Bacteria can develop resistance to certain antibiotics and this resistance can be transferred from one generation of bacteria to another generation genetically. Therefore, strains of antibiotic resistant bacteria can develop. This is usually based on past experience and specific signs of disease. Also, the number of times per day that treatment must be given is very important. Whether or not the drug is given in an extralabel manner and thus would require a veterinarian's prescription can be important The potential adverse side effects must be considered also.
All common antibiotics have advantages as well as disadvantages.
There are a number of newer drugs as well as some that have been around for many years. Also relatively new are the long acting tetracyclines which can now be given subcutaneously instead of intramuscularly. There is no best drug to use, as is obvious from all the above considerations. Discuss the best approach to treatment of pneumonia with your veterinarian. Come up with a plan to identify pneumonia cases, which drug to use, and by what route of administration to give this drug.