What is the positive control and negative control?

[ilovebiology77]

I'm looking at whether the sugar content affects the shelf life in yoghurt.
I will count the number of lactic acid bacteria in each sample.
The more lactic acid bacteria, the longer the shelf life.
My hypothesis is that the higher the sugar yoghurt, the more lactic acid bacteria (preserves yoghurt).
I have one yoghurt with high sugar content and another with low sugar. This will be my experiment.

But I'm a little confused as to what my positive control and negative control would be?

Would appreciate the help!

Many thanks!

[JorgeLobo]

Can you elaborate on why the level of lactic acid bacteria determies shelf life? I've read that pH is a primary determinant as maintenance of a low pH inhibits growth of potential spoilage bacteria and that pH is established and mainained by the lactic bacteria present.
For controls, you;d have to know the variable involved in sopilage - perhaps eliminating lactic acid bacteria, buffering the pH toward neuatrality, etc.

[ilovebiology77]

Hi there,

Many thanks for replying!

Hmm I thought that if the numbers of lactic acid bacteria are higher, the more lactic acid produced. But... maybe this idea is flawed...

The one you mentioned for a control 'buffering the pH toward neuatrality' sounds really good, would this be a positive or negative control?

[JorgeLobo]

If you decided that was the critical variable, pH buffering could be both - buffered low as positive control and buffered to neutrality for a negative control.

[JackBean]

the bacteria eat sugar, so theoretically, the more bacteria, the less sugar there should be. But that also depends on how long they were living in it and what was starting concentration of sugar.

[JorgeLobo]

Perhaps the greatest number of bacteria would have consumed the carbohydrate and associated at the time of sampling with a lesser - perhaps - the least amount of sugar. A high bacterial titer would best indicate a greater level of carbohydrate at some time in the past.
But the number of bacteria would be secondary if shelf life is only a fnction of pH - though some lactic acid bacteria do produce bacteriocins..

[ilovebiology77]

Hi again, thanks for the reply.
Your suggestions for the positive and negative control are really helpful but I think they make more sense if I was counting the actual spoilage bacteria. I'm counting the lactic acid bacteria so from what I understand about negative and positive controls, I need to know one factor that will definitely increase it and one that will inhibit growth.
LAB is pH sensitive and doesn't function at v.low pH according to research.
So I'm planning on using a buffer solution of pH 2 for negative control. Would this make sense?
I'm confused about the positive control. I'm not sure what it'd be.
Do all experiments require a positive control?

[JorgeLobo]

Just what benefit or criterion are you evaluating? As they establish the pH that preserves the product - why would you consider lactic acid bacteria per se "spoilage " bacteria? As you say, laxctic acid bacteria do not grow and slowly die off at pH around 4 . Suggest pH 2 is unrealisticaly low (you should shoot for pH of the product 4-4.5) but in any case this would be the positive control The negative control would be one buffered to neutrality that would fail preservation and spoil.

[JackBean]

The question is, what exactly do you want to check by your controls?

[ilovebiology77]

Ahhh I see, you're controls make sense now!

Thanks to both of you, you helped a lot !