One in four plates grew nothing at all.
A strict anaerobe, thirty plates per medium, and a result that would have been reported as a negative culture.
The result that started this
We plated a standardized suspension of Porphyromonas levii, a strict anaerobe, onto thirty replicate plates of each of three Brucella agar preparations, incubated all of them anaerobically for 72 hours, and counted every colony.
On the aerobically manufactured Non-PRAS plates, eight of the thirty grew nothing at all. Not few colonies. None. Twenty-six of the thirty grew fewer than five.
On OxyPRAS Plus, every plate grew, and none yielded fewer than nine colonies.
The organism was there every time. The inoculum was the same suspension. The incubation was identical. The only variable was the plate it landed on.
A plate that grows nothing does not report low recovery
It reports no growth. And in a clinical workflow, nothing distinguishes that plate from a genuinely negative culture. There is no discrepancy to investigate, no repeat triggered, no complaint filed. The organism simply does not appear, and the report goes out.
The failure mode is invisible from inside the laboratory. A missed anaerobe leaves no evidence unless a Gram stain happens to contradict the culture.
It also has a downstream cost. An organism that never forms a colony cannot be identified and cannot be susceptibility tested, so there is nothing for antimicrobial stewardship to narrow therapy to. The patient stays on broad empiric coverage against a culture that reads negative for the organism actually present.
B. fragilis is the wrong organism to validate on
The study ran three organisms spanning the oxygen-tolerance spectrum, and the pattern is the point. The more oxygen-sensitive the organism, the more the medium decided whether it grew.
For aerotolerant Bacteroides fragilis, the three media performed comparably and no comparison reached statistical significance. For moderately sensitive Fusobacterium nucleatum, the unprotected plate lost 35 percent of recovery. For strict-anaerobe P. levii, it lost 82 percent.
A media comparison run on B. fragilis alone would have concluded that all three preparations were equivalent.
And B. fragilis is the organism most laboratories reach for, because it is robust and easy to keep. The properties that make it a convenient QC organism are exactly the properties that make it uninformative for this question.
Post-reduction narrows the gap. It does not close it.
One arm of the study took the aerobically manufactured plates and held them 48 hours with an oxygen absorber before plating, which is what many laboratories already do.
80 percent of the F. nucleatum shortfall came back
For the moderately sensitive organism, the post-reduced arm became statistically indistinguishable from the pre-reduced medium. This is a real remedy, and more effective than we expected.
Only 36 percent came back for the strict anaerobe
Three plates in thirty still produced no growth at all. How adequate the hold is depends entirely on what you are trying to recover.
A controlled step with its own failure mode
A 48-hour hold adds a documented process, a hold time, and a way for the workflow to go wrong, to a bench that is already busy.
What is in the full paper
Download the full white paper
Fifteen pages, plus a six-page supplement with the full methods and every one of the 270 individual plate counts, so you can check the analysis yourself.
- White paper, 15 pages, PDF
- Methods and data supplement, 6 pages, PDF
The paper opens the moment you submit. No inbox hoops, and no sales call unless you ask for one.
OxyPRAS Plus
OxyPRAS Plus Brucella agar combines a traditional pre-reduced anaerobically sterilized base with the Oxyrase enzyme system, an active oxygen scavenger built into the medium. The enzyme removes oxygen by consuming it, rather than by gas displacement or chemical reductants.
Plates arrive ready to streak, with no post-reduction step required before use.
The questions we get asked
Is this vendor data?
Yes, and the paper says so on the funding page. That is also why all 270 individual plate counts are published in the supplement rather than summarized. You can recompute every number in the paper, and we would rather you did.
What was the comparator?
A Brucella agar built to the same formulation with the reducing agent omitted, prepared in house and manufactured and packaged aerobically. It is disclosed in the methods. This is a study of reduction state, not a head-to-head against a named commercial product.
We already hold plates anaerobically before use. Does this apply to us?
That is the post-reduced arm in the study, and it did help. A 48-hour anaerobic hold recovered 80 percent of the F. nucleatum shortfall. But it recovered only 36 percent of the P. levii shortfall and still left three plates in thirty with no growth.
Why P. levii? We do not chase that organism.
It was used as an oxygen-sensitivity challenge organism, not a clinical target, and the paper says so. The argument is that a medium that loses P. levii will also struggle with the fastidious organisms you do chase.
How long were the plates incubated?
Seventy-two hours, which matches the 2 to 3 day growth window in the product insert. Extended incubation was not tested, and that is stated as a limitation.
Can we try it?
Yes. Request an evaluation and we will send plates for your own bench, with your own organisms and your own workflow.
The rest of the evidence
This paper is one study. If you are working through a media decision, these are the other places to look, including the ones where we are not the only party with data.
The full library
Every Oxyrase white paper, including the oxygen-exposure study on protected and unprotected Brucella agars.
Browse the papers → Comparison studiesPublished side by side
Recovery comparisons across media and methods, including work published by laboratories other than ours.
See the studies → Media typesHow anaerobic blood agars differ
What separates a pre-reduced plate from a gas-flushed or conventional one, and what each approach costs you at the bench.
Compare the types →Run the comparison on your own bench
We will send OxyPRAS Plus plates for evaluation with your organisms and your workflow, at no charge and with no obligation on the other end. If they do not outperform what you use now, you have lost an afternoon.
Talk with a rep · +1 (419) 589-8800 · r.garrison@oxyrase.com