Question from a student 20170503e

Q:

Sorry for bothering. May I know what is the minimum pH limit of

i) E.coli

ii) Salmonella?

Is adjusting pH to 4.2 effective in controlling their growth respectively?

Since there are many different data that confuse me.

A:
pH 4.2 should be effective in controlling the growth of most E. coli and Salmonella strains. However, some E. coli serotypes, such as O157:H7, can survive at pH 4.2. You have to note that minimum, optimum, and maximum of whatever microorganisms are affected by many factors, including their own biology, other environmental factors, and growth stages, there is no absolute limit. They are at best approximations. That is a reality. We can only note the limits as general features not absolutely applicable to all strains of a species.  

Question from a student 20170503d

Q:

I have some questions concerning 2014-2015 Past Paper B2.

I think the implicated microorganism should be Staphylococcus aureus but the incubation period of 11-40 hours is not so consistent with that of the Staphylococcus aureus. Does it mean that as long as the calculated incubation period is longer than the incubation period of the microbe itself, it can be considered as consistent?

And the difference in attack rate of roast turkey is also quite high and there are also Clostridium perfringens detected in the roast turkey. The incubation period also matches. Can we also deduce it is the agent that cause the outbreak?

One more question is that if the microbes mentioned in the questions do not fit in the analysis, we can suggest some possible microbes by ourselves?

And may I also ask if Listeria monocytogens can grow at 40C? If yes, that means heat treatment at 40C is not effective in suppressing its growth?

A: 

The incubation period is a little too long. You have to discuss this point when you answer the question. But the pizza has the highest attack rate difference, which is an important factor for the implicated food. The roast turkey can also be a suspected food but it is less likely than the pizza. 

If the microbes resulted from the investigation do not fit totally with the deduction, you can of course suggest other possibilities.

Listeria can grow at 40C, so 40C is not effective in suppressing its growth

Question from a student 20170503c

Q:

From the outbreak investigation questions in the past papers, part (a) asks about the suspected food and part (b) asks about the implicated food.

I would like to ask:

     1) for answering the suspected food, do we ONLY use the attack rate difference to deduce the answer?

     2) is it necessary that the suspected food MUST BE THE SAME as the implicated food?

For 2012-2013 Q8, are these answers correct?

       (a) Pizza is the suspected food as it has the highest attack rate difference among the others. One point needs to be aware that there is no bacteria found in pizza.

       (b) B. cereus is the implicated microorganism as it is consistent to the shortest incubation period (5 hours) while C. perfringens is not. Therefore, the implicated food is bread

A: 

I do not have the 2012-13 question at hand, so I just answer according to my memory of the questions.

1. You can use whatever evidence you think appropriate to deduce the suspected food. The attack rate difference is an important factor in the deduction or elimination. Especially when the difference is very small or negative.
2. Suspected food and implicated food can be different. 

a. No bacteria does not mean the food is clean. There can be toxins.

b. The bread, if having a very small attack rate difference, or negative attack rate difference, would probably not the implicated food. It is possible that the pizza had Staphylococcus aureus grown on and toxin produced. 

Question from a student 20170503b

Q:
I found that the incubation periods (IP) of some of the bacteria are inconsistent in concept maps and the food microbiology notes. For instance, the IP of Clostridium botulinum is 8hours to 8 days as written on the concept map and said during class but that in the note of p.A-1 is 1.5-3days. For Staphylococcus aureus, the IP written on concept map is 2-6hrs and that on p.A-1 of the note is 1-6hrs. For Campylobacter jejuni, the IP is 2-9days on ppt notes but that on p.A-2 of note is 2-7 days. Should I refer the incubation period of bacteria and virus on the notes or on the concept maps? Thanks for your kind attention!

A:
As I have mentioned many times in my lectures that the incubation period for each pathogen is only an approximation. You will find that the incubation periods from different sources are not exactly the same, even from my concept map and notes. Due to two reasons, at least,

1. Many environmental factors would affect the incubation period.
2. Each species may have many strains and incubation periods for them may be different.

