La bacteria láctica participa en la fermentación natural.

In Spanish, it is very common to use the definite article when speaking about something in a general or class-like way. So La bacteria láctica can mean lactic bacteria as a kind of organism or the lactic bacterium depending on context.

English often leaves the article out in general statements, but Spanish usually does not.

In Spanish, bacteria is singular. Its plural is bacterias.

So:

• la bacteria = singular
• las bacterias = plural

This can feel surprising to English speakers because in formal English bacterium is singular and bacteria is plural, but Spanish does not work that way.

Because adjectives in Spanish usually agree with the noun they describe.

Here:

• bacteria is feminine singular
• so the adjective must also be feminine singular
• láctico becomes láctica

You can compare:

• el ácido láctico
• la bacteria láctica
• las bacterias lácticas

Here láctica means lactic.

In scientific or food-related contexts, bacteria láctica refers to lactic acid bacteria, meaning bacteria involved in producing lactic acid during fermentation.

So it does not simply mean milky in an everyday sense. In this sentence, it has a technical/scientific meaning.

Participa is the third-person singular present indicative form of participar.

That means it matches a singular subject such as:

• la bacteria participa
• el microorganismo participa

If the subject were plural, you would use participan:

• Las bacterias lácticas participan en la fermentación natural.

Because participar en is the normal pattern in Spanish for saying to participate in or to take part in something.

So:

• participar en un proceso
• participar en una actividad
• participar en la fermentación

This is just the standard preposition used with this verb in this meaning.

In Spanish, descriptive adjectives usually come after the noun.

So the normal order is:

• fermentación natural
• vino tinto
• proceso químico

Putting natural before the noun would sound unusual here and would generally add a special stylistic or emphatic effect. In a neutral scientific sentence, fermentación natural is the expected order.

Because not all Spanish adjectives change form for gender.

Natural is one of many adjectives that have the same singular form for masculine and feminine:

• el proceso natural
• la fermentación natural

It only changes in the plural:

• los procesos naturales
• las fermentaciones naturales

The accent marks show the correct stressed syllable.

• láctica is stressed on lác
• fermentación is stressed on the final ción

These written accents are not optional; they are part of the correct spelling.

They also help with pronunciation, especially for learners, because Spanish stress is regular but not random. When a word does not follow the default stress pattern, an accent mark often appears.

A rough pronunciation guide for Spain Spanish is:

la bak-TE-ria LAK-ti-ka par-ti-THI-pa en la fer-men-ta-THYON na-tu-RAL

A few useful points:

• ci in participa is pronounced like th in much of Spain
• ción in fermentación also has that th sound in Spain
• the r in bacteria and fermentación is a tapped Spanish r, not a strong English r

If you are learning Peninsular Spanish, that th sound is important:

• participa ≈ par-ti-THI-pa
• fermentación ≈ fer-men-ta-THYON

Yes. That is also correct.

The difference is mainly one of perspective:

• La bacteria láctica participa... presents it as a type or category in a general way
• Las bacterias lácticas participan... talks about them as a group

In scientific and explanatory writing, both patterns can appear. The plural version may feel more natural if you are thinking of many bacteria rather than the category as a whole.