Why is it sometimes necessary to use more than one antibiotic?

Practical creation of small series for you to sort out the answer:

Sometimes when an animal is infected with multiple disease-causing organisms, multiple drugs have to be used instead of one antibiotic. Sometimes two antibiotics can "help each other" by using less to achieve a better antibacterial effect, reducing or avoiding toxic side effects, the most typical example being penicillin in combination with streptomycin. A combination of antibiotics can reduce the "fish that slip through the net" and avoid or delay the development of bacterial resistance.

Of course, in most cases, only one antibiotic is used, and combination is only suitable for a few cases, and generally two drugs (two drugs) can be used, triple or quadruple is not necessary, and the simultaneous use of two antibiotics of the same category and the same antimicrobial spectrum should be strictly restricted.