What type of bacteria did Fleming find were especially susceptible to penicillin?

Fleming found that Gram-positive bacteria were especially susceptible to penicillin.

In his early experiments, the substance released by the Penicillium mould inhibited several Gram-positive organisms, including staphylococci, streptococci, and the diphtheria bacillus, now called Corynebacterium diphtheriae. These bacteria have a thick peptidoglycan cell wall and lack the extra outer membrane found in Gram-negative bacteria, features that help explain penicillin’s early activity pattern.

The result did not mean that penicillin affected every Gram-positive bacterium or no Gram-negative bacteria at all. Fleming’s original observations showed a strong preference for mainly Gram-positive organisms, while organisms such as Salmonella typhimurium and Haemophilus influenzae were not inhibited in his tests.

This is why “Gram-positive” is the best quiz answer rather than “all bacteria.” Later penicillin derivatives, including ampicillin and other broad-spectrum drugs, expanded activity against some Gram-negative bacteria. Acid-fast bacteria and anaerobic bacteria are separate microbiological categories and are not the general answer to Fleming’s observation.