Aerobic Phenol Red Dextrose (PRD)
Anaerobic Phenol Red Dextrose (AnPRD)
This media is very simple in nature since it contains basic nutrients for bacterial growth and a colorimetric pH indicator. It has been designed to detect acid producing bacteria (APB) in both aerobic and anaerobic environments. If APB are inoculated in this media, these microorganisms will start growing, and due to the organic acids, they produce as part of their metabolism the media will become more acidic, bringing the pH down and changing the color from red to orange and yellow. It could take up to two weeks to see a major change in color due to differences in salinity and bacterial concentration. Even though most APB are facultative organism there are some species that prefer more anaerobic environments, so AnPRD will provide a perfect environment for these organisms.
Frequently Asked Question
1-What is considered a false positive for this media?
A false positive is considered a vial that turns yellow immediately after a sample is inoculated. This sudden color change is due to acidic water from the sample and not from bacterial growth. There is the possibility that there are bacteria in the sample but making the distinction could be challenging since turbidity will become the main form to determine bacterial growth. If this happens we recommend measuring the pH of the sample to determine how acidic it is before continuing with a serial dilution. Biotechnology Solutions TX can also provide a more in-depth analysis to determine bacterial activity in water samples with low pH.
2-I inoculated PRD vials, but they are just turbid and no color change has happened. Why is that?
When PRD or AnPRD become turbid with no color change it usually means that there are general heterotrophic bacteria (GHB) in your system and not a significant concentration of acid producing bacteria (APB). GHB are pretty much everywhere and they will take advantage of the nutrients in the media. There are even some species of fungi that could grow in this media and won’t show a color change. These bacteria do not represent a threat by themselves, but they could work with other kinds of bacteria that could potentially cause MIC problems.
3-I have noticed that some PRD and AnPRD vials have different shades of red, why is that? Is there something wrong with the media?
There is nothing wrong with the media. Both PRD and AnPRD are submitted to rigorous quality checks before they are shipped out to our customers, so they’ll always experience a color change when inoculated with the right kind of bacteria. Phenol red, which is the pH indicator, is sensitive to temperature changes, so any small variation during the process of sterilization, shipping and storage can cause the color to have different “shades” of red. It is important to remember that higher percentages of TDS tend to have darker “shades” of red as well. However, if you think the product you just received is too orange, or too magenta, please feel free to contact a Biotechnology Solutions TX representative and we’ll be happy to assist you with any extra questions you may have.
4-Which media should I choose if I don’t know too much about the conditions of my system/field/well?
If the conditions of the field are not well known, it is recommended to acquire both media and inoculate them with the same sample in order to determine which kind is more prevalent. However, if a choice still needs to be made, starting with PRD is better. Most acid producing bacteria are facultative and can grow under aerobic and anaerobic conditions, AnPRD is more specific for organisms that will have a difficult time growing under aerobic conditions.
5-I performed a serial dilution, but I noticed the color change skipped one vial. What does it mean?
There many factors that could cause a vial to skip the color change in a serial dilution. The most common ones we have encountered are human errors. Sometimes one vial is skipped unintentionally from the serial dilution, or there is air left inside the syringe while performing a serial dilution with anaerobic media. Residual biocides can also affect growth, specially in the first vials of the dilution series. There is also the possibility of a statistical anomaly, which means that all the bacteria got transferred from one vial to another without being diluted. To avoid this, we recommend being consistent with the mixing while performing a serial dilution.
6-Can I incubate this media at high temperatures in order to mimic the conditions I find in my system?
PRD and AnPRD are heat sensitive, so the pH indicator that is responsible for the color change will get significantly affected by high temperatures. It is not recommended to incubate this media at temperatures higher that 45°C for a prolonged period of time. Most APB will grow significantly well at temperatures ranging from 28°C to 35°C, but if it is needed to incubate at high temperatures just make sure a vial is kept intact as a negative control so it could be easy to compare any possible positive vials to the untouched vial.