Final answer:
The antibiotic matching the description of binding to the 50s subunit and being effective against chlamydia and legionella is erythromycin, a broad-spectrum, bacteriostatic macrolide antibiotic.
Step-by-step explanation:
The antibiotic described in the question with the characteristics of having a lactone ring and sugars, being broad-spectrum, and effective against organisms such as chlamydia and legionella, is one that inhibits protein synthesis by binding to the 50s subunit of bacterial ribosomes. This binding prevents chain elongation, making it bacteriostatic, which may lead to bactericidal effects depending on the concentration. Given the information, the correct answer is erythromycin, as it was the first macrolide isolated from Streptomyces erythreus and it acts by inhibiting peptide bond formation and preventing translocation during protein synthesis.