Then you must get one tested. They don't have to be sacrificed.
Take them to ANY vet - have that vet swab their nostrils and do a "culture and sensitivity". They will grow out that bacteria and identify it exactly, by name. They then grow it out on a plate with antibiotic discs. The discs around which they don't grow are the antibiotics to which they're susceptible. That's the exact antibiotic you use.
Please do this. It only takes one sick chick to do the culture and sensitivity. Ask your vet specifically do to this. Write it down if you must - that's what I do. Tell her why. Insist. It's your money. It's about $60 here - way less expensive than culling a flock or wasting money on antibiotics you don't know are working.
You really seriously need to know what this is now.
Let me know if you need help finding a vet. An avian vet isn't necessary, just a vet who is willing to do a C&S. An avian vet is preferred as they can correctly dispense the antibiotic that YOU ONLY WILL ACCEPT AFTER A SENSITIVITY.
Otherwise, you just treat with Tylan. If it's not effected within 3 days, it's either resistant to Tylan or not MG.