenate was then serially diluted and 0.2 cm3 aliquots of a number of dilutions were spread plated on duplicate nutrient agar plates. After incubation, the following colony counts were recorded:

Dilution / Replicate 1 / Replicate 2
10-3 / 356 / 314
10-4 / 31 / 38
10-5 / 3 / 2

Calculate the average number of colony forming units per cm3 in the homogenate.

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Calculate the average number of colony forming units per gram of cream cheese.

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The following details were recorded about the growth of one of the isolates from the cream cheese:

Colony
characteristics / On Nutrient agar / On Blood agar
Size / 0.5-1.0 mm / 0.5-1.0 mm
Shape / Smooth / Smooth
Pigmentation / Translucent / Transparent with narrow zone of  haemolysis

A number of additional tests were then carried out on this isolate and the following observations made:

Test / Observation
Methylene blue stain / Rod-shaped cells
Gram stain / Purple cells
Colony colour in reflected light / Blue-green
Ability to grow at 4ºC / Yes
Motility / Tumbling motion

Identify this cream cheese isolate, explaining your reasoning in full.

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What are the possible consequences of ingesting food contaminated with this microorganism?

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7. A 22-year-old male student was brought to the emergency room after a 2-day history of headache and fever. On the day of admission, the patient had failed to come to school or to phone friends to explain why. When notified of this fact, the patient’s friends had gone to his flat, where they found the patient in bed, confused and highly agitated. The patient was comatose when he arrived in the emergency room. Purpuric skin lesions were present on his trunk and arms. Gram stain of cerebrospinal fluid showed many Gram-negative diplococci, and the same organism was isolated from blood. The patient died despite prompt initiation of therapy with penicillin.

What is the most likely microorganism responsible for this fulminating disease? Explain your reasoning in full.

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What is the most likely source of this organism?

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What other diseases are caused by this microorganism?

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8. Forty-two people in the same town reported to their GPs with fever (40°C), jaundice, and tender abdomen. All 42 had eaten an ice-slush beverage purchased from a local convenience store. Liver function tests were abnormal. Over the next several months, the symptoms subsided and liver function returned to normal. What is the disease? Explain your reasoning in full.

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What pathogen is causing it? Explain your reasoning in full.

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9. Twenty-two percent of the patients in a large hospital acquired Clostridium difficile diarrhoea and colitis during their hospital stay. These patients required longer hospital stays than uninfected patients. Epidemiological studies provided the following information.

Rate of infections for patients: Single room 7%

Double room 17%

Triple room 26%

Rate of environmental isolation of C. difficile: Bed rail 10%

Commode 1%

Floor 18%

Call button 6%

Toilet 3%

Hands of hospital personnel after contact with patients that were culture-positive for C. difficile: Used gloves 0%

Did not use gloves 59%

Had C. difficile before patient contact 3%

Washed with non-disinfectant soap 40%

Washed with disinfectant soap 3%

Did not wash hands 20%

What is the most likely mode of transmission of this bacterium in hospitals?

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How can transmission be prevented?

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