So use the approximate incubation period from either notes or concept maps would be acceptable.

Question from a student 20170503a

Q:
Hello Mr Kwan, i would like to ask if we could bring self-prepared notes to the exam?
Thank you so much!

A: Yes, of course

Question from a student 20170503a

Q and A

• 2014-2015 B2. If I know that the suspected food is pizza from (a), do I also need to calculate the incubation period assuming dinner is the problem? 

Yes, you should check every piece of information to ensure your conclusion is close to the truth.

• I dont understand what you mean by elimination. Do I need to refer to every other food served at the picnic and its attack rate? (eg. Although Salmonella entericaserotype Typhimurium is found in both the sushi and faeces of patients, it seems that sushi is not likely to be the food of problem as it has the lowest positive difference between the attack rate of those who ate and those who didn’t. ) 

Yes, eliminate other possibilities with information at hand. For example, S Typhimurium in this case would probably not the agent because of the attack rate and incubation time are not consistent with it being the agent. You have to do this because disease may not be caused by a single pathogen but may be caused by more than one. You also need to explain any inconsistence observed.

• 2013-2014 QB1 (b) x : Occurred frequently in child-care centers : Human rotavirus. Is this correct? 

No, should be Norovirus

• 2014-2015 QB1 (b)viii : Occurrence and growth in fresh cream even stored in refrigerator: Listeria monocytogenes

Yes, it can grow at refrigeration temperature and may exist in fresh cream

• 2014-2015 QB1 (b) Occurred frequently in child-care centers and caused vomiting: Is it nora virus or human rotavirus? 

Norovirus is the better choice.

Question from a student 20170502a2

Q & A:

Thank you Professor Kwan. I am sorry that I would like to ask 2 more questions. 

1) Can Salmonella and E. coli tolerate high salt content? And in specific, can E. coli survive/grow in food with salt content 7%?

E. coli cannot grow at 7% salt, but it may survive.

2) Clostridium botulinum cannot grow in food with oxygen 5% since it is an obligate anaerobe?

It cannot grow with 5% oxygen.

Question from a student 20170502a1

Q: 

I would like to ask how to determine 'growth' and 'survival' of microorganisms from some given data. 

For 'growth', if the parameters are out of the minimum or maximum limit of microorganisms, it means that they cannot grow?

A: They cannot grow

Q:

For 'survival', I remembered that you have mentioned microorganisms can actually survive at low temperature and low water activity, even if the conditions are out of their limit. How about pH? Will they just die if the pH is below minimal limit/above maximum limit? And for temperature that is higher than the maximal temperature for growth, does it mean that the microorganisms will die?

A: 

pH below minimum or above maximum: Die.

temperature higher than maximum: Die.

But of course, the length of time for the microorganisms expose to these parameters needs to be taken into consideration. If brief exposure, there is D value or similar factors affecting the survival.

Question from a student 20170502d

Question about  pastpaper 2013-2014

I would like to know  how to calculate new D value like question in B5 (d).
And do you mind checking if the following ans in B5 is correct?
a. no. of D's 
Spoilage bacteria = 100D
Bacterial spores = 10D
Fungal spores = 50D
b.

Spoilage bacteria = 10^-95

Bacterial spores = 10^ -6

Fungal spore = 10^-49

c. the standard canning process for  Spoilage bacteria and  Fungal spore are sufficient, but not for bacterial spores

A: You are correct, but you should show your calculation too, also need to explain.

Please see https://fnsc3180hsk.blogspot.hk/2017/05/question-from-student-201617-20170501a.html?showComment=1493708599366#c8295958764595042514

Question from a student 20170502c

Q:
     I have a question popped up in my mind when I looked at the notes of fermentation.  "Lactic acid production can lower the pH to make the fermented food resistant to spoilage." If this is true, then why do fermented foods like cheese, yoghurt still subject to spoilage? Thank you for your attention.

