This is the 10082nd most frequent Russian word.
бактерии
Bacteria.
The word 'бактерии' is used here as the subject of the sentence, referring to bacteria in general and introducing them as the topic.
Бактерии являются микроскопическими живыми организмами.
Bacteria are microscopic living organisms.
In this sentence, 'бактерий' is in the genitive plural case because it follows the verb 'уничтожения' (destruction of).
Антибиотики часто используются для уничтожения вредных бактерий.
Antibiotics are often used to kill harmful bacteria.
Here, 'бактерии' is used as the subject in the nominative plural case to indicate a positive aspect of certain bacteria.