For a first screening of antibiotic properties in a given substance, which bacterial strains (risk group 1) would you test to get a first general indication on whether the substance has a broad antibiotic spectrum?
There is a very extensive literature on labs screening natural isolates for antimicrobial agents, I would suggest you review some of those and follow. That way in case any reviewers question your choices, you can provide citations and get strains that are standard for these types of tests. Having said that, I would probably pick a few G+ strains and a few G- strains, for example E.coli and Strep or Staph as good starting points.
If you want to compare the antibacterial activity of your substances to well-known antibiotic drugs, it might be a good idea to use the reference strains that are recommended for the drugs. See the EUCAST protocols for each drug to learn which reference strains are used.
It depends entirely what you are trying to assess. Certain antibiotics and antimicrobial agents are able to diffuse through agar. In that case spores of Bacillus subtilis are added to agar while cooling and poured onto plates. When it sets, standard holes can be punched in the agar and different strengths of material can be added to the wells. This is compared to a standard strengths. Then the antimicrobial agent can be compared to a standard. The other organism I used was Pseudomonas auruginosa but because it is a pathogen, other Pseudomonas could be used. They are often the most resistant to antimicrobial agents. E coli is also another organism I have used (not 0157). But it would be worth testing a Gram positive coccus, a Gram positive rod, and a Gram negative rod to compare different effectiveness.