Recently ran some environmental samples based off locally collected river water. Samples were filtered and cultured first on a MacConkey agar with subsequent growth being selected and then streaked onto TSA plates. From there bacteria were identified via gram staining and MicroScan with several being Staphylococcus species. Is this possible despite MacConkey agar being said not to be able to support Gram positive bacteria?
MacConkey agar (without crystal violet) will grow enterococci, and most staphylococci. Colonies tend to be smaller than on BA and of course may be pink (enterococci are pink/red on MacConkey).
Obviously, this medium can't support gram-positive bacteria especially, S. aureus due to the presence of bile salts and crystal violet, which disrupt their cell walls. However, some gram-positive bacteria, such as Enterococcus and Streptococcus, can still grow on MacConkey agar and Staphylococci produce pale pink to red colonies and enterococci produce compact tiny red colonies either on or beneath the surface of the medium.
Thank you, everyone! I had gone back to check more of the literature and the agar ingredients and crystal violet was present.
Yes, there are instances in the lab where you may see gpo growth on a Mac plate. Usually it is just a bad batch of media or a few bad plates or old plates. I have seen Staph, Dips, and I believe even Enterococcus species show up on a Mac. It is not the only selective media to make mistakes either. Gnb will show up on CNA and PEA and chromogenic agar particularly for MRSA have had white colonies that were in fact Saur. It is rare, but again, it is usually because of a bad batch of media.
- Badges
- Science topic
- Similar topics
- Phytochemistry
- Extracts