During which growth phase are gram-positive bacteria most susecptible to penicillin?

During the logarithmic phase of growth

Penicillin and other beta-lactam antibiotics are most effective against rapidly growing bacteria(log growth phase)

Bacteria are most susceptible while in the log phase of growth

You can use a "selective" medium that will inhibit the growth of Gram positive bacteria and only allow Gram negative bacteria to multiply.A medium which is commonly used for this is the McConkey agar...