Webinar Highlights: Choose Your Membrane Wisely: Selecting the Best Filter for Your Bioburden Testing Application
23 Mar 2017
Learn how you can find the best membrane for your bioburden testing application
In a recent webinar, Tommaso Ronconi, Global Product Manager, In-Process Testing, Merck KGaA, Darmstadt, Germany, revealed some of the secrets of membrane filtration.
Over the years, membrane filtration has become a standard for microbial enumeration of colonies in the pharmaceutical and beverage industries. This webinar provided important information on how to select the best membrane for your bioburden application, as well as how to deal with hard to filter matrices and antibiotics.
Read on for highlights from the Q&A session, or if you missed it, watch the webinar on demand.
Q: I am using a reusable stainless steel funnel for my microbial tests, why would I switch to a single-use device that will hurt my budget?
A: There are two considerations. First, reusable funnels take much more time to set up on the manifold, if you include decontamination and flaming - you need to flame the inside, flame the base, the manifold head and cool it down with sterile water - which obviously impacts the number of tests which could be done daily or during an afternoon.
The second consideration - if we consider all costs, we must also include the autoclave runs, water, energy consumption and eventually, as well, motivation of the system.
On the other side, screens for recycling funnels are really evolving right now. If we consider this whole picture I still think that single-use is the best choice, also in order to protect results, as we have seen in the webinar, and ensure the highest reliability.
Q: Can you elaborate about the antibiotics neutralization during suitability testing?
A: It is important to select a low-binding membrane, for example a PVDF membrane. What we fear here is that the active ingredient in the antibiotic could slow down or inhibit the growth of the samples. It is also very important to rinse the membrane at least three times with 100 ml, just to make sure that all the sample’s residues are removed and that we retain only the microorganisms that we would like to grow. These are two very important aspects – membrane type and also the rinsing aspect.
I encourage you to contact your sales rep to define what we can do based on your application.
Q: During suitability and qualification, does it matter if 1 x 100 ml or 3 x 100 ml is used to rinse the ATCC bacterials used?
A: The Pharmacopeia advises not to exceed five times 100 ml/filter based on the sample type (water, antibiotic, in-process). For suitability studies, you also need to inoculate the same ATCC MO using the same media as a positive control. If you don’t see any visible growth it means that the antimicrobial activity of the product is still present, and therefore you have to adapt the rinsing/or the method.
Q: Why does the 0.45 µm filter have a higher retention than the 0.22 µm filter?
A: This is a very important topic. If we only consider the capabilities of the membrane to retain the microorganisms on the surface, the 0.22 µm has a higher retention compared to the 0.45 µm. But, in microbiology we want to retain and also grow the microorganisms in order to enumerate them. 0.45 µm is the best option for this, as it provides a 4-log reduction and then we have the possibility to grow the microorganism. This is why the 0.45 µm is advised as the best choice, because not only can we retain all the bacteria, we can also grow to enumerate them. The results we have seen show the 0.22 µm is not as good as the 0.45 µm.
Q: When you showed PVDF membrane is symmetric and PES membrane is asymmetric, what do you mean by that?
A: I’m talking about the membrane structure and the pore distribution. For example, with PVDF, normally pore distribution is more or less a line on the thickness of the membrane – you have the same size of pore on the surface and in the internal part of the membrane. On the PES it’s a little bit different because the pores are more open on the surface and at the end they are more stretched. It doesn’t really impact the results because when a membrane is graded as 0.45 µm the result will be the same.
In laboratory applications, PVDF is more interesting in terms of recovery, even if PES membranes might give a better flow rate. What is important to mention to fully answer this question is that, at the end, when the membrane is graded 0.45 µm, whether the membrane is symmetric or asymmetric has rarely a strong impact on the microbial results.
Q: Is it possible to construct a filter for industrial use (200 liters of product), beginning with a large pore size 1 µm outermost layer, to 0.45 µm middle layer and ending in 0.22 µm innermost layer, so that filter clogging is avoided?
A: Many options are possible to adapt the sizing of a clarification line, and this will depend on the clogging index of the product, the turbidity, the flow rate and the expected results in terms of bioburden reduction or sterility. What you propose is possible, starting with a depth filter as the first 1 µm stage. Filterability and sizing tests like Vmax/Pmax/Tmax would be advised to determine the appropriate filter type and pore size.
Q: Is it possible to use membrane filtration for creams or shampoos for example? How should I proceed?
A: It is advised by USP Chapter 71 to use membrane filtration whenever it is permitted. It is true that filterability might be a concern. We recently conducted some trials and it took a while to find the correct protocol. You have to prepare 100 ml saline at 40°C and keep it for one hour after adding 1 ml of shampoo, then you add 0.5% of Polysorbate 80 emulsifier. After one hour, the shampoo will dissolve and the solution will be filterable on the MC membrane. Again, that needs to be prepared with a PH and saline solution. This should also work with other types of solution, like creams for example.
Q: Any recommendations for filtering oily products, such as sesame oil?
A: Difficult to answer since oil could be really tough to manage, but I’d find it difficult to filter more than 1 ml, diluted in 20 ml of saline and 0.5% of Polysorbate 80. You can vortex the solution before adding enough Fluid K to make it 100 ml.
Do not forget to use a low-binding membrane material, like PVDF!