What is MLT in Pharmaceuticals?

MLT is “Microbial Limit Test”.

This test provides the estimation of the number of viable aerobic microorganisms present and for freedom from designated microbial species in pharmaceutical articles of all kinds, from raw materials to the finished forms.

This test is designed to determine total aerobic microbial count and yeast and mold count. This test demonstrates that the product is free of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa , Salmonella , C.albicans and A.niger. This test takes 7-8 days.


The qualitative phase of testing examines samples per U.S. FDA criteria for “objectionable organisms.” For minimum coverage, these criteria require testing for the following:

  • The bacteria Escherichia coli (E. coli)
  • The bacteria Staphylococcus auseus (S. auseus)
  • The yeast Candida albicans (C. albicans)
  • The bacteria Pseudomonas aeruginosa (Ps. aeruginosa)
  • The fungus Aspergillus niger (A. niger)
  • The bacteria Salmonella

The first step in Microbial limits testing is the preparatory testing which validates the product with the method used. This ensures the product has no inhibitory effect on the recovery of organisms from the product.

Once the product has passed the Preparatory Test, the product is validated for routine screening. The routine Microbial limits screening test consists of the total aerobic and total yeast and mold counts as well as the test for objectionable microorganisms. If the total aerobic and total yeast and mold counts are acceptable and no objectionable organisms are found, Microtest then gives the “go-ahead” for swift release of the product or raw material.

Full microbial limits testing then inoculates specific organisms into general nutrient media. After allowing time for any growth, our microbiologists transfer the results to specific media that show differential reactions (usually via color changes and other morphological characteristics of the organisms) to the presence of a given organism.

One important aspect to keep in mind is every media has its own characteristic growth of an organism, there may be slight change in the organism morphology hence it is always advisable to use standard recommended media to be used for specific organism, and always avoid microbial interaction/mutation. If something visualized like this then it’s strongly recommended to discard the microbial culture.

Microbial limit validation test is required only once per product unless the product formula or manufacturing process has gone through a modification.

Reporting value of MLT is cfu /g (Colony Forming unit/ g).


Microbiological counts work on the principle that each single bacteria present will grow to produce a colony which will be visible to the naked eye after 24 to 48 hours (longer for yeasts and moulds). Then the number of colonies is counted, multiplied by the dilution factor and this gives us a count per gram of the organisms present.

The complication to this plan is that sometimes bacteria stick together in small clumps, even if mixed well, so when they multiply overnight the colony formed may be the result of more than one bacterium. We have no way of knowing whether a colony has actually come from one bacterium or from a clump of bacteria, hence the term colony forming units. The colony forming unit is the bacteria (single or clumped) that gives rise to a single colony on culturing. The colony count obtained may represent the number of single bacteria present, but it is more likely to include a proportion of clump.

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