Emotions as commonly understood and defined require a nervous system, which bacteria plainly don’t have. That definition of emotions makes this an uninteresting question that really is not worth answering.
But if we look beyond the machinery of emotions to their purpose (although inferring purpose is always dangerous in biology), we can fashion a definition of emotions that makes this question a bit more interesting.
The definition I’ll propose is this: emotions are “tags” on memories that allow them to influence future behavior in an adaptive manner. If behavior A makes you happy, you are more likely to do A again. Conversely, if B makes you sad, you will avoid B in the future. And it is well-established that emotional content affects the persistence of memories – we quickly forget mundane unemotional events, while memories of highly emotional experiences may persist for our entire lives.
So for bacteria to have emotions in this scheme, they would require (1) the ability to form memories, (2) some plasticity in behavior and (3) the ability of memories to influence behavior. This last requirement may be redundant, as I don’t know how we would infer the existence of a memory if it did not have a behavioral correlate. But I think it helps to clarify our thinking nonetheless.
There are several reports which demonstrate that bacterial behavior can be influenced by their history: past experience of phosphate starvation leads to faster and more extensive induction of alkaline phosphatase[1]; frequent switching of carbon sources results in derepression of lactose-metabolizing enzymes[2] are two examples, and there are several more[3] .
So bacteria clearly have memory. But these examples appear to be strictly algorithmic, essentially IF…THEN statements which I think no one would construe as evidence of emotion. By my definition above, we need some evidence that these memories can be modified, either in sign (positive vs negative) or strength of response, in order for them to count as evidence of emotion.
The machinery for “tagging” memories exists in bacteria in the form of various signal transduction systems. The composition and extent of these systems varies greatly between bacterial species, leading Michael Galperin to classify the bacteria which possess a high number of such signal sensors as “high IQ” and to further classify bacteria as extroverts or introverts depending on whether these sensors are directed primarily at the external or internal environment[4] .
Experimental results from the sporulation response of Bacillus subtilis with respect to its history of nutrient access and deprivation suggests that these signal transductions systems do indeed modulate the strength of bacterial memories. The decision to sporulate is a critical one for an individual cell – it is energetically costly and results in a high probability of cell death. Episodes of nutrient deprivation increase the likelihood that a second episode will trigger sporulation. More severe deprivation not only increases the likelihood and the speed of the commitment to sporulation, but the memory of this earlier deprivation persists for much longer[5]for severe deprivation than for mild deprivation .
I would argue that these observations fulfill the criteria for emotion in bacteria, albeit a non-neuronal one. They provide evidence for the intensification and persistence of the memories of past events. At a minimum, they constitute evidence of “valence”, the ability to assign existential value to experience[6], considered a key component of emotion . Bacteria don’t have our emotions, but they do react emotionally.
Update: the same sort of reasoning has been applied to bees
Footnotes
[1] Autoamplification of a two-component regulatory system results in “learning” behavior.
[2] Memory and Fitness Optimization of Bacteria under Fluctuating Environments
[3] The cognitive cell: bacterial behavior reconsidered
[5] Memory in Microbes: Quantifying History-Dependent Behavior in a Bacterium