Bacteriile benefice sunt în filtru.

In Romanian, the definite article is usually attached to the end of the noun, not written as a separate word.

So here:

• bacterie = a bacterium
• bacterii = bacteria
• bacteriile = the bacteria

The ending -le is the definite article for this plural form.

It is not a typo. The form comes from the plural noun bacterii plus the definite ending -le.

So:

• bacterie = singular
• bacterii = plural
• bacteriile = definite plural

The two i's are part of the normal spelling. In pronunciation, they are both heard: roughly bac-te-ri-i-le.

Romanian adjectives must agree with the noun in gender and number.

bacterie is a feminine noun, and bacteriile is feminine plural, so the adjective must also be feminine plural:

• benefic = masculine singular
• benefică = feminine singular
• benefici = masculine plural
• benefice = feminine plural

That is why the sentence uses bacteriile benefice.

Sunt is the present tense, 3rd person plural form of a fi (to be).

Because bacteriile is plural, the verb must also be plural:

• este = is
• sunt = are

So Bacteriile benefice sunt... means The beneficial bacteria are...

Romanian usually leaves subject pronouns out when they are not needed, because the verb ending already shows the person and number.

So:

• Ele sunt în filtru = They are in the filter
• Sunt în filtru or Bacteriile benefice sunt în filtru are both perfectly natural

In this sentence, ele would be unnecessary unless you wanted extra emphasis.

This is a very common point of confusion for English speakers. Romanian often uses a bare noun after a preposition where English would use the.

So în filtru can naturally mean in the filter or inside the filter, especially when the context already makes it clear which filter is meant.

You can also have în filtrul... when the noun is made explicitly definite, especially if more information follows, for example:

• în filtrul acvariului = in the aquarium's filter

So în filtru is a natural Romanian way to express the location here.

Yes. Romanian word order is more flexible than English word order.

The given sentence is the neutral order: Bacteriile benefice sunt în filtru.

But you could also say: În filtru sunt bacteriile benefice.

That version puts more emphasis on în filtru. The meaning stays basically the same, but the focus changes.

The singular version would be:

Bacteria benefică este în filtru.

Here:

• bacteria = the bacterium / the bacterium cell
• benefică = feminine singular
• este = is

A detail that often surprises English speakers: Romanian bacteria is singular definite, while English bacteria is plural.

Filtru is a neuter noun.

In Romanian, neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So you get:

• un filtru = a filter
• filtrul = the filter
• două filtre = two filters

This does not change the form în filtru in the sentence, but it is useful to know when learning related forms.

Usually, yes, in this kind of sentence. Benefic means beneficial, good for, or helpful, depending on context.

So bacteriile benefice is a natural way to say beneficial bacteria.

In everyday Romanian, this is a normal and common adjective, especially in scientific, medical, or technical contexts.