This game is incredibly creative, and I think what it scores highest on for me is the theme and the way it implements it. And as a slight biology nerd, I have to say the idea of fighting bacteria is pretty awesome as well. However, I think it has some flaws.
The audio is.. Not the best, though the music seems to be made from in-game sounds or the like, and it sounds incredible. However, the sound of the bullets firing felt a bit shrill and hurt my ears slightly. I think it might need to be toned down slightly.
As for the "fun" aspect of the game, this nails it pretty well! Getting that basic gameplay loop of dodging and weaving as well as nailing a far-off enemy is really satisfying. I think the gameplay could do with some more polish and juice, though. For example, killing an enemy is not as satisfying as it could be, with just a dull thud and the said enemy vanishing. Perhaps you could add some screenshake and some particle effects to make the enemies be funner to kill? Furthermore, you could also add some particle effects and screenshake on death as well. However, the game is still very fun and interesting.
As for the visuals, I think they struggle from lack of consistency. For example, the text is in a completely different style to the game's pixelated art style, and the amount of pixelation seems to vary quite a bit. However, the pixel art itself is pretty good! For the sides of the game, I would recommend adding some sort of texture to them to help them be more visually interesting.
To summarise, this game has an incredible concept and nails the theme head-on, but struggles in some of the other areas and may need some more polish and juice. However, it's still a great game, and don't let my criticism pull you down!