A: 
1." Resistant to spoilage" does not mean never spoiled, only less likely to be spoiled, not high risk for spoilage.
2. In fact, cheese and yogurt do not spoil easily.

Question from a student 20170502b

Q&A:    

 I would like to ask that whether notes content in pages below will be covered in final examination? Coz I am not sure about these contents are related to lectures topics or not. Thanks.

1. Soybean paste 

NO

2. Clostridium difficile 

NO

3. Notes P.34-40 Protozoa and parasitic worms

NO

4. Notes P.41-45 fungal toxins in foods 

NO

5. Notes P.101-106 controversial preservation: nitrite,nitrate,antibiotics and food irradiation 

NO

6. Notes P.107-111 legal considerations and food safety programs

Yes, we have discussed HACCP. HACCP is included.

Question from a student 20170502a

Q&A:
 I would like to ask for the answer of B2 and B3a in 14-15 pastpaper. For B2, is the most suspected food Roast Turkey?

No. According to differences in attack rates and incubation period, and other evidences, it is probably Pizza and Staphylococcus aureus.


 For B3a, drying means heat drying that lowers the aw? For the comparison made, that means in this case, we have to point out lowering pH is more effective in preserving food? 

Yes, lowers Aw and water content.

No, lowering pH is not more effective in preserving food.



Lastly, can I answer all the questions in exam? Thanks a lot!

Yes, but if you answer all the questions, I will only count the number of questions required which have higher marks.

Some more Q&A from another email:


Also, as I see from the approach of doing 14-15 B4(e), the last question about justifying the answer and explain, that means we have to first find out the D value at 120°C, then calculate the treatment time required for both bacteria in regular blot ( spoilage bacteria: 10D x 0.0079= 0.079min, spores: 9D x 0.25= 2.25min), both bacteria in contaminated blot (spoilage: 0.1027min, spores: 3mins), then we use 3 mins at 120 degree celsius as the new time, then we follow part a to c to calculate the corr. D values, cell densities for both bacteria, then we come with conclusion to see if new method is ok. Am I correct?

You have to carry out all the calculations and answer both (i) and (ii). Your calculation to reach the two treatment times already shows whether the times you concluded to use are long enough to achieve the goals. "Justify" means you have to indicate why the times you use are good ones, "explain" means you need to give some details on your calculation and justification.

Question from a student 201617, 20170501a

Q:

Dear Professor Kwan,

I have some problems regarding the past papers. Thank you for your attention.

‘What are the number of D’s targeted in the standard canning process for each of these microorganisms?’ has been asked in 2013-2014 and 2014-2015 past papers. However, as I check the answers from the blog, I find that 2 different approaches are used to answer the same type of question.

1) 2013-2014: Use ‘time in standard canning process/D value for each microorganisms’ for calculation

2) 2014-2015: Use data of ‘approximate densities of microbial flora in the ham before canning’ and ‘acceptable cell density in canned ham’ for calculation

Which one should we use in the examination? The question is asking about the ‘real target of the canning process that can be achieved’ (1) or the ‘ideal target of the canning process’ (2)?

Can you please also help me check the answer of the question below? Thank you very much.

2013-2014 B5 (d) Cell densities of the microorganisms of the regular lot of ham after canning with the new process

	New D value at 120C	No. of D	Cell densities
Spoilage bacteria	0.0025	3min./0.0025=1200	100*10^(-1200) =10^(-1198)/ml
Bacterial spores	0.25	3min./0.25=12	10*10^(-12) =10^(-11)/ml
Fungal spores	0.05	3min./0.05=60	1*10^(-60) =10^(-60)/ml

Since 10^(-11)/ml < 10^(-8)/ml (acceptable cell density in canned ham), the new process can replace the standard canning process.

A: 

‘What are the number of D’s targeted in the standard canning process for each of these microorganisms?’ has been asked in 2013-2014 and 2014-2015 past papers. However, as I check the answers from the blog, I find that 2 different approaches are used to answer the same type of question.

