Learn Spanish (Latin America): Esa bacteria habría alterado lo que descubrimos ayer. - That bacteria would have altered what we discovered yesterday.

This sentence uses the conditional perfect to talk about a hypothetical past event. It is formed by conjugating the auxiliary verb haber in the conditional tense (which gives us habría to match the third-person subject esa bacteria) and adding the past participle of the main verb, alterado.

Why does it say esa bacteria in the singular when "bacteria" is usually plural in English?

In Spanish, bacteria is a singular feminine noun, and its plural is bacterias. While English speakers often use "bacteria" as a singular collective noun in everyday speech (even though the true singular is "bacterium"), Spanish is strict about its singular form. That is why it uses the singular feminine demonstrative adjective esa.

Why use lo que instead of just qué to say "what"?

You use lo que when "what" isn't a question, but rather means "that which" or "the thing that". If you used qué, it would incorrectly imply a direct or indirect question. Since this sentence is stating a fact about the thing you discovered, the relative pronoun lo que is required.

How do we know descubrimos is in the past tense and not the present?