Why is Chloramphenicol problematic for microbial testing on cannabis and hemp?

Chloramphenicol is an antibiotic used in DRBC plates to prevent bacteria growth. Unfortunately, chloramphenicol also inhibits the growth of dangerous fungi (Aspergillus, Penicillium, and Fusarium)

The State of Michigan is in the process of scrutinizing molecular methods and culture-based methods for the detection of Total Yeast and Mold (TYM) on cannabis.

They chose to run this study on DiChloro Rose Bengal with Chloramphenicol (DRBC) plates.

Chloramphenicol is an antibiotic. This can be helpful to eliminate the bacteria which can form obscuring colonies in 24 hours while yeast and molds can take 5-10 days.

Unfortunately, chloramphenicol also inhibits the growth of fungi and it inhibits the most dangerous fungi on cannabis (Aspergillus, Penicillium, and Fusarium). Below are peer-reviewed studies that demonstrate this effect:

Comparing DRBC to PDA and qPCR

To confirm the peer-reviewed literature, we acquired organisms from ATCC and plated them on
Potato Dextrose Agar (PDA) and DRBC.

 

ATCC glycerol stocks performed better than Microbiologic pellets.
Microbiologic desiccated pellets required more time to grow on DRBC (12 days)

As expected the Aspergillus species either did not grow or took 12 days to grow on DRBC. PDA was able to detect all the Aspergillus species. qPCR failed to detect Rhizopus oryzae.

Aniger 1-10 dilution 4 days-1

Aniger 1-10 dilution 4 days-2

Aniger 1-100 dilution 7 days-1

Plating Results-1-1

Aspergillus and Penicillium are some of the most commonly found pathogens on cannabis, making the results of these experiments relevant and concerning for the industry. Below are peer-reviewed studies that confirm the presence of these species on cannabis. 

Conclusions

These data demonstrate that using DRBC as a gold standard for benchmarking other TYM detection technologies is flawed. The failure of DRBC to detect the most dangerous human pathogenic organisms found on cannabis was noted with organisms ordered directly from ATCC. This problem is further exacerbated in real-world cannabis samples as these organisms are usually endophytes which live inside the plant. It is difficult to extract Aspergillus from within the cannabis plant and still maintain its viability. However, in this case, even if the Aspergillus were viable, it wouldn’t grow on DRBC.

The use of chloramphenicol selection in cannabis microbial testing presents a clinical risk to cannabis patients and patient safety.