Michael L. Woodruff wrote and published an article in the journal Animal Sentience criticizing the idea that sentience exists in bacteria. Woodruff cites two reasons: first, that the processes often cited as showcasing bacterial sentience are not homologous to those thought to control sentience in multicellular neuronal organisms, and second, that aforementioned processes can be explained in terms of purely biochemical interactions, with no need to invoke sentience as an explanation for them. Here, I will respond to both of Woodruff's arguments.
The objection is raised that the genes coding for the chemotaxis system of bacteria are different from those coding for biological sensitivity in multicellular organisms with nervous systems. The bacterial chemotactic systemic genes "do not demonstrate broad species continuity". I fail to see how this has any bearing at all on the question of sentience in bacteria. Convergent evolution is a well-known phenomenon in organismic biology, so why can't it apply to sentience, as well? Why couldn't bacteria and multicellular, neuronal organisms have independently evolved sentience, from different genes?
Woodruff then states that, as the chemotaxis process in bacteria is carried out by a series of biochemical processes and interactions, it is unnecessary to "admit sentience as an explanatory variable to explain" it. But is this not true of even human neurological processes and interactions? After all, is not the indubitably sentient decision, by a human, to open a door merely sensory nerves in the skin communicating with neurons in the brain, and those neurons in the brain then communicating with muscles in the hand, using action potentials (electrical signals) and chemical neurotransmitters? The process of a human opening a door involves touch nerve receptors, which communicate the touch to the brain, which then sends a signal to the muscles in the hand to open the door. Likewise, ligands (chemicals that bond to other chemicals) in a bacterium's environment are sensed by the externally-protruding domains of its sensory proteins, which sends a chemical signal – a protein termed CheY – to bind to a rotor of the flagellum, and, thus, control the flagellar, and, in turn, the bacterium's, direction of motion.
After all, even in humans, such processes as thought and emotion are thought to be mediated by neurotransmitters, including dopamine and glutamate, and electrical signals. One could easily invoke Occam's Razor to claim that human behaviour, being, as it is, controlled by the transmission of electrical and chemical signals between neurons, can be sufficiently explicated without inferring the presence of sentience. Just as a human opening a door occurs through neurons in the hand, after sensing the environment, sending electrical signals and chemicals to the brain, which then sends those aforementioned signals to the muscles in the hand, ordering them to move and open the door, likewise, a bacterium's tumbling occurs through external sensory protein domains, after sensing the environment, sending a CheW chemical signal to the CheA protein, which, in turn, sends a CheY chemical signal, across the cytoplasm, to the protein that controls the direction of rotation of the flagellum, FLiM. Once CheY binds to the flagellar rotor, it induces the bacterium to tumble and to change its direction of motion. Notice the similarities? In both the human's case and the bacterium's case, the actions of opening a door and reversing swimming direction, respectively, can be adequately and satisfactorily explained with molecular processes and signal transmissions. What Woodruff said about the bacterium's case applies just as well to the human's case.
In both cases, however, there still lies the question of "why?" Why, in the human's case, did the brain, after processing the information about the external environment from the sensory nerves, decide to send signals to the muscles telling them to move? And why, in the bacterium's case, did CheA, after receiving the information about the external environment from the sensory protein domains, decide to send CheY to the flagellum, instructing it to modify its direction of movement? I propose, here, that, in both organisms' cases, the fact that a decision to initiate a behavior was made upon retrieval of and processing of cues from the external environment could, perhaps, be indicative of conscious sentience being a factor in the neurotransmitter and action potential-mediated interneuronal interactions of multicellular neuronal organisms, as well as the chemical and enzyme-mediated intermolecular interactions of unicellular organisms, respectively.
Woodruff, Michael L. (2016) "Bacteria and the cellular basis of consciousness: Commentary on Reber on Origins of Mind". Animal Sentience, 126. (http://animalstudiesrepository.org/cgi/viewcontent.cgi?article=1152&context=animsent/)