What is the difference between Bioburden and Microbial limit testing (MLT)?

Dear Friends,

In microbiology, we generally use two terms, bioburden and Microbial limit test (MLT). Do we know the difference between Bioburden and Microbial limit testing? Here is a little explanation about Bioburden and MLT testing which you can understand very easily.

Bioburden: The term bioburden means total bioload (living microorganisms) on the surface or within the material. It is a combination of bacterial count and fungal count and generally TVAC (total viable aerobic count) term is used for that. Bioburden is performed for non sterile material, unfiltered or filtered solutions. Bioburden is a quantitative testing in which we only detect number of colony forming units (cfu).

Membrane filter showing colony forming units

Microbial Limit Testing (MLT): Microbial limit testing comprises of detection of total aerobic microbial count (TAMC) and total yeast and mold count (TYMC) seperately in the material. MLT is performed for non sterile materials. MLT also includes detection of specified pathogens which means MLT is quantitative as well as qualitative testing. Quantitative testing is related to only the counting of number of cfu's and qualitative testing is related to detection of specified microorganisms in which we only detect whether the specified microorganism is present or absent.

Mannitol salt agar showing growth of Staphylococcus aureus

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"I am a Microbiologist"

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I am a Microbiologist

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What is 50:50 and hot spike method in BET testing and their importance?

Bacterial Endotoxin Testing is very important test for parenteral preparation. In BET testing generally two methods are followed as mentioned below.

1. 50:50 method
2. Hot spike method


1) 50:50 Method: 

Let's discuss about 50:50 method. In 50:50 method, in negative product control (NPC) sample volume and LRW volume are taken as 50 µL. If we prepare solution of MVD/2 then final results will be given at MVD because after adding same volume of LRW e.g 50 µL and the concentration reduced to half. That's why in 50:50 method we generally perform the BET at MVD/4 or MVD/2 dilution to get the results at MVD/2 or MVD because sample can not be diluted beyond MVD.

2) Hot Spike Method:

In hot spike method,  100 µL sample is required for analysis in negative product control (NPC). Sample is not diluted in hot spike method as there is no addition of LRW in negative product control (NPC). It means if we take MVD/2 sample then testing will be performed at MVD/2. If we take MVD/4 sample then final results will be given at MVD/4. Hot spike method is generally used in the water sample analysis. 

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