Pharma Microbiology: Are All TSA Agars Really the Same? Find Out Here!

Disclaimer: This article isn’t a peer-reviewed scientific publication. It’s “just” feedback from a lab experiment.

This article objective is to get us thinking about our practices. Enjoy the read! 😉

During our visits to laboratories, we’ve often heard:

“All TSA agars are the same; the formula is in the pharmacopoeia.”

Well, think again!

This time, we have proof that not all TSA agars are created equal.

Let’s dive deeper into this in the article below.

Agar Quality: A Key Factor Confirmed by a Study at a Leading Pharmaceutical Company

After publishing our article about the impact of the hockey stick spreading on microorganism enumeration (showing a loss of 80% of Gram-negative bacteria after just 20 seconds of raking), we received many messages.

One comment, in particular, caught our attention. It came from Stéphane Guyonnot at Sanofi, who wrote to us:

“A few years ago, we conducted a similar study on different types of TSA.

The recoveries of Pseudomonas aeruginosa after 15 seconds of spreading varied from 0% to 90% depending on the supplier”

This instantly caught our attention!

We reached out to Stéphane to learn more.

Impact of Hockey Stick Spreading Duration on Microorganism Enumeration

We’re lucky because Stéphane is a SuperMicrobiologist who loves to share his experience.

He shared the results of a study conducted in his lab in 2020.

This study was part of the validation process for a supplier of calibrated strains used in fertility tests for culture media.

As part of the validation, and to ensure standardization, Stéphane wanted to check if the duration of raking impacted microorganism enumeration.

Here are the results:

• Strain: Pseudomonas aeruginosa (a Gram-negative bacterium known for being the most sensitive).
  
• 3 independent trials, 3 replicates per trial.
  
• TSA agar from Supplier A.

N° tRIAL	5 seC SPREADING	10 sec SPREADING	15 sec SPREADING
Trial 1	35	39	0
Trial 2	32	39	0
Trial 3	28	32	0

These results confirmed what we had observed in our article: after 15 seconds of raking, all P. aeruginosa were lost.

Second Test: When a Different Agar Changes Everything

But Stéphane didn’t stop there.

He tested a TSA agar from another supplier, which had different properties (shelf life, storage conditions, and agar volume).

Here are the results:

• Strain: Pseudomonas aeruginosa
  
• 3 independent trials, 3 replicates per trial
  
• TSA agar from Supplier B

N° TRIAL	5 sec SPREADING	10 sec SPREADING	15 sec SPREADING
Trial 1	42	40	40
Trial 2	46	44	46
Trial 3	43	41	42

This time, the raking duration had no impact on enumeration.

So, perhaps the raking tool isn’t the only culprit?!

The Physical Properties of Agar Make All the Difference

This suggests that the agar’s properties allowed the bacteria (P. aeruginosa) to recover more easily after spreading and grow better.

According to Stéphane, the physical properties of agar play a critical role.

A Petri dish with more agar, higher Aw, and better raw materials likely provides a less stressful environment. This helps bacteria resist the stress of raking and recover to multiply.

Conclusion: Challenge Your Agars!

Producing good agar isn’t easy. It might even be the most complex product to manufacture in microbiology.

As this study shows, even though they share the same “pharmacopoeia formula,” the two TSA agars didn’t deliver the same performance.

Making agar is a bit like cooking: everything depends on the choice of quality ingredients and the expertise of the maker. And not everyone is a three-star chef!

These results show that choosing a culture media supplier is not a decision to take lightly. If you want to go further, we recommend reading this article, which explains how to select the right agar for your environmental controls.

A big thank you to Stéphane for this fantastic experience sharing!

If you have similar insights to share, don’t hesitate to send them to us. 😊