Polymers are disclosed that are chemically modified to retard bacterial growth. Such modified polymers (e.g. ePTFE and polypropylene) are produced by first creating acid groups on the polymer surface through reactions with an anhydride. The acid groups are then linked to polyethylene glycol (PEG) through esterification or other reactions such as amidation. Preferably, at least two different molecular weight PEG species are employed. The antimicrobial surface is completed by linking antibiotics (e.g. β-lactam antibiotics) to the PEG extensions. One preferred embodiment of such a modified polymer is produced using ePTFE, maleic anhydride (MA), and penicillin (PEN) to yield PEN-PEG-MA-ePTFE, which inhibits gram-positive bacteria. The PEG spacer is critical for PEN function in this context, since PEN-ePTFE is ineffective against bacterial growth. Another preferred embodiment incorporates ampicillin (AMP) and a heterobifunctional PEG, HOOC—(CH2—CH2—O)n—NH2, to yield AMP-PEG-MA-ePTFE. This latter example inhibits both gram-negative and gram-positive bacteria.