What term describes the bacteria penicillin mainly affected in Fleming's tests?

The bacteria penicillin mainly affected in Fleming’s tests were Gram-positive bacteria.

Fleming tested his mould-derived substance against several organisms after noticing that a Penicillium mould had cleared nearby staphylococcal colonies. It inhibited staphylococci and other Gram-positive pathogens, including organisms associated with illnesses such as scarlet fever, pneumonia, meningitis and diphtheria.

Gram-positive bacteria have a relatively thick peptidoglycan cell wall and lack the outer membrane characteristic of Gram-negative bacteria. Penicillin interferes with enzymes involved in building and cross-linking that wall, making actively growing susceptible cells vulnerable.

The answer is “mainly,” not “exclusively.” Fleming found little or no effect on typhoid and paratyphoid bacteria, which are Gram-negative, although penicillin also affected Neisseria gonorrhoeae, another Gram-negative organism. This exception is why “Gram-positive” is the best canonical description rather than an absolute one.