This is the 10015th most frequent Polish word.
bakterię
"Bakterię" is the accusative singular form of "bakteria," meaning "bacterium" in English.
Here, 'bakterię' is in the accusative case, indicating that it is the object of the verb 'widzę' (I see).
Widzę bakterię w probówce.
I see a bacterium in the test tube.
In this sentence, 'o bakterii' uses the locative case 'bakterii', due to the preposition 'o' (about).
Profesor mówi o bakterii w wykładzie.
The professor talks about the bacterium in the lecture.
This sentence uses 'bakterie' in its plural nominative form.