1) 2013-2014: Use ‘time in standard canning process/D value for each microorganisms’ for calculation

2) 2014-2015: Use data of ‘approximate densities of microbial flora in the ham before canning’ and ‘acceptable cell density in canned ham’ for calculation

The calculation should use 1). 2) was a mistake I made during discussion in review session or email responses this year.  

Which one should we use in the examination? The question is asking about the ‘real target of the canning process that can be achieved’ (1) or the ‘ideal target of the canning process’ (2)?

Should be (1) the ‘real target of the canning process’ (1)

In fact, don't mind whether it is "real or ideal target", as long as the question asked about the standard process, use the standard process to calculate the D value numbers.

Can you please also help me check the answer of the question below? Thank you very much.

2013-2014 B5 (d) Cell densities of the microorganisms of the regular lot of ham after canning with the new process

	New D value at 120C	No. of D	Cell densities
Spoilage bacteria	0.0025	3min./0.0025=1200	100*10^(-1200) =10^(-1198)/ml
Bacterial spores	0.25	3min./0.25=12	10*10^(-12) =10^(-11)/ml
Fungal spores	0.05	3min./0.05=60	1*10^(-60) =10^(-60)/ml

Since 10^(-11)/ml < 10^(-8)/ml (acceptable cell density in canned ham), the new process can replace the standard canning process.

Correct

thank you for pointing out the inconsistency that was a mistake in the review session this year.

Question from a student 20170429b

Q:

I am trying 2014-15 past paper and have the following question on B4(e): I would like to know what is expected for "justify and explain" after calculating the required values. 

A: 

“justify and explain” means you need to justify the bases for your making the conclusion--which should be based on deduction and elimination. Also you need to explain how the conclusion was derived. In this case, you calculate and get some numbers. You need to explain how you use the numbers to come to the conclusion.

Question from a student 20170429

Q:

From: LLLLL

Dear Prof. Kwan,

Will ALL the topics of the 3 sets of microbiology notes be covered? Are there any exceptions?

Best regards, 

A:

Only the topics covered in the lectures will be included in the examination. Of course, the materials in the sets of notes under the topics will be included too.

Questions from a student 20170425

From LLLLL,

I have come across the following problems during my revision. I hope you don’t mind that the questions may be a bit long. Thank you very much.

1.               From the notes I jotted during lesson, I have written down Clostridium perfringens can survive in oxygen but can’t grow in it, while Clostridium botulinum can’t survive in oxygen. I am wondering if I have jotted notes wrongly as they are both obligate anaerobes. So what is ‘obligate anaerobe’ actually?

A. "obligate anaerobic": organism that can only grow or survive in anaerobic conditions and not in aerobic conditions. Those die in aerobic condition can definitely NOT able to grow in aerobic condition.

2.               Regarding 2012-2013 past paper, I have checked the answers in the blog that ‘Aspergillus oryzae’ may most preferentially grow in bagel with redox potential -50mV. However, on notes p.95 about characteristics of molds, molds are obligate aerobes and they require high redox for growth. I don’t quite understand why molds can grow in the bagel with such low redox-potential. Does it mean that the bagel actually rely on the oxygen in the atmosphere to grow, but not rely on the bagel?

A. Molds are not all obligate aerobes. They can ferment and grow on sugar. P.95 refers to spoilage fungi that are obligate aerobes growing on organic acids. It is unclear and needs to be updated.

3.               Regarding naked virus and enveloped virus, naked virus has a cell wall while enveloped virus does not? Cell wall=protein coat? If yes, enveloped virus also has a protein coat… If not, then what is the main structural difference between them except the envelope membrane?

A. Naked virus has protein coat, not cell wall. Enveloped virus has both membrane and protein coat. Membrane is the main structural difference between them.

4.               According to the factsheet of WHO dated 2002, Salmonella was considered as one of the emerging foodborne pathogens. Is it not applicable nowadays?

