Sentence Correction

Here is a sentence from the.
McGraw Hill conquering GMAT Verbal & Writing.

Surviving in an antiseptic environment, the bacteria found in hospitals
are some of the most virulent and dangerous known to mankind.

The above sentence i believe that bacteria is the collective noun for a group of micro-organisms
so using 'are' would be incorrect 'is' would be better, these were my first thoughts when i removed
the Phrases "Surviving in an antiseptic environment" and "found in hospitals" which left the sentence

the bacteria are some of the most virulent and dangerous known to mankind
it sounded better if i put it as
the bacteria is the most virulent and dangerous known to mankind.

Although i agree with the answers published in McGraw Hill, what went wrong in the step by step thoughtprocess??