I've always heard "sentience" defined, especially in contrast to sapience, as the ability to simply perceive one's environment. You may be using it under a broader sense, though, because bacteria can undoubtedly sense and react to their environments, and even communicate information to other nearby bacteria. It's a pretty fascinating area.
Here's a summary of a paper in Nature about Bacterial perception:
Better still is the communication, one example is quorum sensing. Bacteria sometimes take little polls before acting, like little mobs trying to rile each other up:
To the bigger point, putting an ethical line in here somewhere always struck me as a bit of a sorites paradox, the endpoints are fairly easy (don't torture humans, don't worry about bacteria), but there's no obvious line in the middle where you couldn't make an argument to include or exclude just one more species, especially when you confront the staggering biodiversity out there, and start to see everything as more of a spectrum and less discrete.
I avoid smarter animals, like pigs, squid, and octopi (dogs, cats, monkeys, or ravens if they ever came up). Cows and chickens are fair game. This is basically arbitrary, but I don't believe it's any more or less tenuous than anyone else's system, we're all making a call with limited information. (As Thomas Nagel put it, no amount of human thinking reveals what it is like to live as a bat.)
Here's a summary of a paper in Nature about Bacterial perception:
http://biosingularity.com/2006/06/04/how-bacteria-sense-thei...
Better still is the communication, one example is quorum sensing. Bacteria sometimes take little polls before acting, like little mobs trying to rile each other up:
http://en.wikipedia.org/wiki/Microbial_cooperation#Quorum_se...
To the bigger point, putting an ethical line in here somewhere always struck me as a bit of a sorites paradox, the endpoints are fairly easy (don't torture humans, don't worry about bacteria), but there's no obvious line in the middle where you couldn't make an argument to include or exclude just one more species, especially when you confront the staggering biodiversity out there, and start to see everything as more of a spectrum and less discrete.
I avoid smarter animals, like pigs, squid, and octopi (dogs, cats, monkeys, or ravens if they ever came up). Cows and chickens are fair game. This is basically arbitrary, but I don't believe it's any more or less tenuous than anyone else's system, we're all making a call with limited information. (As Thomas Nagel put it, no amount of human thinking reveals what it is like to live as a bat.)