A. In some cases such as relating to produce, Salmonella is emerging. Emerging is not a tight definition.

5.               Some questions about endospore-forming bacteria

è Take the preparation of fried rice as an example (Bacillus cereus):

a) rice cooking activates endospore germination (if cooking temperature is high enough, endospores can possibly be eliminated? Since I remember in the case of roasting turkey, you have mentioned that the cooking temperature may not be high enough to kill the endospores. That means cooking at improper temperature will activate spore germination, while at the same time can’t kill the endospores?) A. Yes

b) cooked rice that is held under room temperature favours growth A. Should be Germination of endospores and secret toxins

c) cooked rice is then fried at temperature that is not high enough to kill the endospores thus become problematic (or should it be temperature is not high enough to destroy the toxins?)  A.  Toxin is a problem and also vegetative cells after germination.

Do the events above correctly describe the situation?

6.               Will the final examination cover the content below?

-               Soybean paste

-               Clostridium difficile

A. No.

Sorry for asking so many questions. Thank you very much for your attention.

Question from a student 20170426

Q:  I would like to ask whether we could bring the printed powerpoints for the final exam (besides the 3 sets microbiology notes)?

A:  

All paper materials can be used in the open-book examination.

Review session 2

Question from a student 20170423

TO BE ANSWERED IN REVIEW SESSION 2 ON 24/4/2017 MONDAY

From CCCCC, a question on the exercise..........................

The student requested email answer and not on the blog. So I deleted her email here and I will work the question in the review session with you students and then I will send my answer via email to the student.

The exercise is as follows:

	a.	45 of the patients joined a company picnic together with another 115 employees.  Lunch was served during the picnic: Menu—Pizza (with toping of  ham, olive, pineapples, and green pepper), shrimp salad, roasted turkey, and bread.
	b.	Lunch in the picnic was served at 12:00 noon on Saturday.
	c.	45 patients started to have diarrhea at  2:00pm on Saturday followed by severe vomitting.  They went to the hospital immediately.  The last patient went to the hospital at 4:00pm on Saturday.
	d.	The hospital laboratory detected low levels of Salmonella enterica subspecies typhimurium (serotype O14) from the feces of 15 patients.
	e.	The laboratory of the Health Department detected low levels of Salmonella enterica subspecies typhimurium (serotype O14) and low levels of Staphylococcus aurues from the shrimp salad that was left over from the food served during the picnic and then refrigerated after lunch.  Low  levels of Clostridium perfringens  spores were found in the vegetable stock, part of  which was used to make the turkey stuffing.  No bacteria or spores were found from bread or pizza.
	f.	The laboratory traced the sources of the food.  Some dough was collected from the pizza house which supplied the pizzas for the picnic.  No bacteria were found in the dough.
	g.	The turkey and bread was prepared in the morning and transported to the picnic area without refrigeration.  The shrimp salad was also made in the morning, stored in a foam box with some ice and transported to the picnic area.  The pizzas were baked right before lunch time and transported to the picnic area while they were kept at 65-70°C.
	h.	Most of the patients recovered after one day of rest.

        

i.	The foods consumed at breakfast and lunch and the illness records are listed in the following table, calculate the attack rates.  Which food would be the suspected food?  Explain briefly.                                                                                                       (4 marks)

Foods served	Ate				Did not Eat
	Well	Ill	Total	Attack rate	Well	Ill	Total	Attack rate
Pizza	78	10	88		37	35	72
Shrimp salad	56	33	89		51	10	61
Roast turkey	78	25	103		30	20	50
Bread	96	40	136		19	5	24

ii.	Using the information provided,  suggest a hypothesis to describe the outbreak.  Describe the implicated microorganism(s) involved, their transmission, food implicated, and events that caused the outbreak.  Explain your answer.                                 (12 marks)

iii.	How could the outbreak be prevented?                                                                  (4 marks)