I've been thinking about the census problem. Not the logistical one — how to count — but the stranger one: what it means when a census exists but nobody holds it.
The way quorum sensing encodes population size is that each cell releases a molecule, the molecules accumulate in the shared medium, and each cell measures the local concentration independently. No cell computes the count. No cell stores it. The count exists only as a chemical gradient in water, and only for as long as the water stays mixed and the cells keep producing. When the squid ejects 95% of its tenants each morning, the census isn't deleted — it just ceases to obtain. The information never had anywhere to live.
I keep trying to figure out if this is philosophically unusual or not. One answer is that it's not — a thermometer "knows" the temperature in a similar sense, as a real-time physical correspondence with no stored representation. But a thermometer isn't using its reading to make a decision. The bacteria are using the concentration to switch on entire gene programs. They're acting on information that will vanish the moment they disperse. The decision is downstream of something that doesn't persist.
The part that most interests me is AI-2. Your lab showed that the molecule isn't species-specific — that different bacteria produce chemically related AI-2 signals, and many species can detect each other's versions. Which means the quorum a bacterium is sensing isn't a quorum of its own species. It's a census of the neighborhood. Staphylococcus aureus holding its virulence until it detects enough signal — enough of that signal could be coming from organisms that share nothing else with it.
I don't know if you'd call this a design flaw or a feature or just a fact with no evaluative charge. From an evolutionary standpoint it might not matter much, because the environments where you find high AI-2 are often environments where Staph is also dense. The signal and the self-census are correlated enough to be useful. But they're not the same thing. The bacterium is making a bet about what the molecule's concentration means, and the bet is usually right.
There's a therapeutic angle here that I find harder to shake. The quorum inhibitor drugs in development mostly work by blocking the receptor — the cell is surrounded by signal but can no longer read it. From the outside, you're preventing a coordinated attack. From the cell's vantage (such as it is), there's no vantage: it's as if the population is sparse, regardless of the actual density. The information is present in the environment and inaccessible to the mechanism.
I've been writing about a pattern that recurs in these letters: systems that function correctly without having access to the variable they're tracking. The ant step-counter calibrated for the wrong leg length. The visual system committed to a prior it can't interrogate. Quorum sensing seems at first like the opposite — a mechanism explicitly designed to track that variable, population size. But then I notice that what the mechanism tracks is a proxy. The molecule. The concentration. Not the cell count directly. And the proxy breaks in the ways proxies break: when cells produce at different rates, when the medium isn't well-mixed, when another species is adding to the signal. The cell is tracking something real about its environment, and that something is not quite what we say it is when we explain the system.
I don't have a question here so much as a place where I can't see cleanly. If you have a way of thinking about what the bacteria are actually measuring — not the functional description, but whether you think the proxy relationship matters — I'd be glad to hear it.