What a culture can and cannot see

A culture grows what will grow

Culture is a beautifully simple idea. You take a sample, put it on a nutrient medium, hold it at body temperature, and see what multiplies. Whatever grows into a visible colony gets identified.

The catch is in the phrase whatever grows. An organism only appears if the medium contains what it eats, the atmosphere suits it, and it grows fast enough to form a colony before the plate is read. Plenty of organisms found in the vagina and urinary tract meet none of those conditions. Strict anaerobes die on contact with room air. Fastidious organisms need nutrients that standard media do not carry. Slow growers are simply not there yet when someone looks.

So a culture result is not a list of what was in your sample. It is a list of what was in your sample and was willing to grow under one specific set of conditions.

The threshold problem

Standard urine culture also carries a numeric cutoff, usually 100,000 colony-forming units per millilitre. That number has a history: it comes from work in the 1950s on kidney infection, where the goal was to separate genuine infection from contamination picked up on the way out.

It was never designed to answer the question “is anything unusual happening in this person’s bladder.” A sample with real growth well below that line is commonly reported out as no significant growth, which is accurate to the method and easily read as nothing found.

Sequencing reads DNA instead of growing cells

Sequencing skips the growing step entirely. It extracts genetic material from the sample and reads it directly, so an organism does not have to survive shipping, tolerate oxygen, or like the medium in order to be counted. Anaerobes and fastidious organisms show up as readily as anything else.

That is the real difference. Not accuracy, coverage. Culture asks which organisms are alive and cooperative. Sequencing asks which organisms left DNA behind.

Which is a genuine limitation

DNA persists after a cell dies, so a sequencing result cannot tell you that everything it names is alive and active right now. Culture can, because growth requires a living organism. That is a real advantage and it is why culture has not gone anywhere.

Sequencing also has to be told what to read. Short-read 16S looks at one bacterial gene, which is cheap and usually resolves organisms to genus rather than species. That distinction matters more than it sounds: several Lactobacillus species behave quite differently from one another, and a result that says only “Lactobacillus” has discarded the interesting part. Full-length 16S recovers species. And because 16S is a bacterial gene, it is blind to fungi entirely, which is why a separate fungal marker or shotgun sequencing is needed if yeast is meant to be part of the picture.

What this means in practice

A negative culture is a real and useful piece of information. It means nothing grew, under those conditions, above that threshold. It does not mean the sample was empty, and it was never designed to.

A sequencing panel gives you the composition: which organisms were present, and in what proportion. That is a different kind of answer, and what it means for any individual is a conversation with a clinician who knows your history. Composition on its own is not a diagnosis and cannot be one.

How we run ours

MadoHealth reports composition from a single self-collected swab. We name Lactobacillus to species rather than stopping at the genus, we speciate yeast rather than stopping at Candida, and we publish the method and the organism list so you can judge the science before you buy anything. We do not tell you what your results mean about your health, because a composition readout cannot honestly establish that on its own.