The"broad spectrum"penicillins have a greater effect on many gram-negative organisms. They include ampicillin, amoxicillin, carbenicillin, and ticarcillin. This group is still susceptible to penicillinase. The penicillinase-resistant penicillins include oxacillin, cloxacillin,dicloxacillin,methicillin,and nafcillin. Penicillin preparations are standardized on the basis of their growth-inhibition potency against test organisms such as Bacillus subtilis or staphylococci. Activity is expressed in units and is measured in comparison with a standard preparation by determining the zone of inhibition of bacterial growth on an inoculated agar plate. The amount of activity represented by I unit is sufficient to prevent multiplication of a susceptible organism such as bacilus subtilis or certain staphylococci in as much means that 1 unit is equivalent to 0.6 ug of penicillin G. The enormous activity of penicillin may be appreciated from the fact that if 1 mg of the antibiotic were placed in about 5 gallons of broth, the growth of several susceptible organisms would be prevented by the resulting minute concentration of the antibiotic by contrast it would be necessary to add 2 to 20g of a sulfonamide to this volume ofThe “broad spectrum” penicillins have a greater effect on many gram-negative organisms. They include ampicillin, amoxicillin, carbenicillin, and ticarcillin. This group is still susceptible to penicillinase. The penicillinase-resistant penicillins include oxacillin, cloxacillin, dicloxacillin, methicillin, and nafcillin. Penicillin preparations are standardized on the basis of their growth-inhibition potency against test organisms such as Bacillus subtilis or staphylococci. Activity is expressed in units and is measured in comparison with a standard preparation by determining the zone of inhibition of bacterial growth on an inoculated agar plate. The amount of activity represented by 1 unit is sufficient to prevent multiplication of a susceptible organism such as Bacilus subtilis or certain staphylococci in as much means that 1 unit is equivalent to 0.6 μg of penicillin G. The enormous activity of penicillin may be appreciated from the fact that if 1 mg of the antibiotic were placed in about 5 gallons of broth, the growth of several susceptible organisms would be prevented by the resulting minute concentration of the antibiotic .By contrast it would be necessary to add 2 to 20g of a sulfonamide to this volume of