Final answer:
The best explanation is that the two strains of Streptococcus aureus are resistant to the antibiotic streptomycin. Some strains of bacteria can acquire resistance to streptomycin through genetic mutations or acquisition of resistance genes from other bacteria.
Step-by-step explanation:
The best explanation for the two strains of Streptococcus aureus not growing on the petri dish with streptomycin-containing agar is that the two strains are resistant to the antibiotic streptomycin. Streptomycin is an antibiotic that inhibits bacterial protein synthesis by targeting the ribosomes. Some strains of bacteria can acquire resistance to streptomycin through genetic mutations or acquisition of resistance genes from other bacteria.
In this case, the two strains of S. aureus have acquired resistance to streptomycin, which allows them to survive and grow on the streptomycin-containing agar. The absence of growth of these strains on the dish indicates that the streptomycin effectively inhibited their growth, while other strains of bacteria that are susceptible to streptomycin may still grow on the dish. It is possible that the student unknowingly streaked two strains of S. aureus that had acquired resistance to streptomycin.