I wish to offer up a possible identification of Alexander Fleming's long mysterious "HAY" bacteria and a possible explanation why it was among the very first bacteria he tested penicillin on.
It is well accepted that Fleming seeded that famous Petri dish at the end of July 1928, discovered that a mold had dissolved mature staph colonies on it in early September 1928 but that he only recorded his first experiment with the mold juice on October 30th 1928.
The results of this (quick) experiment implied to Fleming, I believe, that penicillium juice clearly inhibited the growth of staph-type (gram positive) bacteria .
However it had no effect on coli-type bacteria (gram negative) or acid-fast HAY-type bacteria (ie mycobacterium, the group of bacteria that causes Tuberculosis.)
The Timothy Hay Bacillus, Mycobacterium Phei , I suggest was that "HAY" bacteria and was used as a safe stand-in for the deadly tuberculosis bacteria that had killed so many lab workers in the past.
It is almost tuberculosis-on-steroid in some ways, but is almost completely harmless to humans.
Like all mycobacterium, it is one of the hardest types of bacteria to kill, because it is covered in mycolic acids that resist anything the chemical industry or the immune system can throw at them.
Perhaps Fleming thought an agent that could dissolve even mature staph colonies would be up for dissolving the tough coat of the mycobacterium group - if so he was disappointed.
Because it is not only harmless to humans, but is also the fastest growing of the very slow growing mycobacterium - always important for increasing lab productivity - Mycobacterium Phei seemed a useful addition to the well equipped Bacteriology Lab.
It also has a very unusual nucleic acid composition - 75% GC content - making it of interest to early workers in what we now call DNA research.
Parke Davis, the drug company that bought Fleming's institute's vaccines and serums , was very interested in this bug in the summer of 1928.
They had assisted a group at Yale in investigating some of its characteristics as they related to and contrasted with the deadly TB bug that was killing so many people world wide.
This was a potentially fresh approach to solving the age old TB problem - approaching the beast from an indirect angle.
This Yale team was led by Robert D Coghill - another famous leader in the penicillin saga.
Perhaps the Detroit drug firm had also communicated their interest in the Timothy Hay Bacillus to all researchers connected to Parke Davis International that summer --- including Fleming ?
I would be interested to hear what more experienced writers on Fleming and penicillin think of this suggestion...