La masa madre contiene la bacteria láctica.

Spanish uses articles more often than English does. In a sentence like this, la can refer to:

• a specific sourdough starter already being discussed, or
• sourdough starter in a general sense

So La masa madre contiene... is very normal Spanish. English often drops the article in general statements, but Spanish often keeps it.

Masa madre is a fixed noun expression, basically a compound noun. It means sourdough starter.

Here, madre is not acting like a normal adjective such as grande or blanca. It is part of the set term masa madre. So learners should usually treat masa madre as one vocabulary item.

Contiene is the third-person singular present indicative form of contener.

It is used because the subject, la masa madre, is singular.

• la masa madre contiene = the sourdough starter contains
• las masas madre contienen = the sourdough starters contain

So the verb matches the singular subject.

Yes. Contener is irregular, and it follows patterns similar to tener.

Some useful forms are:

• yo contengo
• tú contienes
• él/ella contiene
• nosotros contenemos
• vosotros contenéis
• ellos contienen

So contiene is not formed in a completely regular way.

In Spanish, bacteria is singular.

• la bacteria = the bacterium / the bacteria as a singular unit
• las bacterias = the bacteria

This can confuse English speakers because scientific English often uses bacterium for the singular and bacteria for the plural. Spanish does not use bacterium in normal grammar. The singular is bacteria.

Spanish often uses the definite article with nouns in a general or classifying sense.

So la bacteria láctica can mean something like:

• the lactic bacterium, or
• lactic bacteria as a type/category, depending on context

That said, in more natural scientific wording, many speakers would be more likely to say bacterias lácticas in the plural, because sourdough normally contains many bacteria, not just one.

Because in Spanish, adjectives usually come after the noun.

So:

• bacteria láctica = lactic bacterium / lactic-acid bacterium

This is the normal order in Spanish for many descriptive and technical adjectives.

Because the adjective must agree with bacteria.

Bacteria is:

• feminine
• singular

So the adjective must also be:

• feminine
• singular

That gives us láctica.

Compare:

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

Yes, it is grammatically correct and understandable in Spain.

However, depending on context, a more natural scientific sentence might be:

• La masa madre contiene bacterias lácticas.

That version sounds more natural if you mean that sourdough contains lactic bacteria in general. Your original sentence is still useful and correct for learning the structure.

You might hear article omission in some technical or headline-like styles, but in a normal full sentence, contiene la bacteria láctica is more complete as written.

Still, many native speakers would more naturally choose a plural or less specific version, such as:

• contiene bacterias lácticas

So the main issue is not just the article, but also whether singular bacteria is the most natural choice in context.